HYI)1RQI-'Y'E3IE5 0 F Z I N C A ND RE L. A I' ED

MEI'AL 0,0—DIALKYL DITHI01-H0S1-HATES

m.

SUARWAN KtJMAR DEWAN

Thesis presented for the degree of Doctor of Philosophy University of Edinburgh October, 1986

/ To my dearest brother Partap Kumar Dewan for his undying encouragement. Declaration

I declare that this thesis is entirely my own composition, that the work of which it is a record has been carried out by myself, and that it has not been submitted in any previous application for a Higher

Degree.

Postgraduate Lecture Courses

The following is a statement of the courses attended during the period of research :-

Organic Research Seminars (three years attendance). Current topics in Organic Chemistry (over three years) Organo-silicon Chemistry (5'lectures). Chemistry at ICI and Beecham Pharmaceuticals (5 lectures). Postgraduates' Techniques' Course (5 lectures).

German Language Test

I have passed the stipulated Departmental German Language Test. Acknowledgements

First of all I would like to thank British Petroleum p.l.c. for the major financial support, Overseas Research Scheme Committee of the Vice-Chancellors and Principals of the U.K. Universities for an ORS award and to the Principal, Daya Nand College, Hisar, University of Kurukshetra, India for the leave of absence. I would like to cordially thank Ian

Gosney, Anthony J. Bellamy, and Alan J. Burn for assigning the research problem and for placing the research facilities at my disposal. Ian Gosney and Alan Burn are also thanked for their valuable comments on and appreciation of my regular reports on this work. Alan Burn is also thanked for providing some of the background literature and for arranging to get some elemental analyses done at Sunbury. Deep thanks are also due to the discoverers of NMR Spectrometer and to the Chemistry department (E.tJ.) for allowing me to use JEOL FX60 NMR Spectrometer for 31 p nuclei, without which the work would have been beyond my capability. Thanks are also due to the discoverers of zinc dithio- phosphates and related compounds used in this study. I would like to express my gratitude to Anthony Bellamy and family for their warm affection which I enjoyed for almost a term on my arrival before they left Edinburgh for Sweden. A short term affection, which gave me a sense of belonging in a land some four thousands, miles away from home; a short term affection, which has become a life long memory. Thanks are also due to John Broom and Evin Williams of the departmental glass blowing service for their instant help. John T. Sharp, Tom Brown, Robert Baxter, Stephen Newlands, Donald Cameron, William Henry, Rita Collins, Robert Ramage, Robert Don%an, Kate Karse, Prabhakar Gupta and family, Abdul Rashid and family for their moral support. My great friend, Qibla Ayaz of Peshawar (Pakistan), is also thanked for his warm friendliness which has become a treasure of memory. Thanks also go to the postgraduates of Pollock Halls of Residence (E.U.) who gave me a chance to act as their representative, thus adding to my experience of the British culture. The Indian and other students of the University of Edinburgh are also thanked for assisting me in founding the E.U. Indian Students

Society. I am also grateful to K.S. Sharma (Rohtak), B. Vig (Rohtak), S.K. Arora (Hisar), and Naresh Kumar (Hisar) for encouraging me before this work. Edwin and

Salina are also thanked for typing this thesis. Last, but not the least I wish to express my deep gratitude to my beloved sister, brothers and parents whose living memories helped me to •tide over this period of great homesickness and to my dear wife for joining me in the

final year and for uncomplainingly bearing my long stay at work and for her constant encourament and to my baby, for his lovely smiles upon my return from the department.

ABSTRACT

The hydrolysis of commercially important

lubricant additives, Zinc and related metal 0,0-dialkyl

dithiophosphates, both neutral and basic forms, has been

investigated prinicipally by application of 31 P n.m.r.

spectroscopy. These hydrolyses followed similar

pathways and gave rise to a plethora of products,

including a precipitate which has been shown to be a

mixture of the sulphide, oxide, phosphate, and

pyrophosphate of the respective metal by elemental

analysis. Three of the major soluble products from the

hydrolyses have been identified as O,O-dialkyl

monothiophosphoric acid, monoalkyiphosphoric acid, and

phosphoric acid by direct comparison with authentic

samples.

The hydrolyses proceeded via the

formation of 0,0-dialkyl dithiophosphoric acids, which

prior to conversion into products, formed a highly

unstable and unidentifiable intermediate with a

chemical shift at 58.Oppm.

In the case of basic Zinc O,O-diisopropyl

dithiophosphate, hydrolysis resulted in its initial

conversion into the corresponding neutral salt, whose

subsequent hydrolysis was found to be unexpectedly

inhibited by the concomitant formation of zinc oxide.

Other studies have shown that the basic salt exists in

equiiibrium with its neutral form and that the presence

of water favours the latter. CONTENTS

INTRODUCTION Page No.

General 1

Programme of Research 20

RESULTS AND DiSCUSSION

Hydrolysis of neutral zinc and related metal

O,O-dialkyl dithiophosphates 22

Hydrolysis of basic zinc O,O-diisopropyl

dithiophosphates (Hexakis-(O,O-diisopropyl

phosphoro-dithioate)-t-4-tetra oxo zinc) 77

Future Research Work 85

EXPERIIIENTAL

Symbols 86

Instrumentation and General Techniques 87

Preparation 90

Hydrolyses 99

REFERENCES 106 I NTRDDUCT I JN INTRODUCTION

CONTENTS

A. General l3ackqround Page No.

Oxidation 6 Thermal degradation 12

Reactions at metal surfaces 17 Interactions with other additives 18

Hydrolysis 18

B. Programme of Research 20 -1-

A. GENERAL BACKGROUND

For the past thirty years, complex metal compounds containing metal-sulphur bonds have been the subject of extensive chemical investigations due mainly to their practical usage in a wide variety of areas ranging from biochemical to purely chemical fields. They are used as anti-oxidants, oil-additives, pesticides, colourimetric analytical agents, and in flotation processes 1 ' 2 .

The metal dithiophosphates (NDTPs), in particular, have a reputation of being efficient lubricating oil additives 1 . Mineral. oils are required to lubricate widely diverse mechanisms, from hinges to the giant diesel engines of large ships, which subject the oils to correspondingly diverse operating conditions. In a single application a lubricating oil will often be expected to satisfy many different requirements, i.e. in addition to controlling or reducing friction, mineral oils are usually expected to reduce wear and often to prevent overheating and corrosion, etc. Often it is not economically possible to obtain a mineral oil, even by the most refined techniques, that can, by itself, satisfy all of these requirements. Additives are used as an attractive method of achieving the required performance level -2- consistent with economic production. In fact mineral oils without additives are rarely sufficient as lubricants for modern equipcinent.

The economic importance of lubricants is often underestimated. About 30% of the total energy produced is wasted by friction and beyond that, wear causes considerable losses of materials. Up to 4.5% of energy consumed could be conserved by better lubrication. Since the first use of additives in 1850 to improve the performance of lubricating oils, their consumption has grown enormously and now forms the basis

of a multi- million dollar industry.

The additives perform a variety of functions and hence are differently known - detergent/dispersant additives, pour-point depressants, anti-foam additives, rust inhibitors, corrosion inhibitors, anti-oxidants and load-carrying additives

(i.e., anti-wear (AW) and extreme pressure (EP)], and viscosity-index improvers. However, one type of additive may perform more than one function. For

example, metal dithiophosphates (MDTPs) (1) are used mainly as anti-oxidants, corrosion-inhibitors and load-carrying additives. These include zinc, antimony,

molybdhum, lead, nickel and cadmium dithiophosphates.

The most common and economical MDTPS are the zinc -3-

dithiophosphates (ZDTPs) (2).

S OR RO S S OR ," \ / " • / Zfl /P\ /N S OR RO \

(1) (2)

R = alkyl or aryl R = alkyl or aryl

N = Zn, Ca or Ni

ZDTPs are classified as either primary, secondary, or aryl, depending upon the alcohol from which they are derived. Based on their relative performance levels, the ZDTPS are selected for a particular application 3 For example, because of their greater degree of thermal stability, aryl ZDTPs are extensively used in those applications where high temperatures are needed, e.g in diesel engine oils.

Primary ZDTPs are widely used in both engine oils and hydraulic oils, whilst secondary ZDTPS are used mainly

in hydraulic oils. Primary and secondary ZDTPs thus have relatively good anti-wear performance, good anti-oxidant qualities and low cost. The commerical

ZDTPS usually contain mixed alkyl groups, e.g. ethyl or butyl. The ZDTPs are either used alone or in combination with other additives depending upon the requirements of the engine. Both the anti-wear and thermal stability characteristics of the alkyl-type -4-.

ZDTPs vary with different alkyl substituents.

Zinc dithiophosphates are, known to exist in two forms referred to as simple neutral (2) and complex basic (3). Commercially manufactured ZDTPS usually contain both these types because the manufacturing process for the neutral forms from ZnO and

(RO) 2PS 2K (R = alkyl or aryl) usually leads to the formation of the basic salts as by-products to varying extents.

r RO S Zn 40 R = alkyl or aryl I / P \ LRO S 6 (3)

Mineral lubricating oils are usually in contact with air whilst in storage or when being used in

situ at elevated temperatures. As a consequence, they can undergo oxidative breakdown and thermal degradation thereby producing oil-soluble and oil-insoluble products that may appear as resins, sludges and acidic materials.

Apart from these two forms of degradation, oil may deteriorate further because of a plethora of contaminants, including unburned fuel in an engine, dust

from the atmosphere, rust products, etc.. -5-

One very common contaminant is water which may gain entry into the system during storage, from combustion of the oil, and from the atmosphere through leaks into system. Not only are all of these contaminants undesirable in themselves, but two or more in conjunction with each other can cause further deterioration leading to diminuition in the performance of the mineral oil. For example, it is recognised that the oxidation products of the oil and water can form a corrosive mixture and as a result produce more corrosion products. It is also recognised that the oxidation products of the oil and water can form a surface-active mixture which will emulsify with the oil and block feed holes and filters. The same emulsion makes an ideal medium for micro-biological growth (bacteria and fungi) which feeds on the oil itself and also blocks oil pipes, valves and pumps.

It is with the view to preventing the contamination of the lubricating oil due to corrosion and oxidation that ZDTPS and other additives have enjoyed widespread us-age for the last forty years. Since the ZDTP additives in a formulated oil are exposed to a variety of influences that lead to decomposition, it is possible that the ZDTPs function by undergoing (1) oxidation, (2) thermal degradation, (3) reactions at metal surfaces, (4) interactions with other additives, -6- and (5) hydrolysis. The following is a chemical description of the different modes of decomposition of ZDTPS when subjected to various chemical agents that are formed in engine oils under operating conditions.

(1) Oxidation Antioxidants can function in two different ways either as radical scavengers (inhibitors) by reacting with chain-propagating alkyl peroxy radicals, or by destroying alkyl hydroperoxides by a non-radical

RO SOR P / /\ RO \S Zn—S OR (2)

RO 2

RO sf S OR RO /S SOR

R02 -i- P P (or) /\ /\ RO S —Zn— S OR RO i —Zn—S OR (4) t

RO S S OR RO S S OR \\/ P P - Pt . + p /\ /\ + /\ RO S—S OR RO S Zn—S OR

( 8 ) (8) ( 7 )

Scheme 1 -7-

mechanism. MDTPs have been shown to react by both ways.

In 1964, Colcolough and Cunneen 4 suggested that the zinc

complexes react with alkyl peroxide radicals by an

electron transfer mechanism as oitlined in Scheme 1,

whereby an electron is abstracted from an electron rich

sulphur atom leading to the formation of the disuiphide

(8) as the major product. It was argued that the

radical cations (4) and (5) decompose rapidly to give

the dialkyl dithiophosphate radical (6) which in turn

undergoes mutual combination to form the disuiphide (8).

Burn5 discarded this mechanism in favour of Scheme 2 by

RO S: S OR

P P /\ /\ RO S

::0. S OR

/\ /\ /\ /\ RO S —Zn— S OR RO S —Zn— S OR

(9a) R62 (9b)

RO S—S OR

P P + 2R62 + Zn 2 /\ RO S S OR

(8)

Scheme 2 -8- arguing that the dithiophosphate radical (6) would not be formed under the conditions described. Instead, he invoked a stabilised peroxy radical-zinc complex (9) which on attack by a second peroxy radical R0 2 gives rise to the disulphide(8). Burn suggested that the function of the metal in the above Scheme was to provide an easy route for the heterolysis of the proposed intermediate(9). This is evidenced by the fact that both the disuiphide (10) and the zinc complex (11) containing no metal and sulphur, respectively do not react with t-butyl peroxy radicals. Burn's proposal

((PrO) 2PS 2 ) 2 1C 12H 25P0 2) 2Zn (10) (11)

in turn was disputed by Howard et a1 6 who argued that alkyl peroxy radicals react with ZDTPs and related complexes at the metal by an electron transfer mechanism

ROO + E(RO) 2 PS2 3 2Zn-3 ROOZn - S 2P(OR) 2 + (R0 2 )P5 2 . . . (1) to generate the dithiophosphate radical (6) (equation

1). They also showed that dialkyl dithiophosphoric acids, (R0) 2 PS2H react rapidly with t-BuOO and thereby inhibit autooxidation of styrene. The most probable mechanism for inhibition by the free acid involves abstraction of the hydrogen attached to sulphur by R02 with (RO) 2PS2 being less active than R0 2 towards propagation (equation 2). e identification of the disulphide (8) as the major product has recently been confirmed by other workers7 .

(RO)PSSH + R02 ---> (RO) 2 PS2 + ROOF! .... (2)

In a different investigation 8 ' 9 , it has been demonstrated that anti-oxidant activity of ZDTPS was due to a process involving heterolytic hydroperoxide decomposition. This was supported by the formation of phenol from cumene hydroperoxide (12) by the action of ZDTPS. Because no obvious mechanism for peroxide

Me Ph — C -----O—OH Me (12) decomposition by ZDTPS was clear, the authors concluded that it was the intermediates formed by the decomposition of ZDTPz that were responsible for the ionic decomposition. Burn et al' ° observed that catalytic decomposition of cumene hydroperoxide occurred via a three stage reaction. The first stage is rapid and involves the formation of the disulphide(8). The second stage (induction period) is very slow and - 10 - probably involves the formation of. the unknown active catalytic intermediate. In the third- stage rapid decomposition of the hydroperoxides gave products which were characteristic of ionic decomposition.

Since the disuiphide (8) was found to be formed during the first stage of the reaction, these workers concluded that this stage of the reaction was primarily the reduction of cumene hydroperoxide (12) to dimethyl benzyl alcohol with the concomitant oxidation of the ZDTP to the corresponding disuiphide (8). The mechanism that they proposed is outlined in Scheme 3. Their work was subsequently confirmed by other groups 12L The products derived from the cumene hydroperoxide were identified as acetone, phenol, and

RO S S. OR PhCHe OOH RO S. S OR 2 p " p > p P +PhCNe2 O RO S-Zn-S OR -OH RO S-Zn-S OR (2) PhCHeO

RO S S OR OH 1W St IS OR

P P +.Zn ( /\ /\ /\ /\ RO S - S OR OCMe 2Ph 1W S—Zn—S OR (8) (13)

Scheme 3 - 11 -

10 dimethyl benzyl alqhol . Burn et al 10 put forward the following general mechanism which is well established as a rearrangement reaction involving catalysis by various

electrophilic species (X) such as H,SO2 , FeC13 17

(Scheme 4).

PhCMe 200H + X ;, PhCHe 20 + XOH - PhCMe2 O , Ph0CMe 2 Ph0CHe2 + PhCNe200H > PhOH + Me 2CO + PhCHe 20 PhOCMe2 + XOH - PhOH + Me 2CO + X

Scheme 4

Product yield studies showed that the formation of phenol and acetone occurred primarily in the third stage which they arued involved catalytic decomposition as the key step. Consequently, it was inferred that the second stage (induction period) must be the time during which the ionic catalyst is formed from the ZDTP.

Several ideas about the nature of the

catalyst have been proposed by different investigators.

Larson 8 suggested that it is formed by thermal

decomposition. Burn et a1 1 ° discounted this idea because the ionic decomposition occurred at temperatures below

which the ZDTP complexes are known to be thermally - 12 -

stable. In contrast, Holdworth et a1 17 and Scott' 8 favoured sulphur dioxide formation while Ohkatsu 19 suggested that the catalyst was sulphuric acid. Yet others 20 have proposed the radical cation, ZDTP+. The latter idea is reinforced by the fact that free-radical scavenging anti-oxidants inhibit the heterolytic decomposition by interfering with the reaction between

ZDTP and R0 2 to give ZDTP. The fate of the ZDTP is complicated further as shown by the subsequent discovery that it is also oxidized to the corresponding basic zinc dithiophosphate complex 12 ' 1229 . As a result the disuiphide which was observed by Burn et a1 10 to be formed in the first stage of the reaction is formed as represented by equation (3).

E(RO) 2 PS2 32 Zn + R 1 00H ---> [(RO) 2PS 2 ] 6Zn 4O + E(RO) 2PS 23 2 + R 1 0H .... (3)

Basic ZDTPs and the disulphide are reported not to scavenge peroxy radicals

(2) Therral degradation

The most widely accepted theory for the mode of action of ZDTP additives is that they react with the metal surface by undergoing thermal decomposition to form soft, easily sheared layers of a polymeric residue on the rubbed surfaces that preven t metal tometal contact - 13 - and subsequent seizure. Many theories have been put forward to explain the thermal degradation of ZDTPs which occurs at temperatures in excess of 1500 C. Asseff30 has correlated the thermal stability of ZDTPs with the ease with which fi C-H bonds can be broken (see 14). This explains why aryl ZDTPZ are more stable than

- P /S S /OR R' - I /\ H S — Zn — S OR

(14) the corresponding alkyl ZDTPS . Similarly, the less activated hydrogen in a primary alkyl ZDTP makes it OH ____ 2E(RO) 2 P(S)S] 2Zn ) 2(RO)2PSSZnSSP /+ 2R 1-CH=CH 2 OR SH / 2( RO ) 2 PSSZnSOP

I2 3OR

[RO)2PSsznSJ_ S OR

Scheme 5 - 14 -

more stable than a secondary alkyl ZDTP. Assef proposed a single-step mechanism for the thermal decomposition of ZDTPS leading to the formation of an acid and olefin.

Later Feng et al 32,33 expanded these ideas into the multi-step mechanism shown in Scheme 5. They also

identified a polymeric residue which was assigned structure (15).

O 0 11 'I —P--S—Zn--S —P—S

OR OR n (15)

Other workers34 ' 35 have identified the mercaptan RSH as an additional product and also determined the sequence in which the products are

formed: (1) olefin, (2) mercaptan, and (3) hydrogen suiphide. From these results it is clear that H 2S is not formed according to Scheme 5 as proposed by Feng. They suggested an alternative mechanism which is outlined in Scheme 6.

Luther and co-workers 36 have proposed another mechanism for the thermal degradation of ZDTPs in which radicals are thought to be involved (Scheme 7).

Later, they observed3 7 that the reactions were accelerated by the addition of polar substances but not - 15 -

RO S-Zn" 2 P0 S-Zn 1 "2 P0 S-Zn"2

P S. p / /\ RO S H 3C S 0 S 0

HC H3CI I / H3C C H

H /\ H H

RO SZn h' 2 \/ H3C P RO S-Zn 112 \/ HC—Si- 0 P + CH3 -CH=CH2

H3C S 0-H

(16

P0 \/ S-Zn" 2 H3 C \ HC—SH

0 H3 C

RO S-Zn" 2 P0 S-Zn" 2 Zn 1/2 Zn1'2 \/ I P ± ) S S I I S 0_ 0 R-0-P —0-- P-OR

H3c H 3C H 3C \ V 2 HC — SH ) HC - S - CH +1125 / / H3c H3C CH 3

Scheme 6

by radical initiators. This pointed to a heterolytic mechanism as advanced by previous workers. Other

workers38 believe that both free radical as well as an ionic mechanism are operating, although Rounds and Spedding40 favour the latter.

S OR 0 SR 11/__ 11/ - Zn-3- P OR OR

0 SR 11/ - Zn - S - P + R N °• I 0 / - Zn - S - P + RS

R - 3 R'-CH=CH2 + H R +RS -4 R2 S RS+H - > RSH

RSH —> R'-CH=CH 2 + H 2S

Scheme 7

Kuzmina et a1 41 ' 42 have identified several other oil-soluble phosphorus-containing products from the thermal degradation of n-butyl ZDTP at 1600 - 17 -

tributyl-tetrathiophosphate (C 4 H9 S) 3PS, O,S,S-tributyl trithio-phosphate (C4 H9 S2 )P(S)(0C4 H9 ), O,O,S-tributyl dithio-phosphate (C 4 H9 S2 )P(S)(0C4 F19 ) 2 All of these compounds possessed strong anti-oxidant properties which suggested that there might be a correlation between anti-oxidant behaviour and thermal degradation. The findings of these workers have subsequently been confirmed by Coy and Jones 43 ' who have also shown that the oil-insoluble deposit is a mixture of zinc thiophosphates and zinc pyro- and polypyrothio- phosphates.

(3) Reactions at metal surfaces

In 1957, Loeser et a1 4448 observed that films containing Zn, P and S are formed on the worn surfaces of cams and tappets when lubricated with oils containing ZDTPs. The ratio of these elements in the film was lower than that in the original ZDTPs. Larson 49 suggested that the decomposition of ZDTPs involved loss of three or more alkyl groups and one and a half sulphur atoms per molecule leaving a largely inorganic residue as a film on the metal. If this is the case, it might be expected that the more unstable the additive, the greater should be the rate of film formation and the greater its effectiveness. However, it has been shown5° that there is no correlation between thermal stability and extreme pressure (EP) effectiveness. The - 18 - possibility that the adsorption of ZDTPS is dependent upon the metal substrate was explained by

Baumgarten 553 . It was suggested that zinc may diffuse from the rapidly formed monomolecular layer into the metal creating a surface alloy. However, other workers54 ' 55 found that there was loss of zinc from the surface into solution and not into the metal.

interactions Nith other additives

Past experience has shown that both the anti-wear performance and the anti-oxidant behaviour of ZDTPS are dimin,ished by other additives 5660• Examples include amine friction modifiers, metallic dithiocarbainaje inhibitors, sulphur, and phenolic anti-oxidants. On the other hand, thermal decomposition temperatures of ZDTPs have been shown to increase in the presence of barium sulfonate 33,57

Hydrolysis

As mentioned previously, one very common contaminant that can possibly contribute to the deterioration of mineral lubricating oils in an engine environment is water 60 . Water can enter the engine system during storage of the oil and from the atmosphere through leaks into the system. Having gained entry, water can also cause corrosion unless driven off by operating the system regularly at temperatures much higher than 1000C. - 19 -

Whilst water forms a very common contaminant during the service of oils, and although ZDTPS can serve as corrison-inhibitors, so far studies of their hydrolysis have been neglected. Nonetheless, it is recognised that ZDTPs can undergo hydrolysis giving rise to unknown acidic oil-soluble and oil-insoluble products which can block oil pipes, valves and pumps. From this standpoint any knowledge gained about the hydrolysis of ZDTP additives could lead to the identification of both sludge and oil-soluble viscous products which might curtail the lubricating properties of mineral oils. On the other hand, hydrolytic studies could also lead to the identification of potentially useful load-carrying additives for use in water-based fluids. - 20 -

B. PROGRAMME OF RESEARCH

The aim of the project was to determine the mechanism(s) of the hydrolysis of zinc and related metal dithiophosphates, both neutral and basic, under

conditions relevant to their usage in lubricants. Because of the complex structure of the ZDTPS and the feasibility of different modes of nucleophilic attack by water, it was recognised that the hydrolysis could result in the formation of a plethora of

phosphorus-containing intermediates and their decomposition products. This pointed to the use of n.m.r. spectroscopy as the best 'spectroscopic handle' to determine both •the rate of formation and structural

identification of these intermediates and their products. As an instrumental technique, 31p n.m.r. spectroscopy has had an enormous impact on organophosphorus chemistry over the last decade leading to many significant developments in different areas of

research6 . Its benefit derives from the fact that the nucleus with a spin of 1/2 represents 100% natural

abundance, making the direct observation of 31 p nuclear magnetic resonance a versatile process with high overall sensitivity (6.6% of 1H). The potential range for

phosphorus chemicial shifts is over 700 ppm, i.e.

approximately +500 ppm to -250 ppm relative to 85% H 3 PO4 with a shift 0.0 ppm. As shown in Figure 1, the shift FIGURE I. The dependence of 31 v P on the coordination Px number of phosphorus.

Px2

Px3

PxS

I

I

400 200 0 -200 6 - 21 - for phosphorus depends markedly upon its coordination number. It is this large range of chemical shifts which makes it possible to make unequivocal identification of the oxidation state of phosphorus species. Moreover, the absolute value of the coupling constants for phosphorus also covers a large range from near zero to approximately 3500Hz. Because of these features, 31 p n.m.r. spectroscopy has proved to be a powerful tool for attacking difficult problems of structure determination, kinetic analysis (qualitative as well as quantitative), reaction mechanisms, and product purity which otherwise might be difficult to solve using other instrumental techniques. R E U L. 'I' A N D D I C U I ON DISCUSSION

CONTENTS

Page No.

A. Hydrolysis of neutral zinc and related

metal 0,0-dialkyl dithiophosphates. 22

1. Hydrolysis of neutral zinc O,O-diethyl,

diisopropyl, and diisobutyl dithio-

phosphates. 25

Attack at °-carbon or phosphorus ? 31

Attack at metal ? 41

2. Hydrolysis of nickel 0,0-diethyl--

dithiophosphate. 45

3. Hydrolysis of cadmium 0,0-diethyl--

dithiophosphate. 48

4. Kinetics of the hydrolysis of zinc

0,0-diethyl and diispropyl dithio-

phosphates. 49

5. Identification of precipitates from

hydrolysis of metal dithiophosphates. 53

6. Identification of soluble products from

hydrolysis of metal dithiophosphates. 56

a. Hydrolysis of 0,0-dialkyl monothio-

phosphoric acids. 57

Hydrolysis of 0,0-diethyl dithio-

phosphoric acid. 59

Reaction between 0,0-diethyl dithio-

phosphoric acid and ethyl iodide. 71

Identification of the intermediate

derived from the hydrolysis of

0,0-diethyl dithiophosphoric acid. 73 Hydrolysis of basic zinc 00-dilsopropyl dithiophosphates (Hexakls-(O,O-di Isopropyl phosphoro-dIthloate)-p-4-tetra oxo zInc) 77

Future Research Work 0 85 - 22 -

A. Hydrolysis of neutral zinc and related petal 0,0-

dialkyl dithiophosphates.

The metal dithiophosphates, also known as metal phosphorodithioates, are derived from 0,0-dialkyl dithiophosphoric acids, (RO) 2PS 2H. In 1912, Pistschimuka the great Russian chemist pointed out that the reaction between ethanol and phosphorus pentasuiphide did not lead to the formation of ethanethiol and phosphorus pentaoxide (equation 4) as claimed by Kekule 62 . In stead, he stated that the reaction mainly

5EtOH + P 2s5 ) 5EtSH + P 20 5 .... (4) gave rise to 0,0-diethyl dithiophosphoric acid and hydrogen sulphide (equation 5). This reaction of great potential benefit went into the oblivion for almost three decades.

4EtOH + P2 S5 ) 2(EtO) 2PS 2U + H 2S .... (5)

In 1945, Hastin et a163 supported the stoichiometry of equation 5 and prepared the mercuric derivative of 0,0-diethyl dithiophoshporic acid by reacting the potassium salt of the latter with mercuric chloride. This opened up a route to a variety of metal salts of different dithiophosphoric acids. Since then - 23 - both the metal dithiophosphates and the dithiophosphoric acids have been widely used for various purposes, e.g. lubricant additives, pesticides, etc. 1 .

In 1956 Wystrach et a164 prepared the neutral zinc dithiophosphates (2) from the reaction between sodium 0,0-dialkyl dithiophosphate and zinc chloride in aqueous solution (equation 6). They also showed that if the reaction medium was basic, the

2(RO) 2 PS2 Na + ZnC1 2 > [(RO) 2 PS2 ]2 Zn-f-2Na.CL ....(6) reaction led to the formation of basic ZDTPs (3) as by-products. Nowadays, zinc sulphate is preferred to zinc chloride even in neutral medium for the synthesis of neutral ZDTPS.

The neutral ZDPTs and related MDTPs studied in the present work were prepared using zinc sulphate in place of zinc chloride in equation 6 and purified by repeated recrystallisation. They were shown to be pure by elemental and 1 F1 n.m.r. analysis. Their purity was further confirmed by the observation of a single sharp peak in their 31 P n.m.r. spectra. Thus the neutral diethyl ZDTP(2a) and diisopropyl ZDTP(2b) gave signals at 101.5 ppm, (previously unreported) and 98.5 ppm, (lit. 65 , 94.7 ppm, CDC1 3 ) in DME, respectively, - 24 - while the diisobutyl derivative (2c) absorbed at 99.6 ppm (lit. 43 100.6 ppm, CDC1 3 . In the case of the corresponding basic salts (3a) and (3b), the latter absorbed at 98.7 ppm, but the former was too insoluble in DME to record its spectrum. Diethyl Ni DTP (17) unexpectedly showed a broad peak (previously unreported). Although a broad peak characterises paramagnetic compounds, the nickel complex has been shown to be diamagnetic 66 by its square planar symmetry. Diethyl Cd DTP (18) absorbed at 107.7 ppm in DHE, (previously unreported). These chemical shifts are

relative to 85% phosphoric acid, employing C6 D6 as an

external lock.

C (RO) 2PS 2 ]Ni [(R0) 2PS 2 ) 2Cd (17) (18)

The neutral MDT?s can be represented by

the simple structure (Fig. 2)66 determined for the diisopropyl ZDTP by X-ray crystal structure studies.

This revealed that the complex is binuclear. Associated with each metal are two dithiophosphate ligands, one of

which functions as an intra-chelating group bound wholly

to one metal. The other actsasa bridging group linking

two monomeric molecules together to form the dimer. The four independent P-S bonds average 1.970 A which lies in

between that for a single (2.14 A) and a double (1.94 A) Figure 2 :Structure. For[(Et(D)Ps]zn 2 22 - 25 -

P-S bond. The ZDTPs are however known to exist as monomers(2) in solution 66 .

From a mechanistic viewpoint it can be assumed that attack by water on the metal DTPs could occur either at one or more sites, including the

.&-carbon of the alkyl group, the phosphorus and/or the metal atom. As such, the rate of hydrolysis might be expected to be influenced not only by the nature of the alkyl group, but also by the nature of the metal atom.

In order to investigate the effect of the alkyl groups on the rate of hydrolysis, neutral zinc

0,0-diethyldithiophosphate (2a) and zinc O,O-diisopropyl dithiophosphate (2b) were selected as representative examples of straight chain primary and secondary metal complexes, whilst the choice of the corresponding diisobutyl compound provided an example of a branched primary alkyl group. In other studies, the zinc atom would be replaced by nickel (17) and cadmium (18) in order to investigate the effect of a change in the nature of the metal moiety.

1.. Hydrolysis of neutral zinc 0,0-diethyl, diisopropyl

and diisobutyl dithic'phosphates.

In order to ascertain the feasibility of - 26 -

studying the mechanism of hydrolysis of ZDTPs by 31 P n.m.r spectroscopy, initial studies involved the hydrolysis of Zinc 0,0-diethyl ditWophosphate (2a) and zinc 0,0-diisopropyl dithiophosphate (2b) under a variety of reaction conditions on a small scale. In this way it would also be possible to isolate and unequivocally identify the hydrolysis products. Since both the aforementioned ZDTPs proved to be insoluble in water, a more suitablesolvent such as tetrahydrofuran

(TIfF) was chosen. Analytical reagent (A.R. ) grade TIfF was purified by distillation after refluxing over dry calcium hydride in an atmosphere of nitrogen for 3h and was stored under nitrogen over 4A molecular sieve

(preheated at 250 0C).

It is worth mentioning at this stage that all phosphorus spectra were recorded with noise-decoupling of the phosphorus-hydrogen spin-spin coupling. This gave rise to discrete signals due to the phosphorus nuclei. Unambigous identification of all phosphorus-containing products was affected by addition of an authentic sample of the suspected species to the reaction mixture in the n.m.r. tube. This technique referred to as the 'peak enhancement' method was utilized in the present work unless otherwise stated. Spectra totalling about three hundred were run during the course of these studies but for the sake of - 27 - convenience only the most significant have been recorded in the thesis.

Preliminary studies involved small scale hydrolyses in n.m.r. tubes whence (ca 0.0001 mole) (2a) and (2b) were heated with two equivs of water at 660 in 0.3cc THF. The formation of a colourless precipitate in both cases indicated that hydrolytic reaction did take place as no precipitate was formed in control experiments in the absence of water. The reactions were monitored by n.m.r. spectroscopy which showed that hydrolysis was not complete after a period of 3 days. That no thermal degradation took place was also shown by 31? n.m.r. analysis of control experiments conducted in the absence of water.

Later experiments on a preparative scale were carried out employing the apparatus shown in fig.

3, whereupon ca (0.005mole) of the two ZDTP'S (2a) and (2b) were heated with two equivs of water in 15cc THF at 660 C. The apparatus consisted of a one-litre 3-necked R.B. flask fitted with a reflux condenser in the central neck. Into the other two necks were 'inserted two reaction tubes which were sealed by quick-fit stoppers held in place by rubber tubing and copper wiring. In order to obtain a constant, reproducible temperature of

66 0C, TIIF was boiled in the R.B. flask. Throughout the Figure 3 : Apparatus for qualitative hydrolyses

To water condenser

eaction tube

agnetic bar - 28 - hydrolysis experiments, solutions in the reaction tubes were magnetically stirred. Aliquots were withdrawn from the reaction tubes from time to time for analysis by 31P n.m.r. spectroscopy. Cooling of the reaction samples to room temperature immediately ended any further reaction resulting from hydrolysis.

The hydrolyses of both ZDTPs were not

completed even after a period of 3 days as shown by 31 P n.m.r. spectroscopy. When the presence of water was

increased by five-fold, i.e. to ten equivs, the diethyl ZDTP (2a) underwent complete hydrolysis in about 3 days whilst the hydrolysis of the diisopropyl ZDTP (2b) occurred in about 7 days. For both compounds the hydrolysis solutions showed similar 31P n.m.r. spectra which consisted of at least 4 major peaks as shown in Figs. 4 and 5 . This indicated the possibility of a

common hydrolytic mechansim in the two types of complexes. In both cases the hydrolysis also gave rise to a colourless amorphous precipitate which did not melt up to 3000C and was insoluble in commonly available organic solvents pointing to its inorganic nature. The i.r. spectra of the two precipitates resembled each other which further argued for a common hydrolytic mechanism for both the ZDTPs. This was reinforced by

the strong resemblance of the above precipitates in their i.r. spectra to that of zinc suiphide. Moreover, Figure 4 : Hydrolysis of diethyl 2DTP in THF at 66 0C t - Ca 84h

1.6

31Pi +c_

Laopropyl ZDTP in THF

1 PB 0 - 29 - both the precipitates liberated hydrogen suiphide upon treatment with dilute HC1, a common test for sulphides. In the case of the soluble hydrolysis products, attempts to remove the solvent by evaporation proved irritably difficult especially since the 1 H signals for THF (S 3.87m) overlapped with the 1 H n.m.r. signals for the hydrolyses products. These difficulties coupled with the long hydrolysis times, necessitated the use of

1,2-dimethoxy ethane (DHE, b.p .. 85 0 C) as the reaction solvent. It is also worth noting that in the case of (2a) a colourless precipitate was obtained occasionally at room temperature in THF. It is believed that in this instance decomposition. occurred due to the presence of peroxides in the THF since the problem was removed when the reaction solvent was further purified by repeated distillation.

The hydrolysis were carried out under reflux in DNE with ten equivz of water. DME (b.p. 850 C) was of A.R. grade and purified in a similar manner to that described for THF. In order to obtain a constant reproducible temperature, TI-iF was replaced by triethylamine, b.p. 90 0C in the R.B. flask of the apparatus (see Fig. 3). Under these conditions the products obtained were identical with those obtained using TUF, but the rates of hydrolysis were not generally consistent. Moreover, on occasion, as in the - 30 -

case of THF, a colourless precipitate was obtained as

soon as the diethyl ZDTP (2a) was added to the

DNE-water solution. This was identified as being the

corresponding basic salt (3a) by elemental analysis.

This indicated the presence of peroxides, but the

problem was removed when the DNE was distilled again and

additionally percolated through a column of alumina.

Under these conditions the diethyl ZDTP

(2a) underwent complete hydrolysis in about 1.5h

compared to about 2.5h for the diisopropyl ZDTP (2b).

As in the case of the hydrolyses conducted in TI-IF, 31

n.m.r. spectroscopy showed the presence of four peaks

(Figs. S and 7) in the reaction mixture. In both

cases, a colourless amorphous precipitate was obtained

which was again tentatively identified as zinc suiphide

by i.r. spectroscopy as well as by the obsvervation that

hydrogen sulphide was liberated upon its treatment with

dilute HCl.

The successful hydrolysis of the diethyl

• (2a) and diisopropyl (2b) complexes in DME at 850 within

a few hours suggested that the reaction system might

serve as a useful media for studying the kinetics of

their hydrolyzes. The difference in the rates of

hydrolysis between THF at ca 66 0C and DME at ca 85 0C

presumably reflects the increase in temperature since Figure 6 : Hydrolysis of diethyl ZDTP in DPI at 85 °C 9 t m Ca 1.5h

T4*0 "OH

31 +c_ PS Figure 7 Hydrolysis of diisopropyl ZDTP in

DMEat85°C, t=ca.2h.

1.3

31p - 31 -

there is little change in the polarity of the reaction medium on going from THF (dielectric constant 7.6 Debye) to DHE (dielectric constant 7.2 Debye). Nonetheless, it seems likely that the rate of the hydrolysis would be quite slow in non-polar media such as hydrocarbon oils in engines. Evidence for the effect of polarity was obtained from the substantial increase in the rate of the hydrolyses which took place in THF when the presence of water was increased from two to ten equivs. (see earlier).

a.. Attack at ( -carbon or phosphorus ?

The tentative identification of the insoluble precipitate as zinc sulphide opened up the fundamental question of as to where the attack of water occurs in the hydrolysis of ZDTPZ. In theory, there are three possible target sites the 'K-carbon, the phosphorus atom and the metal moiety. It has been speculated previously (without comment) that nucleophilic attack by water on the ZDTPs should occur at the <-carbon atom 67 . Attack at the -carbon can be correlated with hydrolytic studies of the cleavage of P-O-C linkages in phosphate esters which have been the subject of vigorous examination in view of their extensive applications in engineering, agriculture and biochemistry68 . For example, it has been shown that - 32 - water attacks the-carbon leading to the C-O cleavage in e.g. trimethyl phosphate (19), in both neutral and acidic media (equation 7). This result is in

NeO MeO 0 +H0j1 +MeOH (7) / P MeO /\ OtMe MeO \ OH (19)

accordance with Pearson's concept of the interaction of hard and soft acids and bases 69 since in alkaline solution the hard base, H0 attacks the hard phosphoryl centre. By analogy, it might be expected that the -P(S)SR moiety in the MDTPs should behave like a comparatively soft acid due to the lower electro- negativity of sulphur compared to that of oxygen. In essence, this would allow the comparatively softer base, water to preferentially attack the phosphorus centre instead of the -carbon even in neutral medium.

The attack of water at phosphorus could lead to the cleavage of both P-O and P-S bonds but the latter would be expected in view of the relative bond dissociation energies (P-S, 45-50 Kcal mole; P-O,

95-100 Kcal mole 1 ) 70 . This was demonstrated by Hudson and Keay70 who in 1956 observed the first example of P-S cleavage in the hydrolysis of diisopropyl methyl - 33 -

phosphonothiolate (20) (equation 8). This is in line with the established C-S bond cleavage in the hydrolysis of tert-butylthioacetate, HeC(0)SCNe 3 which occurs faster than C-O bond cleavage in tert-butyl acetate. It should be noted that the P-S bond is even weaker than

C-S bond (54 Kcal mole 1 ).

Pr 1 0 Pr 1 0

P + H20 P + MeSH .... (8) /\ Pr 1 SMe Pr' OH

(20)

The cleavage of a P-S bond in preference to a P-0 bond seems quite understandable because unlike oxygen, the ester sulphur is less able to participate in pi-bonding on account of its lower electronegativity. A further fact to be taken into consideration is that the mercaptide ion is a better leaving group than the alkoxide ion. Indeed this d ifference is responsible for the enhanced hydrolytic susceptibility of thiol compounds at the phosphorus centre compared to the corresponding oxygen esters within a homologous series7 1

The formation of Zinc surphide in the hydrolysis of ZDTPs suggested that the attack of water occurted at- carbon and/or at phosphorus involving the - 34 -

C-O cleavage (Scheme 8) and /or at phosphorus Involving

P-S cleavage as outlined in Scheme 9. However, from the above arguments it can be deduced that given the choice,the attack of water in the hydrolysis of the

ZDTPs would occur preferentially at the phosphorus atom, thus involving P-S bond cleavage. Therefore Scheme 8 is ruled out in favour of Scheme 9.

As such it might be expected that the primary hydrolysis product should be 0,0-dialkyl nionothiophosphoric acid (21). Dialkyl phosphoric acid

(22) and monoalkyl phosphoric acid (23) will be formed subsequently as a result of further hydrolysis of (21) as shown in Scheme 9.

On this basis the expected products were synthesised by standard • methods as detailed in the experimental section in order to compare their chemical shifts (by peak enhancement) with those observed from the hydrolysis of the metal complexes.

This technique when applied to the hydrolysis of the diethyl ZDTP (2a), confirmed the presence of both the diethyl thiophosphoric acid (21a) ( S64. 1 ppm) and the corresponding monoethyl phosphoric acid (23a).( SO.2 ppm)

(Fig. 6). Diethyl phosphoric acid (22a) with a chemical shift at S-1.5 ppm(DME), could not be identified as being present in the reaction mixture,

- 35 -

RO S S OR

P P /\ /\ R1—C—O S—Zn—S OR

H2b H 2O

-ROH

Nt

HOS S \ OH

/\ /\ HO S—Zn—S ,,7 OH ~ ,---Ao , HO S SH / P Zn /\

H20 . H:S Zn S

HO 0

P /\ HO OH

Scheme 8 - 36 -

ROS S \ OR

/\ /\ RO SZn OR

RO S SH / P (21) + Zn / \ RO OH SH

H 20 -H 2s -H 2S

ly RO 0 P ( 22 ) /\ RO OH ZnS

H 2O -ROH

RO 0 H20 HO 0

P (23) /\ /\ HO OH -ROH HO OH

Scheme 9 - 37 - presumably because it underwent rapid hydrolysis to the monoethyl phosphoric acid (23a) under the experimental conditions.

In the same way, the corresponding products (21b , 62.3 ppm), and (23b, 0.4 ppm) (Fig. 7) from the hydrolysis of the diisopropyl ZDTP(2b) were identified by their independent syntheses and comparison of 31 chemical shifts. From these results it was inferred that the unidentified peaks seen at 81.2 ppm and 1.3 ppm in the hydrolysis of (2a) and (2b), respectively were due to phosphoric acid which resulted from the further hydrolysis of the monoalkyl phosphoric acids (23a) and (23b). This was confirmed by peak enhancement and moreover, a mixed sample of the hydrolysis products from (2a) and (2b) showed that phosphoric acid was common to both reaction mixtures.

Only the structure of the hydrolysis product which absorbed at 861. 1 ppm in the case of (2a) and at 859.6 ppm in the case of (2b) remained to be identified. These peaks could not be assigned to the thiol forms (24) and (25) of the corresponding 0,0-dialkyl inonothiophosphoric acids (21a) and (21b)

since these compounds are reported to absorb at 24 ppm7 25 ppm75 , respectively. Moreover, authentic samples of

(21a) and (21b) showed only one peak at 64.1 ppm and - 38 -

62.3 ppm respectively (vide supra) in DME. Efforts to separate the hydrolysis products from (2a) using preparative t. l.c. were unsuccessful due to their strongly acidic nature. It seemed reasonable to conclude that the unidentified compounds could result from the mutual combination of the inonothiophosphoric acids (21) or their thiol forms (24 and 25) under the experimental conditions. Structures envisaged initiafly

EtO SH Pr' 0 SH \/

EtO '0

(24) (25) included the pyrothiophosphate cOmpounds (26) and (27). Structure (26) can be ruled out since it is known to be

EtO 0 0 OEt EtO S S OEt \/ P P P P EtO S OEt EtO 0 OEt (26) (27)

EtO 0 S OEt \ 411\\/

E t0 0 \E (28) - 39 - unstable at room temperature and decomposes to give (28) with two 31P shifts at -14 ppm and 54 ppm 70 ' 74 . Tetraethyl dithionopyrophosphate (27) was synthesized according to a literature procedure (see experimental) but was also ruled out because it absorbed at 53.1 ppm (DME). Although the identity of the compound with chemical shift at 61.1ppm (in the case of 2a) remained unknown at this stage, the results obtained pointed to the fact that the attack of water occurred at phosphorus with P-S cleavage and not at the (-carbon. This finding was further supported by the hydrolysis of the diisobutyl ZDTP (2c) which after ca 5h (see Fig. 8), gave four major soluble products, two of which were identified as diisobutyl monoth±ophosphate (21c) and phosphoric acid with 31 P shifts at 66.3 ppm, and 2.0 ppm, respectively and from speak enhancement' method.

The precipitate in this case was also identified tentatively as zinc sulphide from its i.r. spectrum.

If the mechanism proposed in Scheme 9 is valid, then the rate of hydrolysis should depend on the nature of alkyl group and the nature of metal moiety both of which affect the electrophilicity of the phosphorus. The alkyl group can play its role in two ways - electronic and stereochemical. Because the isopropyl group lowers the electrophilicity of phosphorus due to its greater inductive effect and sinbe Figure B : Hydrolysis of diisobutyl ZDTP in DME at 95° C, t = Ca 5h

H3PO4 2.0

-f-0 - 31ps - 40 - it is more sterically crowded than the ethyl group, both effects should impede the attack by water at phosphorus. This explains why under the same conditions the diethyl ZDTP (2a) is hydrolysed more readily (ca 1.5h) than to the diisopropyl ZDTP (2b) which in turn is hydrolysed faster (ca 2.5h) than the more sterically crowded diisobutyl analogue (2c) (ca 5h).

Even though the change in the nature of the alkyl group explains the differences observed in the rates for hydrolyses for various ZDTPs, it does not fully establish the proposed mechanism outlined in Scheme 9. If this mechanism is valid for MDTPs with the same alkyl groups,the following observations might be expected to hold. Firstly, they should give rise to

identical soluble products pattern. Secondly, they will differ in the rates of their hydrolysis provided of course this is determined by the relative ease with which the respective P-S bonds are broken in a rate determining step. In other words 1 those complexes with longer (weaker) P-S bondsshould hydrolyze more readily than those with shorter (stronger) P-S bonds.

A comparison of the X-ray crystal structures for diethyl NiDTP (17) and the corresponding ZDTP complexes (2a) shows that although the H-S bond

length in the nickel complex is much smaller (2.21A)76 - 41 -

than that (2.34A) in the Zinc complex 77 ' there is insignificant difference between their F-S bond lengths

(1.99A vs 1.98A). Nonetheless from hydrolysis studies it was found that while even though the two MDTPs gave the same products, the nickel complex hydrolysed much more slowly (ca 5days)(Fig. 9). As stated earlier, diethyl

ZDTF took only ca 1.5 h to hydrolyse under the same conditions.

b.Attack at metal?

- From the above result it appeared that a different mechanism to that proposed in scheme 9 was operating for the hydrolysis of NDTPs. This supposition was supported by theoretical charge distribution calculations which showed that the charge on nickel, phosphorus, and carbon in the nickel complex (17) is +

0.583e, + 0. 136e and + 0.016e respectively 78 . From this data, it is evident that water is more likely tooccurr at metal moiety as shown in Scheme 10 rather than at phosphorus. In other words, for a MDTP complex with a stronger i.e. smaller H-S bond, hydrolysis shouldoccurr slowly than for a similar complex with a weaker (longer)

H-S bond. This explains why the nickel complex (17) is hydrolysed more slowly compared to the corresponding zinc complex. Figure 9 : Hydrolysis of diethyl Ni DIP in DM( at 85 0C, t - ca 5d

1.2

EtO Eto ''OH It-

+- 3' ps 0

- 42 -

P0 S S OR

P P /\ EU S _IJLS OR

RO S OR \\/ P H2 0 01-1 P /\ I /\ RO SH -M(OH) 2 M - S OR (29)

;:° ;U23 I

\/ I P MS4-H2 0 /\ RO OH

- H2S

\/ P /\. RO OH M3(PO4)2+M (P207)

MO H2 OJ -ROH

HO 0

P HO P /\ /\ HO OH -ROH HO OH

Scheme 10 - 43 -

If the mechanism proposed in scheme 10 does take place for MDTPs in general, then 0,0-dialkyl- dithiophosphoric acid (29) should be the first product to be formed in their hydrolysis. In order to verify this fact attempts were made to observe the changing fate of the phosphorus moiety in the hydrolysis of (2a) by examining its 31 p n.ni.r. spectrum at very short time intervals. It was observed that within ca 15 min of the start of the reaction a minor peak appeared at 88.7 ppm which gradually increased in intensity but then declined although it remained visible until the complete disappearance of the substrate. It is this peak at 88.7 ppm which appeared to be the major hydrolysis intermediate and presumably that which gave rise to the final products (Figs. 10-14). This peak was identified subsequently as being due to 0,0-dialkyl- dithiophosphoric acid (29a) by the 'peak enhanccment' method. Similarly, the appearance of a peak at 86.8 ppm confirmed the intermediacy of the corresponding dithio acid (29b) that gave rise to the final products (Figs.

15-18) as the major intermediate in the hydrolysis of diisopropyl ZDTP (2b). The identification of the intermediate (29) in both reactions confirmed that the two ZDTPs followed a common hydrolytic,path regardless of the nature of the alkyl group. This is also evident from the similarities in the spectra recorded during hydrolysis (Figs. 12-14) and (Figs. 15-18)and the fact Figure 10 z Hydrolysis of diethyl ZDTP in DNE t = Oh

101.5

31 P 0 Figure 11 s Hydrolysis of diethyl ZDTP in DME at 85 C, t ca 15 mm

101.3

I_31 P 0 Figure 12 s Hydrolysis of diethyl ZDTP in DIIC at 85 C, t = ca lh

57 5 0.6

60.7

0.4 99.8 63.9 I &.4

.-I- 31 P 5 0 0 Figure 13 : Hydrolysis of diethyi. ZOIP in DME at 85 C, t = ca 1025h

0.9

31 Figure 14 : Hydrolysis of diethyl ZDTP in DME at 85°C, t =ca 1.5h

, H3P4 1.2

E t O. Eto -

31 - pg Figure 15 : Hydrolysis of dileopropyl ZDTP in DME at 850C 9 t = ca 30mm

97.2

[Po) Ps]Zn 2 22

P'O'SH

86.8

3' 4- pa Figure 16 : Hydrolysis of diisopropyl ZDTP in

DME at 85°C, t = Ca. 1.75 h.

0.9

58.5 55.3

61.4

31Ps ±c_ Figure 17 : Hydrolysis of diisopropyl ZDTP in

DME at 85°C, t = Ca. 2 h.

.3

31p Figure 18 Hydrolysis of dilsopropyl ZDTP in

DME at 85°C, t = Ca. 2.5 h.

0.9

31ps - 44 - that the final products were of the same type.

Similarly detailed studies of the hydrolysis of diisobutyl ZDTP (2c) revealed that the formation of the diisobutyl dithiophosphoric acid (29c)

(885.5 ppm) occuited within ca 0.5h (Fig 19-20). Reference to Fig. 21 and 22 shows quite clearly that the formation of the products in this case was virtually identical with that from the hydrolysis of diethyl and dilsopropyl ZDTPs.

It is perhaps pertinent to note at this stage that the alkaline hydrolysis of a related compound malathion, also yields the dithiophosphate anion as shown in equation 9 by an elimination reaction involving loss of fumaric acid 79 . Obviously in this case the C-S bond (54 Kcal mole) could not be expected to undergo cleavage were there no carbethoxy group in the molecule to catalyse the process. Indeed, under acidic conditions P-S bond cleavage (Ed.45-50 kcal

normally occurrs giving rise to the monothiophosphoric acid (equation 10).

CH 30S -H 2O CH 30S EtOOC-CH II CH-COOEt CH 3 O S—CHCOOC---- 2H 5 - CH 3O S

Oi'H—H---COOC 2H 5 (9) Figure 19 : Hydrolysis of diisobutyl ZDTP in DMEt Oh

103.1

31 4- PS Figure 20 : Hydrolysis of diisobutyl ZDTP in DIVIE at 850C, t w ca 30 mm

103.2

31 ps Figur. 21 s Hydrolysis of diisobutyl ZDTP in DME t - ca 4h

.4

(GJO)ps

19

-I-.. 3 p Figure22 : Hydrolysis of diisobütyl ZDTP in DuE at 85 °C, t - ca 5h

PQ 2.0

31 -+1- ps - 45 -

CH30\/ S CH 3O \,S p P + CH-COOC2 H5 /A+ __ /\ II CH3O) S—CH—COOC 2H 5 )CH 3O OH HS-CH-COOC 2H 5

H2 0 H—CH---COOC 2 H5 (10)

In order to establish the general intermediacy of 0,0-dialkyl dithiophosphoric acids (29) in the hydrolysis of the ?IDTPs, detailed studies were carried out on two metal dithiophosphates, nickel O,O-diethyl dithiophosphate (17) which had already been subjected to some preliminary examinations, and cadmium 0,0-diethyl dithiophosphate (18a).

2.Hydrolysis of nickel O,O-diethyldithiophosphate.

A complete run for the hydrolysis of the nickel complex is recorded in Figs. 23-29. These spectra revealed some distinctive differences compared to diethyl ZDTP (2a), especially in the earlier stages of the hydrolysis. Most interesting was the appearance of two major peaks at 92.5 ppm and 91.1 ppm, neither of which corresponded to diethyl dithiophosphoric acid

(29a) as shown by addition of an authentic sample of the latter to the reaction solution. From this observation it was apparent that the nickel complex underwent hydrolysis by a different mechanism. Upon further figure 23 : Hydrolysis of diethyl Ni DTP in DME J.t = Oh

55.7

P Figure 24 : Hydrolysis of diethyl Ni DTP in DMC at 85 °C, t ca lh

40.3

p b Figure 25 : Hydrolysis of diethy]. Ni DTP in DP1E at 85 0C, t - ca 24h

.3, PS Figure 26 ; Hydrolysis of diethyl Ni DIP in DPt at 85 0C, t = ca 48h

91.4

bf ' Figure 27 z Hydrolysis at diethyl Ni DIP in DP1E at 85 °C 9 t = as 60h

91.4-

- Figure 28 s Hydrolysis of diethyl Ni DIP in DME at 85 °C, t Ca 72h

91.3

.-.if PS Figure 29 Hydrolysis of diethyl Ni. DTP in DME at 85 °C, t - Ca 120h

3p ~ 1- - 48 -

heating over a period of ca 24h (Fig. 25) the peaks at

92.5 and 91.1 ppm intensified, while two minor, albeit broad absorptions, appeared at 72-84 ppm and 55-67 ppm.

The former absorption may have included a signal due to

0,0-diethyl dithiophosphoric acid (29a) which absorbs in this range. In due course of time the latter broad absorption was eventually split into three distinct peaks at 63.7 ppm, 61.9 ppm, and 59.9 ppm (Figs. 26-28).

These three peaks appeared to be identical with those observed in the hydrolysis of diethyl ZDTP (2a). The peak at 63.7 ppm was identified by 'peak enhancement' method as being due to 0,0-diethylmonothiophosphoric acid (21a) as in the case of (2a).

After ca 72h a new minor peak at 84.2 ppm was observed (Fig. 28). Addition of an authentic sample of 0,0-diethyl dithiophosphoric acid (29a) to the sample intensified this peak although it did undergo slight broadening. The fact that starting material was still present at this stage of the reaction indicated that the hydrolysis was very slow. This is in sharp contrast to diethyl ZDTP (2a) which is hydrolysed in ca

1.5h under the same experimental conditions. Prolonged heating of the nickel complex over a period of Ca 120h

(Fig. 29) led to the final spectrum shown in Fig. 9 which_js identical to that obtained from the hydrolysis of diethyl ZDTP under the same conditions - 47 -

(Fig. 6).

On the basis of these results, it is clear that the NiDTP (17) also did seem to hydrolyse via the intermediacy of 0,0-diethyl dithiophosphoric acid

(29a) to give the same final products as those obtained from the hydrolysis of the zinc analogue. However, it would appear that the nickel complex probably undergoes initial complexation with water as evidenced by the ready appearence of two new peaks with 31 shifts at

91.6 ppm and 90.6 ppm. Independent evidence is available to support this concept of initial complexation based on molar extinction coefficient studies which have shown that addition of 20 volume % of water to ethanolic solution of NiDTP decreases the apparent molar extinction coefficient to approximately one fifth of the original value, but without changing the position of the band maxima 80 . After complete hydrolysis the maxima corresponds to that for

Ni(H 20) 62 In fact, for the completely decomposed solution (above 60 % water)the only indication of the possible existence of high-spin NIDTP 2 (H 20) 2 or NiDTP

(1120)4 is the slightly higher extinction coefficients than those of ordinary aquo ion80. - 48 -

3.Hydrolysis of cadmium O,O-diethyldithiophosphate

n.m.r. spectroscopy showed that the hydrolysis of the corresponding cadmium complex (18a) is also much slower (ca 5.0h) than that of the diethyl ZDTP (2a)under the same conditions. Despite the formation of a light-yellow precipitate within ça 15 mm, no apparent change in the spectrum occurred for ca 1.5h (Figs. 30-31). Thereafter, the spectra recorded (Fig. 31-34) were identical to those for the hydrolysis of the ôorresponding ZDTP (cf. Figs. 12-15), thus indicating a common hydrolytic pathway which proceeds by way of the dithiophosphoric acid (29a) (98.0 ppm in Fig. 32, cf.

Fig. 11). The final products were shown to be identical by the 'peak enhancement' method to those obtained from the hydrolysis of diethyl ZDTP (Fig. 14).

From the foregoing results it is evident that the hydrolysis of !IDTPs under the conditions described (t'H of the solution changed from 5.5 at the beginning to 0.70 at the end.) proceeds by attack of water at the metal moiety as outlined In Scheme 10 and not at phosphorus, or at the c<-carbon . However, the rate at which the diethyl Cd DTP (18a) undergoes hydrolysis is much slower than that of the corresponding ZDTP, a fact which goes counter to the mechanism proposed in Scheme 10 in view of the relative Figure 30 : Hydrolysis of diethyl Cd DIP in DME at 85°C, t = Ca 0.5h

107.5

31 4- p Figure 31 : Hydrolysis of diethyl Cd DIP in DPIE at 85° C, t = ca 1.5h

J07.5

3 4- °C, t ca 2h Figure 32 : HydrolysiS of diethyl Cd DTP in DPI at 85

57.4

[Et0PSCd 607

10 7. 0 0.4 9.9

31Pg Figure 33 : Hydrolysis of diethyl Cd DIP in Drit at 85 ° C, t = ca 4h

52.

3, - P Figure 34 ; Hydro]yaie of diethyl Cd DTP in DP'IE at 85 °C, t = Ca 5h

f— H3 PO4 1.2

E tO\01.

31pg - 49 -

metal-to--sulphur bond strengths. Although the M-S bond

length for the diethyl complex has not been reported

this information is available for the corresponding diisopropyl cadmium and zinc complexes and shows that in

the former the Cd-S bond (2.53 A) is weaker than the

Zn-S bond (2.35A). This implies that the cadmium

complex should hydrolyse faster than the zinc complex, although the experimental fact is clearly otherwise.

The answer to this dichotomy may lie in structural differences for the twocomplexes in question. It is

known that the diisopropyl Cd DTP (18b) exists as a ditner (see Fig. 35), both in the solid state and in

solution 66 . On the other hand, diizopropyl ZDTP exists

as a solid dimer (Fig. 2), but like diethyl ZDTP is monomeric in solution. By analogy, It seems likely that

diethyl Cd DTP (ISa) also occurs as a dimer in solution

thereby impeding the attack by water at the metal

centre.

4. Kinetics of the hydrolysis of zinc 0,0-diethyl and

diisopropyl dithiophosphates.

If the mechanism outlined in Scheme 10

envisaging attack by water at the metal moiety of the

HDTPs is valid, then the rate of the reaction should be

of a SN2 type, i.e. it would depend upon the

concentration of both the MDTP and water, provided of Fjgugs 35 s Dim.r of fPr'o)7PSjI2Cd - 50 - course this is the rate determining step.

- d[1IDTP] o

Since the water is present in large excess, the rate expression simplifies to -

- dEMDTPJc

- dCMDTP] = k[MDTP) dt

where k is the rate constant for the hydrolysis of MDTP.

In other words, the rate should follow a pseudo first order rate expression -

k = in a 0 1 a0 -x t where a0 is the initial concentrat ion of MDTP, and x is the amount consumed in time t.

In order to ascertain this point, kinetic analyses were carried out for the hydrolysis of both diethyl and - 51 -

•diisopropyl ZDTPs (2a) and (2b), respectively. 31P

n.m.r. spectroscopy was used to obtain the

quantitative rate data. For this purpose it was

neccesary to choose a suitable internal standard such as

triphenyiphosphine or triphenyl- phosphate , both of

which have been widely used in similar kinetic studies.

The spectra were run under conditions to suppress any

peak enhancement due to nuclear Overhauser effects

(n.O.e.). In this way, the 'ratio of height of the

signal due to the ZDT? compared to that for the

standard, could be used for obtaining (a 0-x) values,

where a0 is the initial concentration of the ZDTF and x is the amount by which it undergoes hydrolysis. The

use of paramagnetic reagents such as chromium (tris)

acetyl acetonate to suppress n.O.e. was avoided, because

they generally cause line-broadening in the spectra,

and thus make accurate quantitative determinations very ob difficult.

The apparatus employed for the kinetic

analyses is ' shown in Fig. 36. It consisted of a

one-litre round bottomed flask into which was inserted a

double walled glass 'thermostat.' having a side arm

fitted with a reflux condenser. The thermostat was

filled with liquid paraffin to a height of two inches

and was maintained at a constant temperature of 90 0C by

boiling triethylamine in the round bottomed flask. For Figure 36 : Apparatus for quantitative kinetics

To water condenser

Liould paraffin Thermostat - 52 -

each kinetic determination several capped n.m.r. tubes containing the hydrolysis solutions and sealed by

nescaff film were placed in the thermostat through the holes in a piece of cardboard which rested at the top of

the thermostat. The n.m.r. tubes were withdrawn at regular intervals and immersed in ice-cold water.

This effectively prevented further hydrolysis until the

spectra could be recorded by 31 Pn.m.r. spectroscopy.

As mentioned above,since water is present

in large excess, the hydrolysis of MDTPZ should be of

pseudo-first order. According to preliminary

deterniinations,this seems to be so as shown by the

straight line obtained by plotting ln(a-x) versus t for

the hydrolysis of diethyl ZDTP (2a) (Fig. 37). As can

be seen from this fig. there IS a large deviation from a

straight line in the case of diisopropyl ZDTP (2b). The

deviation from a straight line in both the cases may be

a consequence of the presence of the insoluble products

which start to form concomitantly with the disappearance

of the parent ZDTPz. The rate constant for the diethyl

ZDTP is 3.lxlO m1n 1 As is evident from fig.37,

hydrolysis of both ZDTPS is preceded by an induction

period. In order to show that free radicals exerted no

effect on the kine:tic runs, parallel experiments were

carried out in the presence of the radical inhibitor,

2,4,6 tris tert-butyl phenol. This produced no change - 53 - in the rate of hydrolysis thereby showing that the induction period observed above was not a consequence of a free radical mechanism.

5 identification of precipitates formed from hydrolysis of metal dithiophosphates.

Although earlier studies had suggested that the precipitate from the hydrolysis of the ZDTPs was ZnS, the identification of 0,0-dialkyl dithio- phosphoric acid (29) as the major intermediate pointed to the fact that it was more likely to be zinc hydroxide on the basis of equation 11.

((RO) 2PS 2 ) 2Zn + 2H 20 - >2(RO) 2PS 2H + Zn(OH) 2 ....(11)

However, the yields of the precipitates are considerably higher than those expected for Zinc hydroxide (see Table 1) while the reverse is true for the soluble hydrolysis products. An explanation for this ambiguity is that the zinc hydroxide undergoes further reactions with possibly hydrogen sulphide and phosphoric acid whose identification has already been confirmed. This idea is supported by elemental analyses carried out for all the precipitates (see Table 2). The data show that the precipitates are clearly not zinc sulphide as originally proposed from i.r. spectroscopy and the common test for - 54 -

TABLE 1 : HYDROLYSIS OF NEUTRAL MDTPS

MDTP bit-taken time tt % yield as wt soluble % yield as (0.005m) hydrolysis ppt Zn(OH) 2 products (RO) 2PS 2H

2a 2. 178g 1.5h 0.765g 154.2 1.272g 92.8 2b 2.458g 2.5h 0.666g 134.2 1.602g 74.8 2c 2.736g 5.Oh 0.785g 158.2 1.789g 73.9 17 2. 145g 5.Od 0.545g 117.7 1.795g 70.0 18a 2.412g 4.Oh 0.787g 106.0 1.444g 76.0

TABLE 2 : ANALYSIS* OF HYDROLYSIS PRECIPITATES

ELEMENT Et-ZDTP P r i_ZDTP Bu 1 -ZDTP Et-NiDTP (2a) (2b) (2c) (17)

M 41.0 49.0 41.0 53.7 S 7.6 15.8 7.1 34.4 P 16.9 9.3 15.2 0.9 C 1.3 1.6 0.3 < 0.5 H 1.5 1.1 1.3 < 0.5 0 31.7 •23.2 34.5 10.0

*All figures are % wt; oxygen figures by difference.

TABLE 3 : ELEMENTAL RATIOS IN PRECIPITATES

MDTP C H 0 P S M M+P/S+O

2a 0.03 0.03 0.77 0.41 0.18 1 1.48 2b 0.03 0.02 0.47 0.18 0.32 1 1.49 2c 0.00 0.03 0.84 0.37 0.17 1 1.35 17 0.00 0.00 0.18 0.01 0.64 1 1.23

TABLE 4 METAL CONTENT IN PRECIPITATES

MDTP REQIRED OBTAINED % YIELD

2a 0.3265g 0.3120g 96.25 2b 0.3265g 0.3263g 99.90 2c 0.3265g 0.3218g 98.50 17a 0.2935g 0.2927g 99.70 - 55 - sulphides. Nonetheless the elemental ratios are found to be different for all the ZDTPs' precipitates. As seen from table 3, only (Zn+P)/(S+O) is reasonably consistent but this has no absolute structural significance. Most likely, the precipitates are a mixture of zinc hydroxide, zinc oxide, and zinc suiphide apparently with some zinc phosphate. Zinc suiphide could result from the reaction of hydrogen suiphide with either zinc oxide or zinc hydroxide. In the case of the nickel complex (17), the precipitate seems to contain comparatively more metal suiphide. The phosphorus content of the precipitates is present in the form of the metal phosphates and pyrophosphate in all cases.

This was proven by 31 n.m.r. analysis, whereby the precipitates were dissolved in 2M sodium hydroxide solution and the spectra compared with authentic samples of the sodium salts which absorbed at 6.3 ppm and 4.1 ppm respectively. The precipitates were dissolved in sodium hydroxide solution because they were found to be insoluble in commonly available solvents. Indeed, these findings are to be expected in the light of the presence of phosphoric acid as one of the soluble hydrolysis products. Since the common test for suiphides, involving evolution of hydrogen suiphide by acid treatment, is equally good for thiophosphates, it is also possible that the precipitates contain some of the respective metal thiophosphates (and/or pyrothio- - 56 -

phosphates). Since 31 P n.m.r. zpectroscopy failed to show any signals due to such species in the precipitates, the evolution of hydrogen sulphide on acid treatment is most certainly due to the presence of a metal suiphide. In view of these observations it is not unreasonable to expect that the yields of the precipitates and the soluble products will increase and decrease, respectively with time as the concentration of phosphoric acid builds up . Consequently, the elemental ratios for the precipitates will also change with time as observed.

Finally, it is important to note that in all cases there is an almost quantitative yield of the metal content in the •precipitates (see Table 4) regardless of their composition. This gives further support to the mechanism outlined in Scheme 10,wherein attack of water occurs at the metal centre and does not give rise to any metal-containing soluble products.

6 ldentification of soluble products formed from hydrolysis of metal dithiophosphates.

Although the question of as to where the attack of water occurred in the hydrolysis of MDTPs had been answered unequivocally by the preceding results, one of the four major hydrolysis products which absorbed - 57 -

at 61.1 ppm still remained to be identified (see Fig.

6). An earlier supposition that this unidentified peak was due to a derivative of the monothiophosphoric acids

(21) was already proved wrong (see earlier ). This coupled with the fact that the monothiophosphoric acids

were present only in minor quantities (see Fig.6 and

Fig.8) in the hydrolysis products led to the hydrolysis of the moncthiophosphoric acids. It was necessary to

investigate whether they contributed to the end

hydrolysis products.

a. Hydrolysis of 0,0-dialkyl monothiophosphoric acids.

It was observed that diethylmonothio- phosphoric acid (21a) did not hydrolyse under the

identical reaction conditions. In the light of this unexpected observation, the diisopropyl (21b) and

diisobutyl (21c) derviatives were also examined and

found not to undergo hydrolysis in aqueous and even

under acidic conditions in DME at 85 0 C.

These observations may be explained on

the simple basis that the monothiophosphate anion is

resonance-stabilized to such an extent that it resists

any nucleophilic attack by water at the phosphorus centre (equation 12). - 58 -

P0 S P0 S 1W S

P P <- > P / \ / \ /\ P0 OH P0 0 P0 0

(21) R=Et, Pr or Bu' . (12)

Since the monothiophosphoric acid (21) did not undergo hydrolysis it is therefore not surprising that dialkyl phosphoric acid (22) could not be observed in the hydrolysis of the dialkyl dithiophosphoric acid (29) and !IDTPs. This implies that monoalkyl phosphoric acid (23) identified as being present among the hydrolysis products of the latter, is formed from a species other than dialkyl phosphoric acid

(22)

From the aforementioned evidence, it is apparent that the monothiophosphoric acid (21) is not formed directly from the primary intermediate, which was identified as the dithiophosphoric acid (29). In fact a more detailed 31 n.ni.r. analysis of the hydrolysis of diethyl ZDTP (2a) showed that the formation of 0,0-diethyl dithiophosphoric acid intermi diate (at 88.7 ppm) is succeeded by the formation of two other unidentified products which absorbed at 60.7 and 57.8 ppm (Fig.12). The latter peak gradually disappeared in - 59 -

favour of the former and another signal appeared at 63.9 ppm due to the monothiophosphoric acid. In order to ascertain the identity of these peaks and to substantiate the mechanism in Scheme 10, it was decided to investigate the independent hydrolysis of diethyl dithiophosphoric acid (29a) under the same conditions.

No details of the hydrolysis of (29a) are known -todate except that it is sensitive to atmospheric moisture62 ' 63 ' 78 On the other hand aqueous solution of the the potassium salts of (29) are stable for longer periods at ambi. temperatures. For example, an aqueous solution of potassium 0,0-diisopropyl dithiophosphate is stable for at leoast ten months but undergoes decomposition in an acidic medium giving off hydrogen sulphide 81 .

b. Hydrolysis of 0,0-diethyl dithiophosphoric acid.

As seen from a comparison of the Figs. cf. 13, 33 and 38, the hydrolysis of 0,0-diethyl dithiophosphor'ic acid (29a) with ten equiv of water at

850 C gives rise to the same major products as those obtained from diethyl zinc dithiophosphate (2a) and diethyl cadmium dithiophosphate (18a). As in the hydrolysis of the latter complexes, prolonged heating led to the gradual diminuition of the peak at 57.5ppm in favour of that which absorbs at 60.5ppm, (Fig. 39). Figure 38 : Hydrolysis of diethyl dithiophosphoric acid in OME at 85 °C, t - Ca 2h

4- 3i PS Figure 39 : Hydrolysis of 0,0-diethyl dithiophosphoric acid in DME at 850C t - ca 2.5h

H PQ 4

0. 8

0

Et0 EtO''

31 - 60 -

Moreover a new peak due to the monothiophosphoric acid

(21a) appeared at 63.5 ppm (vide supra). Besides confirming the intermediacy of 0,0-dialkyl

dithiophosphoric acids (29) in the hydrolysis of metal

0,0-diethyl dithiophosphates, these observations showed

unequivocaUy that the unidentified compound (with 31

chemical shift at 61.lppm, Fig.6 or 60.5 ppm, Fig. 39) was derived from the primary hydrolysis product the

dithiophosphoric acids (29) and not from a secondary

intermediate, i.e. the monothiophosphoric acids (21) as

initially proposed.

An examination of the hydrolysis of

0,0-diethyl dithiophozphoric acid (29) at very short

intervals, revealed that the first intermediate which is

formed within ca 15 mm., is the product with chemical

shift, 58ppm (Fig.40). This is the same product as that

observed (57.8 ppm) from the hydrolysis of the parent

ZDTP(2a) (Fig. 13). As the hydrolysis progressed, this

intermediate gradually disappeared in favour of the

final productz,which included the unidentified peak at

60.5ppm, the monothiophosphoric acid (21a), the

monoethyl phosphoric acid (23a), and phosphoric acid

(Figs. 38, 39, and 41). The intermediate at 58.0 ppm

could not be assigned to the thiol form (24) of the

monothiophosphoric acid (21a) since it absorbs at 24

ppm.75 gA —.o ic

08S

1.11w SI. 3

98 48 ]WQ UT pre o.xoqdaoqdotqtp TM.fl81p-Q'Q JO BTSATOJPAH : op ..zn6çj Figure 41 : Hydrolysis of 0,0-diethyl dithiophosphorjc acid in DME at 85 0C 9

t - ca lh

57.7

ME

3' 4- - 61 -

As expected, the hydrolysis of (29a) with ten equiv. of water at room temperature is a much slower process, but it gives rise within 48h to a single major product at 58.0 ppm (Fig. 42). This product is identical with that obtained at 850 C within ca 15 mm.

Conversion of (29a)into this compound is almost complete within 15 days. Thereafter, the reaction, did not show any appreciable change even after a period of 30 days.

At -tempts to isolate this product from the hydrolysis mixture proved to be unsuccessful since it underwent ready decomposition when efforts were made to remove the solvent by evaporation under high vacuum. No structural information about this intermediate could be obtained from 1 H n.m.r. analysis of the hydrolysis solution because 1 H signals arising from the intermediate overlapped with those of DME.

In other experiments, the hydrolysis of the diethyl dithiophosphoric acid was also effected with only two equiv. of water at 85 0C (see Fig. 43). After ca 15 min incomplete hydrolysis gave rise to five peaks at 79.1, 73.9, 64.6 (identified as 21a), 62.2, and

59.9ppm. By comparison, after the zame length of time but with ten equiv. of water, only the latter three peaks were observed (Fig. 40) as in the hydrolysis of the diethyl ZDTP (2a). Apparently the presence of a large excess of water results in the rapid conversion of Figure 42 : Hydrolysis of 0,0-diethyl dithiophosphoric acid (29a) at ambient temperatures in DME. t = ca 2d

95, .7

3' i- P 5 6L

UTW SL = 3140

U1 ATnbe za;er' ORI q;p' pr3e OT 10 4dsoqdOT41TP IA48Tp-0'0 JO STSAIO1PAH : çp exnbtj - 62 -

the intermediates with shifts at 79.1 and 73.9 ppm into the hitherto unidentified intermediate which absorbs in this case, at 59.9 ppm (58.Oppm in Fig. 40). This idea was supported by the fact that hydrolysis of (29a) generated in Situ with two equiv of water at 85 0C gave after Ca 6.5h the same major products as those obtained with ten equiv of water.

These results pointed to the fact that

0,0-diethyl dithiophosphoric acid (29a) does not undergo direct hydrolysis to the monothiophosphoric acid

(21a) but that its conversion is preceded by other reactions. It seemed likely that these involved P-0-C to P-S-C migrations Since such reactions occur frequently in the thermal rearrangement of organopho- sphoro-sulphur compounds 82 . Details are given later, but in each case the driving force for the migration is the formation of more stable P=0 bond at the expense of the P=S bond. This prompted an examination of the possibility of a P-O---C to P-S-C migrations in the hope of identifying the second major intermediate at 58.Oppm and the remaining product at 61. lppm (Fig. 40).

In the event of any ?-0-C to P-S-C migrations in o,o-diethyl dithiophosphoric acid (29a) three possible structures must he considered, viz. the most likely candidate, O,S-diethyl isomer (30), Its - 63 -

thiono tautomer (31), and the S,S diethyl isomer (32).

From subsequent mechanistic considerations, It was also neccesary to consider the likelihood of structures (33),

(34), and (35) which might form as a result of the above

migrations. Unfortunately, 31 P chemical shifts for

the compounds (30)-(32) are not known, but it is reported that compounds (33)-(35) absorb at 94.7ppm, - 53.5ppm and 61.3ppm, respectively 75 . Based on this data, it IS possible to rule out structures (33) and

EtS 0 EtS S EtS 0 \ P P P

EtO SH EtO OH EtS OH

(30) (31) (32)

EtO S EtO 0 EtS 0 \/ \/ P P P

EtO SEt EtS SEt EtS SEt

(33) (34) (35)

(34). At first sight, structure (35) seemed an

unlikely candidate but it could form as a result of the attack by ot.hanethiol (which would be expected to form

from the hydrolysis of a P-S-Et bond) on the S,S-diethyl

isomer (32) of the 0,0-diethyl dithio-phosphoric acid

(29a). The compound (35) was synthesized 84 for direct - 64 -

EtOS S \ OEt

/\ EtO/\ S—Zn—S OEt

H2 0

EtO S H 20 S OEt P OH P /\ I /\ EtO SH —Zn(OH) 2 Zn—S OEt

(29a) ZnO I EtO 0 \i \/ P P /\ /\ ) EtO SH EtO OH Eto OH

(22)

EtSi'S EtSO \ P P /\ EtO/\ OH EtO OH (31) (36) H20 1_EtSH EtS \/ P P /\ /\ EtS OH EtO OH

(32) (23)

EtSH H2 0 jEtOH EtS

EtS— P=O P ZnO Zn3(PO4)2 + Zn P207 /\ EtS HO OH (35)

Scheme I' - 65 - comparison and was found to be the same as the hitherto unidentified product with 31P shift at 61. lppm by the 'peak enhancement' method. Only the intermediate with shift at 58.Oppm remained to be identified.

Although the formation of (35) as a hydrolysis product came as a surprise its presence gave support to the concept of P-O-C to P-S-C migrations in

0,0-diethyl dithiophosphoric acid (29a). This pointed to a different mechanism requiring the Intermediacy of (32) in the hydrolysis of the diethyl ZDTP (2a). As outlined in Scheme 11, the mechanism now proposed involves an initial intramolecular ethyl migration from P-O to P-S in the key intermediate (29a), leading to the t.hlolo Isomer (30) whIch was believed to undergo a further ethyl shift via its thiono isomer (31) to form the S,S-diethyl isomer (32).

From Scheme 11 it appears that once the isomer (32) is formed, it will react with ethanethiol to give (35). Ethanethiol is suggested to be formed from hydrolysis of (36) which in turn is believed to resul.t from hydrolysis of another intermediate (30). P-O-C to P-S-C migrations are reported to occur in related compounds including the examples (37 and 38), both of which isomerise readily at about 100 0C (equations 13 and 14).82 ,83 These migrations are facilitated by polar - 66 -

media and it was suggested that they are intermolecular

RO S 1000 RO SR

P P (R=Me) .....(13) /\ /\ RO OR RO 0

MeO S 1000 0 SHe \/ __ P P (14) /\. /\ Cl Cl Cl Cl

in nature as depicted in Scheme 12. In both cases, the driving force behind the migration is the formation of the more stable P=O at the expense of a P=S bond. In fact scheme 12 may be regarded as a special case of the

Pistschimuka reaction in which thioriophosphates (37) are

RO S RO S .RO SR RO S \ \ __ \/ P 1 + P2 +P1 + P2 - /. RO OR RO OR RO OR RO 0

RO SR RO SR \/ F l + P2 RO 0 RO 0

Scheme 12 - 67 -

isomerised to the thiolo ester by alkyl iodides

(equation 15).

RO S RI RO SRi -RI RO SR \/ ___ - \/ I P >J !' I , P (15) /\ /\I /\\ RO OR RO OPJ RO 0

Control experiments showed that (29a) did not undergo any changes when heated in DME at 85 0C in the absence of water. It may well be that (29a) readily dissociates in the polar aqueous medium, and once the anion is formed, only then are conditions met for the occurrence of P-O-C to P-S-C migrations as shown in

Scheme 13, to ultimately give the S,S-diethyl isomer

(32).

EtO S H 20 EtO S \/ P P / \ / \ EtO SH EtO

(29a) (29a)

0 SEt 0 S 0 S

P P 4 P

-O SEt EtO SEt EtO SEt

(32) (31) (305

Scheme 13

- 68 -

As seen from the intermolecular pathway

outlined in Scheme 14, the conversion of (29a) into (32)

via its anion requires the intervention of several key

intermediatess, including (33) and (34).

It also follows from Scheme 15 that the

anion of (29a)could also react with (33) to form (32) in

conjunction with (34)9 Careful monitoring of the

reaction by 31 P n.m.r. spectroscopy failed to provide

EtO S ' EtO S EtO SEt 0 S

P + P P + P /\\ /.\ EtO S EtO H EtO S EtO SH

(29ar (29) (33) (39) 1133)

EtO SEt 0 SH

P + P / \ _ /\ ,0 SEtO SEt

(30) (30) 1W) 0 SEt 0 SET 0 SEtO SEt \\/ (34) P + P /\ /\ • /\ -O SEt HO SEt HO SEt 0 SEt

(32) (32) (40) (34)

Scheme 14 - 69 -

EtO S EtO S 0 S Eta S \/ \/ ___ \/ P + P P + P EtO S - EtO SEt EtO SEt EtO SEt (9) (31) (33) o SEt 0 SEt 0 ' S 0 SEt (34) P + P < P + P / \ o SEt EtO SEt EtO SEt EtOSEt (32) (31)

Scheme 15

any evidence for the formation of either (33) or (34), which absorb at 94.7 and 53.5 ppm, respectively.

Besides the formation of (32), the above

Schemes also implicate the intermediacy of (30) and (31), for which no 31P n.m.r. data was available from the literature. The isomer (30) did not show a good candidature to be the unidentified intermediate in question. This is because the presence of P=O bond in this compound would certainly make it absorb at a considerably lower chemical shift [like (24) versus (21a), see Table 51 compared to isomer (31) containing a P=S bond.

The isomer (31) of (29a) was ruled out as TABLE 5 : 31 P CHEMICAL SHIFT (PPM) FOR SOME SELECTED COMPOUNDS RELATIVE TO 85% PHOSPHORIC ACID.

No. Structure Observed in DME Reported in CDC1 3 75

EtO 0 (23a) "P 0.2 -- HO 'NOH

EtON . O (22a) P -1.5 -1.3 EtO" NOH

EtS O P -- 27.0 HO NOH

EtS N O / P 46.8 -- EtS NOH

EtO S (21a) "P 64.1 58.6 EtO" 'OH

EtS\ : S

(31) P -- -- Eta" "OH

EtO 0 (24) ,P -- 24.0 EtO SH

EtS 0

(30) " P -- -- EtO' 'SH

EtO S N (29a) P 88.4 84.5 EtO" NSH

EtO S 94.7 94.1 EtO SEt

EtON O .,/P. -- 53.5 EtS "SEt

EtS\ O ,P 61.1 61.3 EtS - 70 - a possible candidate for the unidentified intermediate at 58.0 ppm since it would be expected to absorb at much lower field. For example, 0,0-diethyl monothio- phosphoric acid (21a) absorbs at 64.1 ppm and replacement of an EtO group by EtS (to give 31) would certainly raise its chemical shift.

On the other hand, the isomer (32) can be expected to resonate close to 58.0 ppm. This is because reference to Table 5 shows that for (23a), replacement of a EtO group with a EtS group leads to an increase in

31 P chemical shift by about 27 ppm (cf 36). It follows that incorporation of a second EtS group in (23a) to give (32) should lead to a further increase by about 27 ppm. The isomer (32) was synthesised by the alkaline hydrolysis of S,S,S-triethyl trithiophosphate (35) as described by Thain84 . The author reported that (35) was hydrolysed via the P-S cleavage to give ethanethiol and

EtS SEt EtS OH \/ ___ P P +EtSH (16) EtS 0 EtS 0 (35) (32)

EtS SH \/ P /\\ EtS 0

(41) - 71 -

(32) (equation 16). The reaction was monitored by 31 P. n.m.r. spectroscopy in this work which showed that (35) underwent only the P-S and not C-S cleavage. No compound (e.g. 41) other than the desired product (32) was found to be formed, and significantly, it resonated at 46.8 ppm (in DME) indicating that it was different from the unidentified intermediate with 31F shift at

58.0 ppm.

C. Reaction betNeen 0,0-diethyl. dithiophosphoric acid and ethyl iodide..

Based on the preceding evidence, it appeared that the unidentified intermediate with chemical shift at 58.0 'ppm was not 0,9-diethyl dithiophosphoric acid (30) nor any of its rearranged

isomers (31) and (32). Because of the implications of

these findings for Scheme 11, other means were sought to verify the conclusion that compounds (30) - (32), were

not implicated in the hydrolysis of diethyl ZDTP (2a).

Reference has already been made to the Pistschimuka

reaction82 ' 86 whereby thiophosphates are isomerised to 4 hiol esters by alkyliodides. It was envisaged that in the presence of an excess of EtI, 0,0-diethyl

dithiophosphoric acid (29a) would undergo similar reaction to give initially isomer (30) which in turn

would rearrange to (31), and finally form its - 72 -

S,S-diiethyl isomer (32) as shown in Scheme 16.

O \ SEt Et-I P Et-I /\ ____ /\ EtO SH EtO SH (29a) (30)

0 OH Et-I"'EtO OH

P P /\ EtS SEt I-Er'S SEt (32) (31)

Scheme 16

In the event, 31 P n.m.r. analysis of the

reaction in dry boiling DHE showed that within ca 0.5h, a new peak appeared at 65.4 ppm which slowly intensified, but then disappeared (ca 30h) in favour of a single peak at 94.7 ppm which was identified as O,S,S- triethyl -trithiophosphate (33) by peak enhancement. Formation of (33) in:stead of (32) as expected from Scheme 16 suggests that the peak at 65.4 ppm most likely corresponds to isomer (31) which upon reaction with more

EtI gives (33) as shown in equation 17. The

intermediate with shift, 65.4 ppm was shown to be different from the unidentified intermediate at 58.0 ppm by the peak enhancement method. - 73 -

EtO OH EtI EtO OEt \/ __ \/ P P ...... (17) S SEt -HI S SEt (31) (33)

The fact that an intermediate with chemical shift at 65.4 ppm is observed in the Pistschimuka reaction of (29a) with EtI, but not in its

hydrolysis, implies that P-O-C to P-S-C migrations do not occur to any appreciable extent in the latter reaction, and therefore, in the hydrolysis of ZDTPZ. Nonetheless, the identification of S,S,S-triethyl trithiophosphate (35), albeit in26.2% yield, as a hydrolysis product requires the intervention of (32) by EtSH (see Scheme 11), unless another pathway exists for its formation. A further quandry in the formulation of

(35) as a hydrolysis product lies in the failure to observe the source of EtSH, i.e. (36) in Scheme 11 which absorbs at 27.0 ppm 75 , in the hydrolysis spectra of both (29a) and diethyl ZDTP (2a).

d Identification of the intermediate derived from the

hydrolysis of 0,0-diethyl dithiophosphoric acid

In view of the above uncertainties further attempts were made to identify the intermediate at 58.0 - 74 - ppm, upon which hinged mechanism of hydrolysis, by chemical means. As mentioned previously, efforts to isolate this intermediate were unsuccessful. It was found to be stable only in DHE since attempts to remove the solvent.resulted in its decomposition. The strategy adopted was to freeze the hydrolysis reaction as soon as the intermediate was formed, and on the basis that it was acidic in nature, attempts were made to convert it into its cyclohexylammonium or anilinium salt.

Conversion into its sodium or potassium salt was avoided in order to avoid the possibility of further hydrolysis in the presence of strongly necleophilic hydroxyl ions. The hydrolysis was monitored therefore by repeated

31 P.n.m.r. analysis. Aliquots withdrawn from the reaction mixture after two minute intervals showed that the conversion of the parent dithiophosphoric acid (29a) into the intermediate with shift of 58.0 ppm was complete within about twelve minutes. The intermediate at this stage was about 95% pure as seen from Fig. 40.

Addition of excess cyclohexylamine or aniline in DME to the reaction residue obtained after removal of solvent at room temperature over a period of 24h., resulted in the formation of a colourless, amorphous precipitate. The filtrate was found to contain many unidentifiable phosphorus-containing products. The precipitate gave off hydrogen suiphide even at room - 75 - temperature presumably by reaction with atmospheric moisture. It was insoluble in commonly available solvents except for water in which it also decomposed with emission of hydrogen suiphide. Attempts to recrystallise the precipitate failed. No definite conclusions could be drawn from its i.r. spectrum (Fig. 4tio)except that it differed very much from that (Fig.ib) of the cyclohexylammonium salt of S,S-diethyldithio

-phosphoric acid (32) which has been proposed as an intermediate in the hydrolysis (Scheme 11). Their 31 P chemical shifts in water also differed (36.7 ppm vs 42.4 ppm) and for comparison, it should be noted that the corresponding salt from O,O-diethyl dithiophosphoric acid (29a) absorbs at 111.3 ppm. Moreover, the former two salts are solids, while the latter is a liquid. A

Yteld*,S 0/0 colourless precipitate also resulted when the intermediate was neutralised with the weaker base, aniline until pH7. It absorbed at 43.8ppm (water). The filtrate showed two unidentifiable peaks in its n.m.r. spectrum at 59.5 ppm and 57.9 ppm. As in the case of cyclohexylamine salt the anilinium precipitate gave off H2S at ambient temperatures, probably by reacting with the atmospheric moisture. It was also soluble only in water in which it decomposed with evolution of hydrogen suiphide. The profound instability of both complexes precluded elemental analysis and their spectroscopic identification by

- 76 -

n.m.r. and mass .spectrometry. Their rapid decomposition

also prevented structural elucidation by means of x-ray

crystallography.

In conclusion, it has been shown that the

neutral zinc and related metal dithiophosphates do

undergo hydrolysis giving rise to a plethora of

products. Most of the products have been unequivocally

identified by means of the novel application of

n.m.r. spectroscopy. Although the formation of the

products is prece-ded by the intermediacy of 0,0-dialkyl

dithiophosphoric acids, the exact mechanism is not yet

clear. Perhaps this requires the corroborative evidence

for the identification of the product with shift 61.1

ppm as S,S,S-triethyl trithiophosphate in the hydrolysis

of diethyl ZDTP (2ä). This is further necessitated in

view of the fact that hydrolysis of 0,0-diethyl

dithiophosphoric acid does not involve any P-O-C to

V P-S-C migrations as shown. - 77 -

B. Hydrolysis of basic zinc 0,0-dilsopropyl dithio-

phosphate (Hydrolysis of Hexakis - (O,O-diisopropyl

phosphorodithioateJ-u-4-tetraoxo Zinc

Zinc 0,0-dialkyl dithiophosphates also exist in a basic form (3). These basic salts were first obtained by Wystrach et a1 64 in 1956 as a by-product of

1(RO)PS2 J 6 Zn 4 O [Zn 2 [S2 P(OR) 2 13 0H]

(3) (42)

R=Et C; R=Bu'

R=Pri d; R=Bu

the reaction between zinc chloride and sodium

O,O-di-n-butyl phosphorodithioate in aqueous solution during the preparation of the neutral salt (3d). Later,

Bacon and Bark 87 isolated other examples of the same series of compounds using zinc oxide and 0,0-dialkyl hydrogen phosphorodithioa-tes (29). The same basic salts can also be prepared by oxidizing the neutral salts with hydroperoxides (see equation 3, P.12).

On the basis of elemental analysis and neutralization equivalents from non-aqueous acid-base titrations, Wystrach'.s group proposed a basic double- salt formula (42) for these basic salts. Their - 78 - formation was found to be dependent upon the presence of hydroxyl Ions, and it was claimed by the same workers that either the normal salt (2) or the basic salt (42) could be exclusively formed according to the stoichOometry of equations 18 and 19.

2(R0) 2 PS2 + Zn 2 ) ZnCS 2P(OR) 21 2 (18) (2)

3(R0) 2 p 2 + 2Zn 2 + 0H ) Zn 2[S 2P(OR) 2 ] 30H ..... 19) (42)

The authors also reported, without comment, that the i.r. spectra of these basic compounds did not contain an absorption due to a 0-H stretching frequency. The structure that they proposed was supported by Bacon and

Bork87 , but in 1965 Burn and Smith 88 keeping in view the apparent 'absence' of OH groups in these compounds ( and their molecular weights) obtained what they believed to be the correct structure for the isopropyl basic compound (3b) by a partial x-ray crystal analysis. The structure contained a central oxygen atom surrounded tettahedrally by four zinc atoms with the six

0,0-dilsopropyl phosphordithioates groups symmetrically attached to the six edges of this tetrahedron (Fig. 44).

Very recently, extended x-ray absorption fine structure studies (EXAFS) have confirmed this finding88. Figure 44 Structure for [(Pr'O) 2PS2 ] 6Zn4O

Zinc 0 Oxygen ij1JJI Sulphur

Phosphorus © • Carbon

Full alkyl group not shown - 79 -

As mentioned in the Introduction, basic zinc dithiophosphates form a significant component of commercial neutral ZDTP additives and their hydrolysis seemed worthy of detailed investigation. Initial hydrolysis studies were carried out with the isopropyl derivative (3b) under the same conditions used for the neutral analogue (2b) which required ca 2.5h for complete hydrolysis. These showed that the basic compound gave a colourless, cloudy precipitate within ca

15 mm. Analysis of the reaction at this stage by 31 P n.m.r. spectroscopy showed the presence of only two peaks (in the ratio 1:3 ), the minor one at 99.7 ppm due to the basic compound itself and another larger peak at

98.4 ppm (Fig.45). The latter signal was identified as being due to the neutral isopropyl salt (2b) by the

'peak enhancement' method. Although the precipitate, was not analysed, it is most likely ZnO that was formed according to equation 20

(+10 equv. U2 0)

[(Pr'O) 2 pg2 J 6 Zn4 O ) 3[(Pr m O) 2PS 2 ] 2Zn + ZnO (3b) (2b) (20)

As the precipitate became more bulky, the

intensity of the peak at 98.4 ppm increased and that of the peak at 99.7ppm decreased with time (Figs. 46-50), but even after ca SOh of heating the process was not ligure 45 : Hydrolysis of basic diisopropyl ZDTP with 10 eiuiv. water

in ONE at 850 C, t = ca lh

98.4 Figure 46 * Hydrolysis of basic diiaopropy], ZDTP with 10 egujv.

0 water in DME at 85 C, t = ca 2h

99.4

1(PrOwS1 l 22j2fl 2

3, - p rigure 47 s Hydroiysi8 of basic dilsopropyl ZDIP with .10 eguiu, water

in DtIE at 850 C, t ca 3.5h

9 IB.4

1I RPs] Zn40 Figure 48 Hydrolysie of basic diisopropyl ZDTP with 10 eguive

water in DME at 85 0C, t =ca 24h

984

3) 4- p Figure 49 * Hydrolysis of basic diisopropy]. ZDTP with 10 egujv, water in DME at 850 C, t = ca 72h

98.0

19

31 4- p Figure 50 : Hydrolysis of basic dilsopropyl ZDTP with

10 equiv. water in DME at 85°C, t = Ca. 90 ii.

98.3

Pr'Q)PS Zn 22 I2

99. [(PO)Ps ]ZnO 22 64

31 k— P5 complete and a small amount of the basic compound still remained unreacted. More surprising was the observation that.the neutral salt, so formed, did not undergo hydrolysis, although under identical reaction conditions, it is totally hydrolyzed in ca 25h (see earlier and Fig. 51).

From the foregoing results, it appeared as if all the water present was consumed to form hydrated zinc oxide. As a result no further water was available for the secondary hydrolysis of the neutral salt. To test this theory, it was decided to hydrolyze the basic compound (3b) with double the number of water equivalents ie. 20 so that 10 equivalents would still be left to hydrolyse the resulting neutral salt (2a).

Under these conditions, it was found that the basic ZDTP was almost completely converted into the neutral salt within ca 25 mm (Fig. 52). Thereafter, the resultant neutral ZDTP underwent complete hydrolysis over a period of 77h (Figs. 52-56) to give products identical with those obtained (after ca 2.5h) from the independent hydrolysis of the neutral ZDTP With 10 equiv of 112 0 under the same conditions (see Fig. 51). In this instance, however, a remarkable induction period of >50 hours was observed during which no secondary hydrolysis took place. It is worth mentioning that the corresponding hydrolysis reaction of the basic ZDTP (3b) FigureS]. : Hydrolysis of diisopropyl ZDTP in

DME at 85°C, t = Ca. 2.5 h.

0.9

+1-. 3Ip Figure 52 : Hydrolysis of basic diisopropyl ZDTP with 20 equiv. water in DP1E, t = ca 25 mm.

97.2

31 +.- p Figure 53 : Hydrolysis of besic dlisopropyl ZDTP with 20 sguiv. water In DME at 950 C 9 t -ca I.5h

97.'

S Figure 54 : Hydrolysis of basic diisopropyi ZDTP with 20 •guiv. water In DE at 95 0 C 9 t -Ca. 23h

97_I

31 PS Figure 55 !dro1ysia of basic dilsopropy]. ZDTP wIth 20 aguiv* water in ONE at 85 0C 9 t = ca 51h

97.'

5 Figure 56 : Hydrolysis of basic diisopropyl ZDTP with 10 equiv.

water at 85°C in DME, t = Ca. 7 d

O.G

+I- 31 - 81 -

with 10 equiv of water (vide supra) was found to give after ca 7 days a 31 P n.m.r. spectrum (see Fig. 57)

identical with that observed from its hydrolysis with 20 equiv of H 20 after ca 77 h (Fig. 56).

The question arises therefore as to why the basic compound requires longer times for conversion into

the neutral salt with 10 equiv of H 20 compared to 20 equivalents. In order to determine the role of water in these hydrolysis experiments a control experiment was

carried out in the absence of water. Reference to Fig.

58 and 59 shows that under identical conditions

(anhydrous DME, 850) the basic ZDTP is in fact converted

into the neutral ZDTP, albeit to a small extent, i.e.

1:0.2 compared to the relative ratio of 1:3 in the hydrolysis with 10 equiv of H 20 for an equal period of

time (ca 1 hour). Addition of excess water to the

reaction mixture led to an almost instantaneous, yet

incomplete conversion, of the basic compound into the

neutral one (Fig. 60). It was further observed that the thermal degradation of the basic salt occurred very

slow, and even after about 12 days of heating, a small amount of the basic ZDTP still remained unchanged.

Other workers89 have previously noted that the basic

compound is transformed into the neutral salt and ZnO upon heating at temperatures > 950C. Figure 57 Hydrolysis of basic diisopropyl ZDTP with 20 eguiv. water in DME at 85° C) L77h

1.4

31 ~I - ps Figure 58 s Thermal degradation of basic diisopropyi ZDTP in DME at 85° C, t 24Ornjn

99.0

EFo) PS] Zn 2 2 2 0P zo

3' p Figure 59 : Thermal degradation of basic dilsopropyl

ZDTP at 85°C in DME, t = Ca. 1.5 h.

99.0

93. 6 [(Prb)Ps] Zn I_i 2 22 - L 22 Figure 60 : Effect of addition of water on thermal

degradation of ZDTP at 85°C in DME

98.3

31 4— Ps -82 -

In concomitant experiments it was shown that the conversion of the basic salt (3b) into its neutral form was promoted by the presence of O,O-diisopropyl dithiophosphoric acid (29b). The conversion was complete within a few minutes and was accompanied by the disappearance of the precipitate of

ZnO. It should be noted that the reaction between ZnO and a dithiophosphoric acid leads to the synthesis of neutral ZDTPS (equation 21) at ambient temperatures as

2(PrO) 2 pS2H + ZnO ) C(Pr'O) 2PS 21 2Zn + H 20 .....(21) (29b) (2b) reported by Bacon and Bork 87 . This tends to suggest that the ZnO may be undergoing a reversible reaction with the neutral salt (equation 22), and that its

C(PrO)2PS2]6Zn4O - 3E(PriO)2PS23Zn + ZnO .....(22) (3b) (2b) removal by reaction with the dithiophosphoric acid (29b) promotes the complete conversion of the basic compound into the neutral salt, ie (3b) -) (2b) as observed.

Whether the ZnO is formed by hydrolysis or thermal decomposition of the basic compound (or both) was not established, but it did appear that the presence of ZnO in the reaction mixture caused inhibition of both the - 83 - conversion of the basic compound (by reversing equation

22) and the subsequent hydrolysis of the neutral salt (by complexing with the water). This latter. point was verified by a control experiment in which the neutal salt (3 parts) was hydrolyzed under the same Conditions

(DME; 20 parts of H2 0; 850 C) but in the presence of ZnO (1 part). It was found that no hydrolysis occurred

until after an 'induction period' of > 50 hours and additionally, that none of the basic compound was formed

during this period as might have been expected from the reverse of equation (22). However, this was not the case when the neutral salt was mixed and stirred with ZnO in the ratio of 3:1 in DME at ambient temperatures

in the absence of water. Under these conditions, a small amount of the basic compound was observed by 31 P n.m.r. spectroscopy. This showed that the equilibrium in equation 22 is inclined towards the right.

Since the literature on zinc oxide shows that it takes up only one molecule of water of 8 crystallistion, one is forced to conclude that the water

is physically adsorbed on the surface of the zinc oxide.

The induction period noted above may be attributed to a gradual •desorption of water, thus allowing the

resultant neutral salt to undergo hydrolysis.

Nonetheless, the reaction between ZnO and the dithiophosphoric acid (29b) is instantaneous even at - 84 - room temperature, and this is also likely to be another factor in the observed inhibition of the hydrolysis of the neutral salts. - 85 -

C. Future Research iJork

Although much progress has been made in the hydrolytic studies of ZDTPs several key as well as puzzling features of their mechanism remain to be explored before a complete definition of the hydrolysis mechanism can be obtained. These include the following

Identification of the intermediate with 3 IP chemical shift 58.Oppm and complete product identity including the product with shift at 61.1 ppm. Information on the effect of aryl group on the hydrolysis of ZDTPs. More detailed investigation into the unexpected retarding effect of ZnO on the rate of hydrolysis of neutral ZDTPs and, closely relatedtothis, the effect of pH. Comparative information on the products and rates of hydrolysis of alkyl and aryl type basic ZDTPs. Quantitative rate data on the formation of products and on the appearance and disappearance of intermediates, especially on other metal ZDTPs. For example, Cd and Ni salts for which preliminary results have been obtained already. These showed that while the end-products of hydrolysis are the same as those from ZDTPs, the mechanism of their formation appears to be significantly different in the latter case. EXPER I MENTAL. EXPERIMENTAL CONTENTS

Page No.

A. Symbols and Abbreviations 86

B. Instrumentation and General Techniques 87

C. Preparation

1) 0,0-dialkyl dithiophosphoric acids and

their potassium salts. 90

2) Neutral metal 0,0-dialkyl dithiophosphates. 91

3) Dialkyl phosphites. 94

4) 0,0-dialkyl monothiophosphoric acids. 94

5) Diethyl phosphoric acid. 95

6) Monoalkyl phosphoric acids. 95

7) Tetra ethyl dithionopyrophosphate. 96

8) S,S,S-triethyl trithiophosphate. 96

9) S,S,-diethyl dithiophosphate. 97

10) O,S-diethyl dithiolophosphoric acid. 97

11) O,O,S-triethyl dithiophosphate. 97

12) Basic zinc 0,0-dialkyl dithiophosphates. 97.

D. Hydrolyses

Hydrolysis of neutral zinc and related metal

0,0-dialkyl dithiophosphates. 99

Hydrolysis of 0,0-dialkyl monothiophsphoric

acids. 101

Hydrolysis of 0,0-diethyl dithiophosphoric

acid. 101 Reaction between 0,0-diethyl dithio-

phosphoric acid and ethyl iodide. 104

Hydrolysis of Basic zinc 0,0-dialkyl-

dith iophosphates. 104

Reaction between neutral zinc 0,0-diisopropyl

dithiophosphate and zinc oxide. 104 - 86 -

A. Syrbols and Abbreviations

m moles/litre Oc degree centigrade

M.P. melting point b.p. boiling point h,min,d hours, minutes, days n.m.r. nuclear magnetic resonance

S,ppm chemical shift, parts per million

s,d,t,q,m singlet, doublet, triplet, quartet, multiplet

i.r,v infra red, wave number

M.S. mass spectroscopy 14+ mass of molecular ion

ni/z mass to charge ratio T.l.c thin layer chromatography - 87 -

B. lnstruentation and General Techniques

Nuclear Magnetic Resonance Spectra : 1 H.n.m.r. routine spectra were obtained at 80 MH on Bru.kerWP80.

31 P.n.m.r. spectra at 24.21 NH 3 on JEOL FX60 FTNHR spectrometer; 13C.n.m.r. on a Varian CFT-20 spectrometer. All chemc:al shifts are expressed in parts per million with respect to tetramethyl silane 31 (TMS) in the case of 1 H and 13 C while P chemical shifts are relative to 85% phosphoric acid, C6 D6 being used as an external lock. Typical instrumental parameters on which 31 P n.m.r. spectra were recorded on a non-(P-H) spin-spin coupling mode (COMP) are spectrum width (SW) 10 KHz, pulse repitition time (PR) Ss, accumulation scan number (SCANS) 108.

Infrared Spectra These were recorded either on a photo Perkin-Elmer 157G grating spectrer or Perkin-Elmer

598 infrared spectro photometer. Solids were run as nujol mulls and liquids as thin films, both either on sodium chloride or potassium bromide plates. Spectra were calibrated with the polystyrene peak at 1603cm.

Mass-spectra All low-resolution mass-spectra obtained on an A.E.I. Ms-902 instrument at an ionising voltage of 70 eV. or KRATOS M.S.50.T.C. with 30M BYTE

DG-30 Data System, D.S.90. - 88 -

Elemental Analysis Microanalysis for C, H and N were carried out on a Perkin-Elmer 240 Elemental Analyser and Elemental Analyser CARLO ERBA MODEL. Analysis for phosphorus and sulphur were carried out by the analytical section of B.P. Research Centre 1 Sunbury.

Melting Points : Determined on a Reichert hot-stage microscope as well as Electrothermal melting point apparatus. All m.ps. are uncorrected.

Thin Layer Chromatography Carried out using

0.3mm layer of alumina (Merck, neutral aluminium oxide GOG, Type E) or silica (Merck Kieselgel 60G), containing

0.5% Woelm fluorescent green indicator on glass plates. The components were observed under ultraviolet light or by their reaction with iodine vapour. Preparative t.l.c. was carried out using 1.0 mm layers of the supports mentioned above.

Drying and Purification Commercially available pure solvents were use4 without further purification unless otherwise stated. Tetrahydrofuran (THF) and 1,2

Dimethoxy ethane (DME) also known as ethylene glycol dimethyl ether were of A.R. grade and purified as described in the discussion P2€, . The purity of the phosphorus containing reagents was additionally checked by 31Pn.m.r.spectroscopy. Organic solutions were dried - 89 - over anhydrous magnesium sulphate for several hours and were evaporated under reduced pressure in most cases by usingahigh vacuum Buchi and whenever this procedure was used, n.m.r. spectroscopy was used to ensure that no change apart from the desired function took place. In all cases nitrogen was rigorously dried by passing the gas through silica gel, concentrated sulphuric acid, silica gel followed by calcium chloride before use. - 90 -

C. Preparation

The preparation of the starting materials follows that of the intermediates from which they were derived and arranged in the order in which the results are discussed.

1)0,0-dialkyl dithiophosphoric acids and their Potassium salts : Prepared according to the method of Mastin et a1 89 . Typically, I mole of an appropriate alcohol was gradually added to 0.25 mole phosphorus pentasuiphide in 0.51 toluene and the reaction mixture boiled under ref lux with constant mechanical stirring at a rate sufficient to maintain a gentle ref lux over a period of

3-5 hours. H2S was liberated and led into saturated FeC1 3 or NaOH aqueous. A viscous layer was deposited at the bottom of the flask. The upper brown solution consisting mainly 90 of the desired 0,0-dialkyl dithiophosphoric acid Ca 85% was decanted and stripped off the solvent. It was neither necessary nor desirable to purify these acids by distillation except in one case which was also the subject of hydrolytic studies i.e.

0,0-diethyl dithèphosphoric acid, b.pt. 800 C/0.5mm, lit.91, 65° C/0.75; 1 H (CDC1 3 ), 1.33(dt, 611), 3.37(S. 111), 4.17(dq, 4H). Its purity was further confirmed by

a single sharp signal in its 31 P n.m.r. spectrum, 8,

84.6ppm (neat), lit.,75 84.5ppm. - 91 -

The crude acids were neutralized with saturated aqueous KHCO 3 till PH7 to form their potassium

salts after extraction of the by-products with ether.

It is advisable to use saturated aqueous KHCO 3 because

it becomes difficult to remove the water after neutralisation. The potassium salts were stripped off

water and recrystallized (hot filtration) from acetone/ether. The potassium diethyl dithiophosphate

melted at 194-95 0 C, lit.92, 152-53 0C (ether/ethanol);

requires (C 4 H 1Q O2 PS2 K),C, 21.43; H, 4.46%; observed; C,

20.8%; H, 4.52%; 31 PS 110.6ppm (H 20), lit.75, 110.5ppm.

Potassium 0,0-diisopropyl dithiophosphate melted at

203 0C,lit. 93 , 193 °C; 31 PS 107.1ppm, lit. 75 107.4ppm.

Potassium O,O-diisobutyl dithiophosphate melted at

188 0C, lit. 93 , 189-90 0C.

2)Heutral Hetal 0,0-dialkyl dithiophosphates : These

were prepared according. to the method of Wystrach et

a164 . Thus, potassium 0,0-dialkyl dithiophosphates were

treated with the required metal sulphate in 2:1 ratio to

give a suspension of the desired metal dithiophosphate

in essentially metathesis reactions, the whole operation

being carried out in a separating funnel with vigorous

shaking. The MDTPs were extracted out with ether and

recrystallized from n-heptane and shown to be pure by a

single sharp peak in 31 Pn.m.r.spectra. Their m.ps,

elemental analysis, and n.m.r. data are given in Tables - 92 -

3 and 4. Their structures were also confirmed by mass spectrometry.

Table 3: H.P's and element analysis for metal

dith iophosphates

Metal M.P.( °C) Elemental Analysis

Dithiophosphate Observed Reported Calculated Observed

C% H% C% H%

7794 22.40 4.59 22.25 4.87 C 8H 200 4P 2S 4Zn(2a) 75-76 95 C 12 }128 04 P2 S4Zn(2b) 144-145 144.5 22.29 5.66 29.27 5.97

C 16 H36 O4 P2 S4Zn(2c) 110-111 112 43 35.00 6.58 34.66 6.36

C 8 H20 O4 P2 S4 Ni (17) 105-106 105 96 22.39 4.49 22.31 4.49

C8 H20 04 P2 S4Cd (18) 144-145 144 97 19.90 4.10 20.50 3.94 - 93 -

Table 4 : NMR Data for Metal Dithiophosphates

MDTP 'H Shifts 13 C Shifts 31 P Shift s*

(CDC1 3 ) (CDC1 3 ) Observed Reported

1.375(dd,3H) 15.66(d,CH 3 ) 101.5 --

4.23(dq,2H) 64.47(d,CH2 ) 96.5** --

1.38(d,6H) 23.44(d,CH3 ) 98.5 94•765**

4.83(dh,1H) 74.13(d,CH) 92.9**

0.885(d,6H) 18.72(S,CH3 ) 99.6 100.6 43**

1.887(m,1H) 28.68(d,CH)

3.86(dt.,2H) 74.25(CH 2 )

(17) 1.396 (t,3H), -- Broad --

4.292 (m,2H)

(18a)1.240(d,3H), -- 107.7

4.090(m2H) 104.2**

* relative to 85% H 3PO 4 using C 6D 6 as an external lock

and in DME.

** in CDC13 - 94 -

Dialkyl Phosphites Diethyl phosphite, 31 Ps 6.4ppm (DME) lit. 5 , 6.2ppm (CDC1 3 )obtained from Aldrich. The isopropyl and isobutyl analogues prepared from the appropriate dry alcohol and PC1 3 (freshly distilled) as described in literature98 These were as such shown to be pure by their single sharp 31 P n.m.r. signals at 6.5ppm (DME), lit. 75 ' 6.5ppm (CDC1 3 ) and 7.5 ppm (DNE), lit. 75 , 7.5ppm (CDC1 3 ), respectively.

0,0-Dialkyl rnonothiophosphoric acids : Prepared by the acidification of N,N,N, triethyl ammmonium 0,0- dialkyl phosphorothioate obtained by refluxing the appropriate dialkyl phosphite, sulphur, and triethylamine

in ether 99 The N,N,N, triethyl ammonium 0,0-dialkyl phosphorothioates were obtained in almost quantitative yields as shown by 31 n.m.r. spectroscopy. Thus, monitoring of the above reaction in case of diisobutyl phosphite showed a single major peak after three hours at S58ppm (ether) due to N,N,N, triethyl ammonium

0,0-diisobutyl phosphorothioate, £(BuiO)2 P(S)0]NHEt 3. Keeping in view its lower field shift, this compound can be assigned to the thiono form in preference to the thiolo form. Thus were obtained

0,0-diethyl monothiophosphoric acid, 31 PS 64.lpprn

(DuE), 62.Oppm (CDC1 3 ),. lit. 100, 58. lppm(neat); sodium salt, 54.9ppm (H20), 56.86ppm(CDC13), lit 75 , 57 & 56ppm. 0,0-diisopropyl monothiophosphoric acid, PS 62.4 - 95 -

ppm (DME), lit., unreported; 1 H(DME), 1. 14(d, 12H), 4.65

(m,2H), 7.21(S,1H). (iii) O,O-diisobutyl monothiophosphoric acid, 31P 965.1 ppm(DME), lit., unreported; 'H(neat), 0.789(d, 1211), 1.75 (m,2H), 3.68(dt,4H), 7.15(S,1H).

S. Diethyl phosphoric acid Obtained by the controlled alkaline hydrolysis of triethyl phosphate according to the method of Bentley' 01 for the hydrolysis of tribenzyl phosphate. Thus to (1.82g, 0.01 mole) triethyl phosphate was added (0.40g, 0.01 mole) sodium hydroxide in 5cc. alcohol and the mixture refluxed for 2h.

Monitoring of the raction by 31 P n.mr. spectroscopy showed that the reaction was almost complete over this period, the peak at (--1.9)ppm due to triethyl phosphate disappeared, while a new peak at (-0. 1)ppm made its appearance. The sodium salt of the diethyl phosphoric acid obtained after stripping off the alcohol was very hygroscopic. Acidification of the sodium salt followed by extraction into ether yielded the title compound, 31 P -1.5ppm (DME), lit. 75 ' -1.3ppm(CDC1 3 ). The acid obtained as such was shown to be identical with that obtained from Kodak Chemical Labs., by means of 31P n.m.r. spectroscopy.

6. Hono-alkyl phosphoric acids Monoethyl phosphoric acid 31p5 0.3ppm, (DME) obtained from K & K Labs. The - 96

acid and monoisopropyl phosphoric acid were prepared in almost quantitative yields according to the method of

Kirby 102 , by the phosphorylation of the appropriate dry alcohol, crystalline phosphorus acid (Aldrich, stored under nitrogen), in the presence of solid iodine and triethylamine. The monoethyl phosphoric acid, thus prepared was shown by means of 31 P n.m.r. spectroscopy to be identical with that obtained from K & K Labs. The monoisopropyl phosphoric acid absorbed at 90.2ppm (DME), lit. unreported and was identified as its anilinium

102, salt, m.p. 158-159 0C, lit. 159-1600C.

Tetraethyl Dithionopyrophosphate Prepared according to the method of Toy 103 by the action of pyridine on a mixture of water and diethyl -thionochiorophosphate

(Aldrich, 31P567.7ppm, neat, lit. 67.7ppm, 68. lppm)

in the presence of sodium carbonate. B.P. 104 0 /1.5mm,

lit. 103 1050 /1.5mm, 31Pg 53.Sppm (DME).

S,S,S-triethyl trithiophosphate Prepared from

freshly distilled phosphoryl chloride ( 31 PS, 3.3ppm,

neat) and ethanethiol (Aldrich) in the presence of triethylamine according to the method of Thain 84 . B.P.

108° C/2.5mm, lit.86, 89 0/0.2mm; correct 6 ' i.r.(VP=0

1203cm, VP-s-C 558cm); 31 P59.6ppm(ether), 61.1ppm

(DME), lit.,84 61.3, 61.4ppm; 1 H(CDC1 3 ), (t,9H), (m,6H).

- 97 -

S,S-diethyl dithiophosphoric acid Prepared by the

alkaline hydrolysis of S,S,S-trithiophosphate according

to the method of Thain 84 and identified as

cyclohexylainine salt,m.p.130 0 (ether/acetone),

(C 10 H24 02 NPS2 ) requires C,42. 1%; H,8.42%; N,4.9%; found,

C,41.3%; H. 8.45%; N, 5.10%). While the sodium salt

absorbed at 38.6ppm (60% aqueous dioxan), the cyclohexy

-lammonium salt at 42.4ppm (H 20), the acid itself

absorbed at 46.8ppm(DME), lit., unreported.

O,S-diethyl dithiolophosphoric acid Generated

in situ by ref luxing (CaC1 2 drying tube) excess Eti

with 0,0-diethyl dithiophosphoric acid in the ratio

of (0.005 mole : 0.001 mole) in/dry DME86 . No effort

was made either to isolate this compound or the final

product identified as 0,S,S-trlethyl dithiophosphate

after ca 30h since it was neither necessary nor

desirable to do so.

O,O,S-triethyl dithiophosphate A twenty-six

years old ex-A.J.Burn sample kindly provided by Ian

Gosney, was shown very stable; 31 P94.7ppm (DtIE),

lit. 90 1 94.1ppm (neat); 1 H(C 6D 6 ), 1.032(dt, 911), 2.68

(dq, 211, CH 2S), 4.0(dq, 411).

Basic zinc 0.0-dialkyl dithiophosphates Prepared

according to the method of Brunton et a1 28 by the oxidation of the neutral zinc 0,0-dialkyl dithiophosphates with t-butyl hydroperoxides in 70% yield. Thus were prepared the ethyl derivative, m.p.

1950 -1960 C, lit.,1950 -1980 C; (C24 H60 0 13 P6 S 12 .Zn 4 )C, requires 20.8%, H,4.3%; observed, C, 20.7%; H, 5.45%;

1 P' 102.5ppm (CDC1 3 ), unreported. The isopropyl analogue, melted at 2040 -2050 C, lit. ,204° -206° C; requires (C 36 H84 O 13 P6 S 12 Zn 4 ) C, 27.7%; H, 5.4%; observed

C, 27.4% H, 5.45%; 31 P98.7ppm (DME), lit., unreported.

Both of these derivatives were shown by 31 n.m.r.

spectroscopy to be identical with those obtained by the method of Wystrach et a1.64 D. Hydrolyses

1. Hydrolysis of neutral zinc and related metal

0,0-dialkyl dithiophosphates In all hydrolysis

experiments, (0.005 mole) metal dithiophosphate was treated with (0.05 mole) water contained in 15cc of the solvent, THF or DME, using the apparatus shown in (Fig. 3). In all the cases, a precipitate started forming in

10-15 min and became bulky with time. The amount of the precmptizate and the residual liquid obtained after evaporating the solvent over the rotary evaporator at the end of the hydrolysis (DME) in each case is shown in

Table 2? (P.5). All the precipitates were washed with 5cc each of ether, methylene chloride, benzene, and finally ether and dried under aspirator till powered before being weighed. Their elemental analysis are

given in Table 2, I'51.

For quantitative rate data experiments carried out on zinc 0,0-diethyl dithiophosphate (2a) and Zinc 0,0-diisopropyl dithiophosphate (2b), an internal standard, triphenyl phosphate or triphenyl

phosphine (TPP) was used in equiv. amount. This was considered. essential for a covenient determination of

(a 0-x) values where a 0 is the initial concentration of the ZDTP and x is the amount which underwent hydrolysis.

The apparatus used is shown in Fig.36. MUSIOM

0.7262g (0.00166 mole) of zinc 0,0- diethyl dithiophosphate (2a) was reacted with ten equiv.(0.3g) water in 5cc - DME. In order to make the solutions of, required strength (0.333H), 0.6g 1120 and

(0.5438g, 0.0016m) triphenyl phosphate ( 31 Pz-17.2ppm) were added to 10cc - DME in a volumetric flask. This solution was then added into a 5cc- volumetric flask containing the required amount of the ZDTP. Values. of

(a 0-x)(moles 1itre 1 determined at the appropriate time lengths (mm) are as follows : -

t t o t 20 t 30 t 45 t 50 t 75 t go

(a 0-x) 0.333 0.295 0.268 0.197 0.103 0.056 0.0119

(-)ln(a0 -x) 1.099 1.219 1.314 1.619 2.2702.874 4.4300

For the hydrolysis of the zinc 0,0-diisopropyl dithio- phosphate (2b), (0.8198g, 0.0016 mole) were treated with ten equiv.(0.3g) water in 5cc; DME. (a 0 -x) (moles litre) values obtained using triphenyl phosphine

-6.Oppm, (DME)J at particular times (mm) are as follows

t t t t 40 t5 1 60 o 20 t30

(a 0-x) 0.333 0.293 0.264 0.163 0.066 0.025 0.011

-ln(a0 -x) 1.099 1.226 1.331 1.830 2.703 3.684 4.454 - 101 -

In other experiments involving the use of radical inhibitors (2,4,6 tri-tert. butyl phenol and galvinoxyl) in the hydrolysis of the two ZDTPS (2a and

2b), the amount of the inhibitor used was 5% by weight of that of the ZDTP. This produced no changes in the rates of their hydrolysis.

Hydrolysis of 0,0-dialkyl monothiophosphoric acids :

(0.005 mole) inonothiophosphoric acids (ethyl, isopropyl, and iso-butyl) reacted with (0.05 mole) water in 15cc

DME and at 850 C. 31 P n.m.r. spectroscopy showed that the hydrolyses did not take place over a period of ca 6h.

The hydrolysis of isobutyl monothiophosphoric acid was also found not to occur even in presence of phosphoric acid (2% by weight of that of the thiophosphoric acid).

Hydrolysis of 0,0-diethyl dithiophosphoric acid

Preliminary experiments included the hydrolysis of Ca

(0.0001 mole) ofthe title compound in 0.3cc DNE in n.m.r. tubes using apparatus shown in (Fig. 2). 31 n.m.r.spectroscopy showed that the compound did hydrolyse to give four major products after Ca 3.5h of heating.(Figure 39), identical with those obtained from hydrolysis of diethyl ZDTP (2a) (Fig.6).

Subsequent experiments included the hydrolysis on a preparative scale i.e. Ca (0.930g, 0.005 - 102 -

mole) with ten equiv. water (0.908, 0.05 mole) in 15cc

DME using the apparatus shown in (Fig. 36). The kinetics of the hydrolysis were determined by withdrawing aliquots from the reaction solution and analysing them by 31 P n.m.r. spectrozcopy. Hydrogen sulphide was observed to evolve during the course of the reaction as shown by lead acetate paper test. No solid was obtained and the amount of the residue obtained after ca 4h of heating was 0.441g.

In other experiments, 0.31g of

0,0-diethyl dithiophosphoric acid (2a) was hydrolysed with 0.3g water in 5cc DME at 85 0C. Aliquots were taken out after about five, seven and ten min and immediately run for 31 P n.m.r. analysis which showed that the conversion of the dithiophosphoric acid into the major intermediate was incomplete. However, an aliquot withdrawn after ca 12 min showed that the conversion was complete (Fig. 39). The reaction was frozen immediately and ca 2gms MgSO 4 added into it.

To the above solution dried over ca 2h was added excess of aniline (Fisons) till p117. The precipitate was washed with dry DME(20cc ) and washings collected. An aliquot was withdrawn from the filtrate and the precipitate was kept drying under aspirator till powdered. The aliquot was immediately run for - 103 - n.m.r. analysis which showed two major peaks (59.5ppm and 57.9ppm). The precipitate when dry weighed 0.269g yield = 48.6% based on 0,0-diethyl dithiophosphoric acid, m.p. 137-1390 C, insoluble in commonly available solvents except in water in which it decomposed giving off hydrogen suiphide. It could not be recrystallised and rapidly decomposed giving off hydrogen suiphide even at room temperature, probably by reacting with the atmospheric moisture. No definite conclusions could be drawn from its infra red spectrum. It absorbed at 44.Oppm (water) in its 31 P n.m.r. spectrum. When the aliquot of the filtrate was returned to the filtate solution, the latter was found to contain a precipitate.

The solution was concentrated over rotary evaporator at room temperature and the amount of precipitate appeared to increase. The precipitate was filtered and washed with DME (5cc-) and washings collected. The precipitate was dried under aspirator till powdery and weighed 0.037g. This precipitate also underwent rapid decomposition emitting H2S at room temperature and could not be recrystallized. It was found soluble in water only and shown to be different from the first crop of the precipitate upon the basis of its different shift (16.Oppm). The filtrate also gave off H 2 S as shown by lead acetate paper test. It contained only one major peek at 58.1 ppm. The second peck at 59.8ppm had considerably diminished. The P-containing products - 104 -

decomposed into unidentifiable products when efforts were made to get rid of the solvent over the rotary evaporator.

Reaction between 0,0-diethyl dithiophosphoric acid and ethyl iodide See the preparation of 0,S-diethyl

cLithiolophosphoric acid.

Hydrolysis of Basic zinc O,O-diisopropyl dithio- phosphate (3b) (7.785g. 0.005 mole) of (3b) were hydrolyzed with ten equiv, (0.90g, 0.05 mole) or twenty equiv.(1.8g, 0.1m) water in 15cc DME at 85 0 , employing the apparatus shown in (Fig.3). Aliquots were withdrawn from the reaction tubes at various intervals for analysis by means of 31 P n.m.r. spectroscopy. The amount of the precipitate and the soluble products obtained at the end (ca 77h) of hydrolysis in the latter case are

2.406g and 4.806g, respectively.

Reaction between neutral zinc O,O-diisopropyl dithiophosphate(3b) and zinc oxide: (0.7376g,0.001426 mole) of the neutral salt was reacted with (0.0406g,

0.000498 mole) zinc oxide and twenty equiv. (0.180g) water in 1.5cc . The reaction was studied at 85 0C and monitored by 31 n.m.r. spectroscopy. In other experiments the same reaction was carried out at ambient temperatures but in absence of the water and the - 105 -

reaction was again monitored by 31 n.m.r. spectroscopy.

Since these experiments were designed from a qualitative view point, no quantitdive determinations were necessary. R E F E R E NC E - 106 -

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103.A.D.F. Toy, J. Am. Chein. Soc., 1951, 73, 4670. Page 1 The following should be read as second paragraph:

The title compounds (MDTPs) are derived from 0,0- dialkyl

dithiophosphoric acids (Ro)PSi which since World War II,

have been widely used not only as pesticides, lubricant

additives but also as war gaseS

Almost all of these toxic acids form overpowering vapours

and it is advised that extra care should be taken while

handling these compounds. It is my personal experience

that these compounds are very toxic. Almost all of these

compounds gave me severe headaches on a number of occasions.

Of these,dimethyl dithiq1ic acid caused such severe

headaches that I gave up studying either this compound or its

derivative.

These acids have properties similar to those of nerve gases,

and inhibit the action of several ester-splitting enzymes in

living organisms. They are singularly effective against

cholinesterase which hydrolyzes the acetylcholine generated

in myoneural junctions during the transmission of motor

commands. In the absence of effective cholinesterase,

acetylcholine accumulates and interferes with the coordination

of muscle response. Such interference in the muscles of the

vital organs produces serious symptoms and eventually death.

Further, extra care should be taken to avoid contact with skin

while handling these compounds. It is my experience that

once these compounds touch the skin it is very very difficult

to remove them from the skin even by washing a hundred times

with soap!