BULLETIN DE L'ACADÉMIE POLONAISE DES SCIENCES Série des sciences chimiques Volume XVIII, No. 7, 1970

ORGANIC CHEMISTRY

Esters of Nitric Acid as Electron Acceptors

by

B. HETNARSKI, W. POŁUDNIKIEWICZ, and T. URBAŃSKI

Presented by T. URBAŃSKI on March 2, 1970

It is well known that C-nitro compounds are strong electron acceptors forming readily charge transfer complexes. Nothing so far has been known as regards possible electron accepting properties of Onitro compounds. However, in a series of papers one of us [1—5] and his co-worker [6] have found using the thermal analysis method that nitric esters such as D-mannitol hexanitrate and erythritol tetranitrate can form additive compounds with some aromatic nitro compounds. The problem arose what is the nature of such additive compounds: whether they are charge transfer complexes, and which of the components is the electron acceptor. A suggestion was advanced that nitric esters can act as electron acceptors and this was substantiated by our present work. We examined now an interaction between nitric esters of mono-, di-, tri-, tetra- and hexahydroxylic alcohols and a model electron donor — tetramethyl-p-phenyl- enediamine (TMPD). The choice of the latter was justified by its low ionisation potential (6.5 eV). The preliminary report has already been published [7]. We found that the solutions of nitric esters, when added to a solution of TMPD, produce an intense violet colour. This colour is due to two absorption bands: at 570 and 620 nm (Fig. 1). Their source is the TMPD cation ("Wurster radical") (I) [8] formed from TMPD through the loss of one electron.

(I) [385] 386 В. Hetnarski et-al.

620 nm

tOO 500 600 Я (Vim) Fig. 1. Electronic spectrum of a solution Fig. 2. Job's diagram in 1,2-dichloroethane; of glycerol trinitrate (NG) and tetramethyl- NG+TMPD, Erythritol -p-phenylenediamine (TMPD) in 1,2-dichloro- tetranitrate+TMPD; /еп-Butyl nitra- ethyne at the concentration of 0.018 M of te+TMPD. Concentration of each com­ each component ponent, 0.01 M, thickness, 1 cm. c-conccn- tration of TMPD in mole fractions

Using Job's method of continuous changes [10] (Fig. 2) we established that three ON02 groups are needed to form one cation (I). Thus, three moles of each of the nitrates of primary, secondary, and tertiary butyl produced (I) according to (1):

N(CH3)2 I N(CH3).

3RON0 + (1) 2 (3 DONQ,fK © )

N(CN3)2

(II)

Here R denotes primary, secondary, and tertiary butyl, respectively. Table I gives values of the absorption of TMPD cation which are characterizing the relative electron affinity of the butyl nitrates.

TABLE I

Electron acceptor, Absorption butyl nitrate at 620 nm *

Primary 0.100 Secondary 0.158 Tertiary 0.530

•Solutions in 1,2-dichloroethane at con­ centration of 0.1 M. The path length was 1 cm.

It follows from the above data that the primary ON02 group creates the weakest electron acceptor properties of butyl nitrates, and the strongest were shown by the Esters of Nitric Acid as Electron Acceptors 387

tertiary 0N02 group. This is most likely the result of the strongest electron delo-

calization when ON02 is attached to the branched aliphatic group. Ethylene glycol dinitrate reacted with TMPD at the same ratio of components: three moles of the dinitrate with one mole of TMPD. Glycerol trinitrate (nitroglycerine, NG), reacted with TMPD at the molar ratio

1:1, i.e. three ON02 groups were needed to form (I). Its absorption produced by the cation TMPD at 620 nm is 0.272 (concentration of the components was 0.018 mol/1., the layer thickness, 1cm). Pentaerythritol trinitrate shows the same electron afinity as NG, the molar ratio being 1:1. Two tetranitrates were also examined: erythritol tetranitrate and pentaerythritol tetranitrate (PETN). Both react with TMPD and the molar ratio of tetranitrate: TMPD was found to be 1:1. Erythritol tetranitrate is more reactive, i.e. has stronger electron acceptor properties (Table П).

TABLE II

Electron acceptor Absorption Concentration, at 620 nm * M

PETN 0.100 0.02 Erythritol tetranitrate 0.540 0.01

* The path length was 1 cm.

It seems that the higher electron acceptor properties of erythritol tetranitrate to form complexes can partly be explained by a more polar structure of that ester, as suggested in one of the former papers [11].

TABLE III

Electron acceptor Absorption at 620 nm *

D-Mannitol hexanitrate 0.499 Dulcitol hexanitrate 0.455 D-Sorbitol hexanitrate 0.468 игио-Inositol hexanitrate 0.530

* Concentration of components was 2xlO-3M, path length, 0.02 cm.

Nitric esters of a number of hexahydroxylic alcohols were also examined: viz. hexanitrates of D-mannitol, dulcitol (D- or L-galactitol), D-sorbitol and mj>o-inositol. Using the same method of continuous changes we found that one mole of hexanitrates reacted with two moles TMPD, i.e. the rule of three ON02 groups for one mole 388 В. Hetnarski et al.

of TMPD is fulfilled. The electron affinity of the hexanitrates is much stronger

than that of nitric esters with a lower number of ON02 groups (Table III). The reaction of hexanitrates with TMPD was manifested not only by the for­ mation of the Wurster radical. The latter was subjected to disproportionation yielding a divalent cation of tetramethyl-/>benzoquinone-diimonium which in turn formed a charge transfer complex. This will be the object of our next paper [12].

Conclusions On the basis of our experiments we can formulate a general rule for the relation between the number of O-nitro groups in nitric esters of alcohols, the number of moles of the ester and number of moles of TMPD, according to the schematic equation:

J a 3 xR(ON02)„+/rMTP^ [xR(ON02),,] ' y[TMPD] where R are hydrocarbon groups. Table IV gives the values of x and y as the function of n.

TABLE IV

Values of

11 X У

1 or 2 3 1

3 or 4 1 1

6 1 2

The general rule reads: at least three O-nitro groups should be present to form the Wurster radical from tetramethyl-p-phenylenediamine (TMPD). It should also be pointed out that inorganic nitrates such as potassium, sodium and aluminium nitrates do not react with TMPD. The observed electron affinity of nitrates is limited to nitric esters only and should be ascribed to délocalisation

of electrons along the bonds С—O—N02.

Experimental

1,2-Dichloroethane (as solvent) was purified according to the literature [13J. Tctramethyl-p-phenylcncdiamine was prepared according to the literature [14] and purified by double distillation in the atmosphere of nitrogen (m.p. 51°). л-Butyl and sec-butyl nitrates. The corresponding butyl alcohol was carefully added

at —10° under vigorous stirring to an excess of nitrating mixture of HN03 (dl.5) and H2S04 Ul 1.84) (1:1 wt./wt.). Subsequently all was poured into water and ice, extracted with ether, washed with aq. sodium carbonate, dried over sodium sulphate and the product was distilled under re­ duced pressure (lOmmHg). /err-Butyl nitrate was prepared by mixing cooled tor-butyl chloride in dry ether with silver nitrate in the molar ratio 1:3. All was left in a refrigerator for two days, the solid phase was. Esters of Nitric Acid as Electron Acceptors 389 filtered off, ether solution was washed first with water, then with 5% solutions of sodium carbo­ nate and sodium hydrogen sulphite, and dried over sodium sulphate. The product was distilled under reduced pressure (5 mm Hg). Ethylene glycol dinitrate and glycerol trinitrate were prepared by standard methods [15] and purified by distillation under reduced pressure (20 mm Hg) and by freezing crystallization [16], respectively. Pentaerythritol trinitrate was prepared and purified according to the literature [17]. Erythritol and pentaerythritol tetranitrates were prepared according to the litera­ ture [17, 14]. The products were purified by crystallization from EtOH.

TABLE V

20 Substance I'D m.p. b.p.

«-Butyl trinitrate 1.4063 25°/8 mm sec-Butyl trinitrate 1.4025 - 22°/8 mm tert-Butyl nitrate 1.4020 - 22°/5 mm Ethylene glycol dinitrate 1.4475 - 105720 mm Glycol trinitrate 1.4732 - — Pentaerythritol trinitrate 1.4936 - - Erythritol tetranitrate 61° — Pentaerythritol tetranitrate - 141° - D-Mannitol hexanitrate — 112° — Dulcitol hexanitrate - 94- 94.5° — D-Sorbitol hexanitrate — 54- 54.5° — ш.г-0-Inositol hexanitrate — 132-132.5° —

Hexanitrates of D-mannitol, dulcitol, D-sorbitol, />i.yo-inositol were prepared according to the literature [15, 19]. The properties of the substances are collected in Table V. The electronic absorption spectra were taken on a Unicam SP 500 spectrophotometer.

The authors are indebted to Dr R. Kuboszek and Mr S. Krzemiński, M. Sc., for the prepa­ ration of ethylene glycol dinitrate, glycerol and pentaerythritol trinitrates.

INSTITUTE OF ORGANIC CHEMISTRY, POLISH ACADEMY OF SCIENCES, WARSAW, KASPRZA­ KA 48/52 (INSTYTUT CHEMII ORGANICZNEJ PAN, WARSZAWA)

DEPARTMENT OF CHEMISTRY, TECHNICAL UNIVERSITY, WARSAW, KOSZYKOWA 75 (KATEDRA TECHNOLOGII ORGANICZNEJ II POLITECHNIKI, WARSZAWA)

REFERENCES

[I] T. Urbański, Roczniki Chem., 13 (1933), 399. [2] , ibid., 14 (1934), 925. [31 , ibid., 15 (1935), 191. [4] , ibid., 16 (1936), 359. [5] , ibid., 17 (1937), 474. [6] M. Witanowski, ibid., 39 (1965), 635. 17] B. Hetnarski, W. Południkiewicz, T. Urbański, Tetrahedron Lett., 1970, 3. 390 В. Hetnarski et al.

[8] G. Briegleb, Elektionen-Donator-Acceptor-Komplexe, Springer, Berlin, 1961. [9] A. C. Albrecht, W. T. Simpson, J. Am. Chem. Soc, 77 (1955), 4454. [10] P. Job, Compt. Rend., 180 (1925), 928; Ann. Chim. Phys., (10) 9 (1928), 113. [11] T. Urbański, Roczniki Chem.. 25 (1961), 257. [12] T. Urbański, В. Hetnarski, W. Potudnikiewicz, Bull. Acad. Polon. Sci.. Sér. Sci. Chim., [sec the following paper in this issue]. [13] J. C. D. Band, Snedder, Trans. Faraday Soc, 53 (1957), 894. [14] J. N. Ashley, W. G. Leeds, J. Chem. Soc, (1957), 2706. [15] T. Urbański, Chemistry and technology of explosives, Vol. II, Pergamon Press, Oxford — PWN, Warszawa, 1965. [16] J. Hackel, Roczniki Chem., 16 (1936), 213. [17] N. S. Marans, D. E. Elrick, and R. T. Preckel, J. Am. Chem. Soc, 76 (1954), 1304. [18] A. Saskisyants, Med. Prom. SSSR, 14 (1960), 117. [19] T. Urbański, M. Witanowski, Trans. Farad. Soc, 59 (1963), 1039.