The kinetics of the thermal decomposition of benzaldehyde and benzyl benzoate

Item Type text; Thesis-Reproduction (electronic)

Authors Reynolds, Dexter Harold, 1902-

Publisher The University of Arizona.

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Download date 23/09/2021 21:27:34

Link to Item http://hdl.handle.net/10150/553204 The Kinetics of the Thermal Decomposition

of Benzaldehyde and Benzyl Benzoate.

by

Dexter H# Reynolds

Submitted in partial fulfillment of the

requirements for the degree of

Master of Science in the College of Letters, Arts, and Sciences, of the

University of Arizona

1933

Approved: cfc £ Major adviser f ) * *

sS in

> t •;r> t ^ o: > >• /

7

s t A £ 9 7?/ /933 & o

ACKN OT/LEDGEMKITT

The writer wishes to express his most sincere

gratitude for the generous advice and assistance

of Dr. Lathrop Emerson Roberts, under whose

direction this investigation was made.

98620 TABLE OF CONTENTS*

Introduction ------Page 1 Review of literature - ~ ------■ « 5

Preliminary work on the oxides of nitrogen - - - « 7 The apparatus and its use - 9-go

a . She thermostat — * 9

b. The thermo-regulator o- - - - - r > - — - ” 10 c. The thermo-couple ------* 11

d. The apparatus for following the ; pressure changes acoomp&ayiag reaction 12

Analysis of the product® of reaction • - - - - - « 21

Preparation of materials ~ ------» 22-23 a. Benaaldehyde ------« 22

b * Benzyl benzoate.'*1' -■* — -;-. ” 23 ■, . ■ .. ' ' ’ I',:/-' - ' ' . Experiaontal data - - - — ------,----- « 24-34 ■ • ' . y : - ' ■ ' ' : . .. . ' ' a* Benzaldehyde — - — — — — — —' — ;B4:-

b • Benzyl benzoate — — — — — — — — — —w . 28;,

6• Benzoic acid — — — — — — — — * , 33- -■ ■ d « Bensophenone — - - — — — — — — —. — ■— — —. —. H 34

Interpretation of results ------> - < 3 5 - 5 5

a • Benzaldehyde — — — ** — — — — — — — — — — .. ^ ■ ■ 35 b. Benzyl benzoate ------n 43

o. Miscellaneous ------« 54

Summary - - • » 56 Bibliography « 58 The Kinetiei of the Thermal Decomposition

of Benzaldehyde and Benzyl Benzoate.

Introduction.

One of the fundamental problems of theoretical chem­ istry is the determination of the exact mechanism of chem­

ical change. The study of the kinetics of a chemical reaction is of prime importance in the study of the reaction mechanism,'and li the study of the reasons for the occurence of a reaction. Thermodynamics gives information as to the energy changes involved in a chemical action, and may show whether a reaction may reasonably be expected to occur or not, but it cannot tell whether a reaction will really

take place, or whether it will take place appreciably in finite time. In ease the reaction really does take place,

thermodynamics can only take account of the conditions of

the Initial and final states, and leaves entirely out of

consideration the question of intermediate reactions and

mechanism of the change. These latter may best be found

out by an interpretation of the results of kinetic studies.

Reaction rates may he studied either in solution or

in the gaseous phase. The literature cites many examples

of kinetic studies in both liquids and gases. Inter­

pretation of the results in the liquid state are extremely 2 difficult becaxuw of lack of knowledge of exact molecular conditions in liquids. The kinetic theory of gases gives rather exact information concerning the gaseous state which may he used in interpreting the results of kinetic studies of gaseous reactions. Often, reactions in the gaseous phase may he followed conveniently hy measuring changes in pres­ sure which take place as the reaction proceeds. These facts tend to make the study of gaseous reactions and the inter­ pretation of results much easier than is the case with reactions in solution.

In the study of a gas reaction it is desirable to determine the influence of a change of initial pressure of the reacting substance, the effect of temperature changes, and of the influence of the surface of the reaction vessel upon the course of the reaction. It is of importance to find out whether the reaction really occurs in the gaseous phase or whether it takes place on the walls of the con­ taining vessel, i.e., whether the reaction is homogeneous or heterogeneous. The order of the reaction is of consider­ able Importance in the interpretation of results.

Unlmoleeular reactions, in particular, are of great

interest theoretically in connection with activation theories.

Only a few homogeneous gas reactions have been found and studied in which the rate of reaction is proportional to the first power of the partial pressure of the reacting substance. Many studies of the thermal decomposition of vapors have given rise to the idea that as a molecule increases in complexity, the greater is the possibility that it will decompose in accordance with the unimolecular law. For example, it is found that the thermal decompos­ ition of acetaldehyde1 is himolecular, while the decom­ position of the next higher member of the series, prop ion- aldehyde? is unimolecular.

The studies mentioned above have all been made with the socalled permanent gases, or with vapors of low boiling liquids. Obviously it is desirable to extend this study to tapers of liquids with molecules of greater complexity. These liquids are in general high boiling. This brings up . several problems in apparatus design which are not present , or are of minor Importance, in the case of the lower boiling liquids. The lack of a stopcock lubricant which will permit the vacuum tight operation of the stopcock at high temper­ atures prevents the use of the usual methods of introducing the vapors into the reaction chamber. Ramsperger3 has designed a stopcock for such operation, but one was not available for the work described in this paper.

Then, too, in the case of vapors of high boiling liquids, pressures may not be measured with an ordinary manometer. To prevent eendensation of thejvapors in the tubes nonnesting the reaction flask and the manometer, it is necessary that the connecting tube and the surface of the manometer liquid be heated to a temperature above the boiling point of the liquid being studied* If this temper­ ature Is in the neighborhood of 800®C., it would be difficult end certainly tepraetloal, to obtain a manometer liquid with a sufficiently low vapor pressure to enable the accurate measurement of pressure#! V . Always it is desirable to keep the vapors being

studied away from contact with metallic surfaces, and from contamination with stopcock lubricant. That these difficulties have been overcome in a satisfactory manner will be shown in the description of the apparatus used.

In the choice of a suitable liquid for study it was observed that the decomposition of the vapors of aoetonef acetaldehyde^ diethyl ether® and trichlormethyl chloro- formate, an ester, in the gaseous state are homogeneous reactions. It would be expected that a higher member of one of the homologous series mentioned would behave in a similar manner. Since the more complex substances appear to decompose unlmolecularly, it was thought that benzal- dehyde would probably decompose homogeneously and unl- molecularly. Benzaldehyde, being easily obtainable, was, therefore, shosen as the subject of the studies to be

described in this paper. The reaction proved to be of

great complexity, and, in the effort to trace the different possible reactions taking place, it was found that benzyl benzoate apparently decomposed according to the unlmolecular

law, homogeneously. The last reaction was, therefore, studied

in some detail. •5—

Review of the literature.

The products of the thermal decomposition of benzol- dehyde were studied by Mile. Peytral; by passing the vapors through a narrow platinum tube at 1150*0., and analyzing the products formed. The chief products of the decomposition were found to be benzene, carbon monoxide, hydrogen, and diphenyl, with small amounts of methane, carbon, and a substance which was thought to be anthracene.

In accordance with the analysis of the reaction products, the mechanism of the reaction was given as, V. • ; : .; :... ' ■ l V : . ' : (1) C5H5CHO m CgHg + CO

(2) 2 C6H5CH0 « (OgHgJg + H2 + 00.

These results have been practically duplicated by Hurd and

Bennetf using a pyrex tube at 650-670*0. They identified the substance called anthracene by Mile. Peytral as 9 p-diphenyl benzene. In a sealed tube at 350*0., Lachaann found that two molecules of benzaldehyde combine to form a molecule of benzyl benzoate, according to the reaction,

(3) 2 C6H5CHO « CgHs-CHg-O-CO-CgHg.

Both Hurd and Bonnet, and Laohmann reported that there is no decomposition of benzaldehyde in a sealed tube at 250*0., even after heating for an eighteen hour period. Hurd and

Bennet reported that no methane was formed at 690*0. in pyrex tubing. The gases formed had the following composition by volume:

00 - 86.7$, Hg _ 12.90, GO, - 0.320. 10 Marc da Henrotlnne studied the photoehealeal

decoiEpdiltlbn of benzaldehyde. He reported the produets of V - the resetion to be the same as in the thermal decomposition 11 Hurd and Bennett studied to some extent the deooepoe

it ion of benzyl benzoate. The sample of 42.5 gras, was

sealed in a tube and heated at 346*350°C. for two hours,

A high pressure was developed in the tube. Ho further mention was made of the gaseous products, butaaaamgtthe

liquid produet# formed are: toluene, benzaldehyde, benzole

acid, benzole anhydride, and a considerable amount of

tar and residues.

The work mentioned above will be considered in more

detail when the data obtained in the present study is

discussed* Preliminary work on the oxides of nitrogen.

Previous to the decision to study the thermal decom­

position of a vapor, an interesting situation was observed

among the oxides of nitrogen. In many cases nitrous ezlde

acts as a more vigorous oxidizing agent in combustions

than does elemental oxygen. An anomaly arises in that

while nitric oxide is readily oxidized at ordinary temper­

atures by oxygen to nitrogen dioxide according to the

equation*

B NO + 0* » 2 NO »,

there is no apparent reaction between nitrous oxide and

nitrie oxide♦

An attempt was made to determine qualitatively

whither the two oxides could be made to react with each

other at elevated temperatures. Nitrous oxide, prepared

by heating ammonium nitrate, was introduced over mercury . vv .X/ . ■ ■■■ ■ into a pyrex tube 15 mm. inside diameter and 50 cm. in

length, until half the mercury was displaced. The ■■■XX', ' remaining mereury was displaced with nitric oxide, gener- XX' XV ' ■ ' a^ed by the action of dilute nitric acid on metallic copper. Tbix; tube . was then sealed and heated in a combustion furnace for several hours at temperatures ranging from room tem- / perature to 500*0. No oolor was developed at any time

fwithlh the tube, showing that no nitrogen dioxide was f X X X ' - fdrmed^ % o n breaking the seal of the tube, the contents of the tube rapidly become brown. According to the best thermodynamic data available, the following reaction should have taken place:

NgO + NO « N a + NOa.

It was then decided, for reasons given above, to study the thezmal decomposition of the vapor of benzaldehyde*

An apparatus was constructed similar to that used by 4 Hinshelwood and Hutchison in the study of acetone vapor.

The apparatus was found to be unsuited for the desired study. The apparatus to be described was then developed# PM e la.

Plate. Ib. The Apparatus and its Use.

a. The Thermostat*

A An electrically operated thermostat was constructed as follows:

105 cm. (41 inches) of Chromel-A resistance wire, No. 18 helix, was stretched to a length of 1280 cm. 30 cm. were turned back at each end and twisted to form leads. A 15 cm. lead was alee made at the center of the length. The wire was then wound on a grooved alundum cylinder of about

15 cm. inside diameter and 25 cm. depth. The outside of ' ' - the cylinder and the wiring was covered to a depth of about

0.5 cm. with alundum cement and dried for three days in an electric oven at 150*0. The core and heating element thus prepared was placed in a sheet iron container and a

10 cm. thickness of infusorial earth tightly packed around

it. A tight fitting cover of transite was made leaving

small openings for the neck of the rmetIon flask, the

thermocouple,and the thermo-regulator.

T&e wiring diagram is shown in Plate I. By proper

manipulation of the double-pole double-throw switch C,

the incoming current may be made to pass from end to end

of the heating element, or the element may be made to

operate as two resistances in parallel. This, in conjunc­

tion with the rheostat B, enables very close control of

the rate of heating and cooling. B is a 9 ohm rheostat,

arranged in one ohm steps, which will safely carry currents

up to 25 amperes. In operation it was found that a current -10- of 2*5 amperes was sufficient to maintain the temperature of the furaaee at 400*0.

b. The Thermo-regulator •

The ihermo-regulator is shown aiagramatioally in

Plate lb.

A is a pyrex tube, 5 cm. inside diameter and 15 cm.

long* The tJ-tube C is made of 4 mm. tubing with a sealed-

in electrical contact at (a). In use the TJ-tube is filled with mercury. E is a battery for operating the relay F .

Four dry cells were found to give satisfactory eerriee, F is a 20 ohm pony/ relay. G'is *a magnetic circuit breaker. H is a sliding Contact hheostat, with a resistance of 9

ohms, and a capacity of 15 amperes.

In operation, bulb A is immersed in the thermostat.

Stopcock B is left open until the furnace is nearly up to

the desired temperature. Stopcock B is then closed. Fur­

ther heating of the furnace causes excess pressure to devel­

ops in A, which causes the mercury column to rise, closing

the relay circuit at D. This breaks the magnetic circuit

of the circuit breaker. This releases the armature of the

circuit breaker, which sends the current being delivered

to the furnace through the rheostat H. H, in conjunction

with B of Plate la, can be so adjusted that with the cir­

cuit breaker eloeed the furnace heats at a very slow rate,

and with the magnetic switch ©pen the furnace eools at a

correspondingly slow rate. In actual practice it was found

possible to regulate the temperature of the furnace so that -li­

lt would not vary more than 0.5*0 on either side of an average for days at a time. The exact temperature main­ tained inside the thermostat is controlled by the amount of air which is left in A, the electrical contacts being set accordingly*

c. The Thermo-eouple. ,

Temperatures inside the furnace were measured by means of a five juneflon Nichrome-Advanee thermo-couple. The junctions were electrically welded. The eeM junc­ tions were maintained at 0*0 by immersion in a Dewar flask which contained ice and water. The leads were insulated by small quartz tubing inside the furnace, and small rubber tubing outside. The potentials developed were measured by means of a Leeds and Rorthrup student type potentiometer*

The thermo-couple was calibrated against the Bureau of

Standards Platinum— platinum-rhodium thermo-couple No. 451.

The potentials developed By the standard thermo-couple were measured with a Leeds and Northrup,type K, potentio­ meter.

The two thermo-couples, the standard in the center of the group of the five base metal terminals, were placed in a pyrex test tube and immersed in a bath of Rose metal itiside^thd f&fnae#. The furnace was then allowed to heat at the slowest possible rate. The potentials developed by eaeh couple was noted at various times. Temperatures up to °C. ^ ™ i ! 550*0 were checked hy means of a mercury thermometer whose bulb was immersed in the Rose metal bath.

The results of the calibration are shown in Table I, and the calibration curve in Figure

Table I.

Potentials Temperatures. Hloh/Adr. Standard Calculated Hg them#

29.3 0.84 , 13810 34.5 0.97 150* 49.0 1.38 . — : . . 200° 64.6 1.86 J250° 2*0* 70.1 2.02 267? 268* 73.7 2.12 279* 280* 80.2 8.81 " 300* , 300* 96.7 2,78 351* 350* 104.0 3.03 ' 380* 113.1 3.28 405* 181 ;l 3.50 425®. 129.2 3.73 443® 138.5 4.01 476® 149.0 4.31 505® 155.0 4.50 525° 161.7 4.68 545* 171.8 4.97 573*

The measurement of the absolute temperature of the thermostat cannot be made closer than 5*0, by this thermo­ couple, since the Bureau of Standards No. 451 couple is certified no closer than 5*0 to the actual temperature.

It was found that a change of 0.1*0 was readily detected by means of the thermo-couple arrangement described.

d. The apparatus for following the pressure changes

accompanying reaction..

The details of the apparatus are shown diagramatioally

in Plate II. The apparatus is constructed entirely of pyrex glass. The tubing,with the exception of the mm* ometers and the seek of the reaction flask, is of 1 mm. capillary, 8 mm, outside diameter.

A is a mercury,manometer, made of 4 mm. tubing, and of sufficient length to measure pressures up to 1.5 atmospheres. The high arm was evacuated by means of a

Cenoo Hyvao pump over a six hour period. Absolute evaoum- tiodjeas not essential, since the readings of this manometer were calibrated against manometer J, which could be evacua­ ted.

The stopcock B was introduced for convenience in glass work upon the remainder of the apparatus. 0 is a three liter flask which would withstand evacuation to 10 mm. of mercury. It is used to reduce the pressure above the diaphragm gauge when desired without the necessity of using the vacuum pump. D is a buffer flask of about 500 ce, capacity, which allows the slow building up of pressures

In the system above the diaphragm gauge F. F is a dia­ phragm or "click" gauge. Its construction is described in detail by Smith and Taylor!2 0 is a finely drawn out

capillary, sealed at the tip. It is used to allow the

slow building up of pressures below the diaphragm after the reaction is complete. I is a capillary leak,made from

a 0.5 mm. capillary tube, finely drawn out. I allows the

slow entrance of air into the system above the diaphragm.

J Is a mercury manometer used in the calibration of the

click gauge and manometer A. The closed arm is evacuated &

,Td 5**mKf*f

• 1e

Plats by attaching to auction and tilting until the mercury column stands above the stopcock. The stopcock is then closed, and the manometer Returned to its normal position.

This procedure is repeated just previous to each cali­ bration of the click gauge.

The constriction K Tras introduced to allow the sealing off of the reaction flask at the completion of the reaction,

in case an analysis of the products was desired. L is a

solenoid wound upon a pyrex tube of sufficient sine to allow it to slip over the side arm of the flask and its

heating element. M is a radio B-battery for operating the

solenoid.

The capsule N contains a weighed amount of the liquid

whose decomposition is being studied. It, together with

a detail of the neck of the reaction flask,is shown on a

larger scale in Plate II. The capsule was constructed of

thin walled 5 mm. soft glass tubing. The capacity was

varied by varying,the length* It is filled with the

liquid,or melted solid, being studied by suction. The

hook end w & sealed carefully to avoid ingress of air. The

other end is then heated near the flame until there is a

rather vigorous discharge of vapors from the finely drawn

out tip. The tip was then sealed off while vapors were

still being discharged. If the liquid was heated to near

its boiling point and no air allowed to enter while sealing,

it was assumed that the contents of the capsule were free

of absorbed gas. -15-

H is the reastion flask immersed in the thermostat. Three flasks were used in this work* All three were con­ structed from 60 mm. heavy walled pyrex tubing. The inside of the tubing was scrubbed with Dutch Cleanser and was well rinsed and dried before construction of the flasks.

Concordant results could not be obtained using a similar flask which had been cleaned with sulfuric aeld-dichromate cleaning solution. The necks of the flasks were made of heavy walled 6 mm. tubing, and were about 10 cm. in length from the top of the flask to the side arm P. One of the flasks had a measured volume of 566 cc* measured at room temperature, the second had a volume of 315 cc., and the third, after packing with pyrex tubing, had a volume of

238 co. The surface/volume ratio of the first two flasks was 0.97, and that of the packed flask was 13.3. 0 is a m a l l tube filled with iron filings. It is of sufficient size to slide smoothly in the side arm. One end is rounded.

The other is drawn out slender, and a small ball formed on the end. The ball serves to prevent the dropping of the capsule prematurely. The side arm S was introduced to allow the introduction of mercury for displacing the

gaseous products of the reaction for analysis.

The neck of the flask, the side arm P, the capillary G

the click gauge and all intervening tubing is wrapped with nlohrome resistance wire and connected to the lighting

circuit through a rheostat. This allows the heating of

all the parts of the apparatus outside the furnace to idiieh the vapors are exposed. The wiring Is so arranged that the parti of the apparatus above the side arm P may he heated separately from the portion of the seek of the flask below P .

In use, the filled capsule of liquid to be studied is introduced into the neck of the flask and hooked over the projecting part of the pin 0. The flask is then placed in the furnace and sealed to the apparatus.

The apparatus, including flask C, is then evacuated to about 10-15 mm. of mercury. During the evacuation the stopcock H is turned so that evacuation is from both sides of the diaphragm gauge. As soon as is possible, the heating eolls are placed in position and heating of the apparatus above the side a m Is started. By the time the desired vacuum is reached the click guage will usually be in thermal equilibrium with its surroundings.

Stopcocks E and C are closed and the calibration of the click gauge is made. Air is allowed to enter through the capillary leak I. The rate at which air enters should be regulated so that at 15 mm. pressure the mercury level in the low arm of A will fall at a rate of not more than 2 ram. per second. If air is allowed to enter faster, the time lag between the hearing of the

click and the closing of the stopcock H is great enough and variable enough that concordant readings cannot be

obtained. As soon as the click is heard, H Is closed.

The readings of the two manometers, A and J, are noted. Click yauye. calibration Correction

gact-ge reading 0)0 MX) ZAO 3oo 3tTo + 00 o~ Oo The reading of A minus the reading of J gives the cor­ rection to be applied to the reading of A during a reaction for that particular reading of A. H is then opened to connect the two manometer systems. When the pressures have equalized, H is closed and opened the other way, and the above procedure is repeated. If it is desired to read pressures near atmospheric inside the reactten flask, it is necessary to build up a pressure above atmospheric in the gasometer and to operate the gauge by use of stop­ cock E. If at any time it is desired to repeat a reading, pressure may be reduced above the diaphragm by opening the stopcock to the evacuated flask C until the diaphragm

Is heard to click back into its normal position. The corrections obtained for the different pressure* are plotted against the readings of A. A smooth curve, usually a straight line, drawn through the plotted points enables the proper correction for any reading of A to be obtained* A typical calibration curve is shown in .

Figure III The accuracy of the click gauge was found to be well within the limits set by Smith and Taylor in their discussion of the gauge and its use. Readings were found to be reproducible within 6.1 mm., and to be accurate within 0.2 m , for the gauge® used. When a new gauge is placed in service it is found thet its character1stio^bhange the first few times it is cooled and reheated, but after a time it attains constant characteristics. One diaphragm was used for ten ©» or more rims without the slightest change In character­

istics* even when heated intermittently to temperatures

above 300°C. Care must be taken to allow the diaphragm

to return to its normal position as soon as possible

after each reading. If* left pushed in at the high tem­

perature, the characteristics change rapidly, destroying

the value of any observations made, the diaphragm tends

to lose its elasticity, if left in the pushed in position.

After calibration the apparatus is washed free of

oxygen by evacuating to about 10 mm., filling with oxygen-

free nitrogen, er other gas, if that gas is to be present

in the flask at the start of the reaction, and again

evacuating;,. Two washes were considered sufficient in

all the work done. After the last wash, the apparatus is

evacuated to less than 0.1 mm. pressure, and the capillary

sealed at T. This isolates the reaction flask and its

contents, end prevents the vapors from coming into contact

with anything but glass. A plate glass shield is then

placed between the operator and the apparatus, the heating

coils below the side arm included in the heating eireuit,

and the stepeoek H opened. H is closed as soon as the

click is heard. The capsule is closely observed until

the liquid expands to completely fill it. The circuit

through the solenoid is then closed, and the capsule

dropped into the reaction flask where it breaks almost

instantly. The stopwatch is started at the instant the

capsule breaks. H is then opened and the exact moment

of the click noted. The reading of manometer A ie- -19- is then oheerved, and the diaphragm allowed to return to its normal position. After a little practice it is pos­ sible to take readings at one minute intervals, or less if desired.

Ihen the resetion is complete, the resetion flask is sealed off at K, and removed from the furnace. The tip is broken from the capillary G, and air admitted through the leak I at the same time. This procedure allows the maintenance of approximately equal pressures above and below the diaphragm, thus preventing the unnecessary rupture of the gauge,

In this work, time was measured with a Meylan stop watch* No attempt was made to read time intervals of less than a second.

Possibly more detail has been given above than is absolutely necessary, but the technique is not simple.

It is desirable that future workers*with the apparatus may profit as much as possible from past experience in its use.

% e design of the apparatus is not entirely free from criticism. There is a considerable, and not entirely negligible volume of apparatus outside the furnace which

is occupied by the vapors. The effect is that a portion

of the resulting products are cooled after reaction,

and that all the reacting substance is not subjected to

the temperature of the reaction flask at one®. Both

effects tend to reduce the apparent rate of the reaction. The use of soft glass In the construction of the capsules may also be mentioned. It was found that cap­ sules made of pyrex glass were not dependable. They would break at unexpected times, sometimes before they were dropped into the flask# and sometimes not until two minutes after dropping. The results were at times disastrous.

The design has some merits which should not be over­ looked. The exact instant of the beginning of the reaction is easily determined, the exact amount of reacting subs­ tance introduced is known, the vapors come in contact with nothing but glass during reaction, the exact instan­ taneous pressure is easily obtained, and the gaseous products of the reaction are readily available for analysis. Analysis of the products of reaction.

The least satisfactory and most uncertain part of the work arose in the analysis of the produet* of reaction.

This was caused by the small amounts of the products formed* From 16 to SO oe« of permanent gases were usually obtained for analysis, and the amounts of non-gaseous materials formed during the reaction were so small that it was,impractical to attempt more than a rough quali­ tative analysis of them*

The methods used in the analysis of the gases were taken from Scott’s, standard Methods of Analysis^3 with frequent reference to Hemple, and to Clowes and Coleman*

The usual absorbents were used in the order listed below.

Carbon dioxide was absorbed by potassium hydroxide solution unsaturated hydrocarbons and benzene vapors with 15$ fuming sulfuric acid, and carbon monoxide by acid cuprous chloride

The residual gases left after absorption were mixed with air and exploded over mercury* The amount of ear bon dioxide formed and the amount of oxygen in excess were determined by absorption with potassium hydroxide solu­

tion and alkaline pyrogallol solution, rsspedtlvsly.

Speculations were then made as to the gases present before explosion, and the amounts of each.

A n unsuccessful attempt was made to use palladiumized

asbestos in the determination of hydrogen. The catalyzed

oxidation could not be made selective. With a synthetic mixture of hydrogen and methane, the catalyst could only be made to oxidize all or none of the combustible gases present. Various means were tried to regenerate the

selectivity of the catalyst, but all met with failure.

The leak of definite knowledge of the products of

the reaction proved to be a severe handicap in the inter­

pretation of the kinetic data obtained. Analyses were

not made with all kinetic runs, particularly during the

first part of the work.

Preparation of materials.

a. Benzaldehyde,

Eastman* s best grade of benzaldehyde was purified

by a method given by Gatterman^6 k large sample was first

distilled, and the portion coming over between IVOnand 180*0

(Bar. pres. « 700 mm.) was saved. This fraction was

treated with a saturated solution of sodium bisulfite.

The precipitate was filtered by suction, washed twice

with alcohol and once with ether, and sucked dry.

This product was decomposed by passing steam into an

emulsion of it in a 10# sodium carbonate solution. The

distillate was extracted with ether and dried over fused

calcium chloride. The ether was distilled off, and the

residue subjected to fractionation. The fraction boiling

at 173*0*0 was saved. The purified benzaldehyde was

transferred to capsules as soon as possible and the

capsules sealed. The capsules were stored in a cool dark

place until desired for use. • a -

b. Benzyl benzoate.

Two attempts were made to prepare benzyl benzoate from benz&ldehyde by the method given by Volume II, Organ!®

Synthesis.17 In the second attempt every possible ear® was used in the preparation of materials. Each reagent was subjected to the best methods of purification available.

The yield was negligible in each attempt.

Recourse was thenhad to the general method for the formation of esters of treating an alcohol with an acid chloride. Carefully purified benzyl alcohol was treated with an equivalent amount of Baker* a C.P. Benzoyl ohlorid* in the presence of a 10$ solution of sodium hydroxide.

The resulting mixture was extracted with ether and dried over fused calcium chloride. The ether was distilled off

and the residue subjected to fractionation. Upon the

third fractionation a product was obtained which boiled at 504*0, and whose melting point was 18.2*0. This material was considered to be of sufficient purity for

the purpose at hand.

The other materials used were the best obtainable

laboratory reagents. No attempt was made to further

purify them. The nitrogen used in washing out the

apparatus was commercial tank nitrogen, purified by

passing through a train to remove oxygen and water vapor.

The carbon monoxide used was prepared by the action of

sulfuric acid on sodium format©, and was passed through

a train to remove oarbondioxlde and water vapor before

use -24'

Ezp@rlmemtml Data*

The following symbols have been used in the tabula* of results:

t « time in seconds measured from tho beginning of

the reset ion.

P e total pressure in m . at any tine t.

P0 * initial pressure of the reacting substane®.

Pf * pressure at the end of the reaction.

X «= . * the fraction of the total pressure

change which has taken plaee a time t .

Any other symbols whioh may be used will be explained at the time of their introduetion*

a. Benzaldehyde.

Table II.

Temperature * 500*6. P 0 » 70,5 mm.

t P P - P0 X 58 88.5 18.0 0.294 120 96.5 25.8 0.422 180 100.5 . 2t ,8 0*487 300 105.8 35.3 0.575 422 109.6 39,1 0.638 600 113.8 43.5 0.707 898 117.2 46.7 0.762 1800 122.0 51.5 0.840 131.8 61.5 Table III«

Temperature *= 500*C P Q *= 120 *0

t P P - P0 X so 149.4 29.4 0.288 120 157.4 57.4 0.366 180 181*9 41.9 0*410 240 167*7 47.9 0.468 299 171.4 51.4 0.90# 480 176.0 56.0 0.847 600 182.0 62.0 0.606 900 187.7 67.7 . 0.662 1800 1*8.2 78.* 0.764 m - 2*2.1 102.0

Table XV*

Temperature = 500*0 P 0 r 164e5

t 1 .P M o X 59 197.7 59.2 0.238 121 210.4 45.9 0.328 181 218.4 53.9 0.386 240 226.2 61.7 0.442 300 230.4 65.9 0.471 420 238.3 73.8 0.528 600 246.2 81.7 0.585 901 255.2 90.7 0.650 1200 261.7 97.2 0.695 2400 275.0 110*8 . 0.792 00 304.0 199.5 ■'

Table V.

Temperature » 500*0 P0 » 195.6

t P « p 2 114 235.9 40.5 0.254 299 262.9 67.3 0.390 422 284.6 89.0 0.516 597 298.1 102.5 0.594 1020 517:7 1 2 2 .1 0.707 1500 329.4 139*8 0.776 2700 343.1 147.5 0.818 3600 9 9 0 ^ 154.7 0 .8 9 7 03 368.0 172.4 Table VI.

Temp. * 500*0 P qo • 120.1 P0 «= 289.0

t P P - P 0 X 62 338.1 49.1 0.332 116 349.1 60.1 0^407 178 356 i5 67.5 0.457 240 363.5 7* .* 0.498 304 367,0 78.0 0*538 421 374*5 85.0 0.578 600 383*0 94.0 0;655 1860 403.5 114,5 0.775 2700 411.6 122.6 0.830 00 436.6 147.6

Table VII.

Temp, » 500*0. Fog m 84.8 P 0 « 245.4

t . P ' P > P0 66 302.2 57.1 0.386 m 311.4 66.0 0.447 180 317,1 71.7 0.485 240 321*7 76.3 0.517 . 299 324.6 . 79.2 0 .63* 422 830.0 84.6 0.578 599 535.0 8 9 .3 0.606 900 343.4 98.0 0.663 1800 • 556.4 111.0 0.750 2700 363.6 118.0 0.300 00 - 393.8 147^

Table VIII.

Temp, » 500*0. P 0 « 151.6 Paoked flask.

t P P - Po - X ;

59 166.7 14.1 . 0*132 120 174.3 88.7 0.213 180 180,0 28.4 0.267 240 184.2 32.6 0.306 299 187.8 56.2 0.340 420 194.1 42.5 0.399 660 204.9 ' 53*5 0.501 904 212.6 61.0 0.578 1200 819.7 68.1 0.639 1800 28711 V • 75.6 0.706 2700 254.0 82.4 0.773 0® 258.1 106.5 #87#

Table DC.

Temp. « 500°C. Pp * 200.0 mm. Packed flask.

t P P - P0 X 63 234.2 34.2 0.217 184 242.9 42.9 0.272 178 249.6 49.6 ,0.314 841 256.2 56.2 0.356 368 8*6.8 66.2 0.420 488 ", 874.8 74 0.470 600 284.7 84.7 0.537 901 292.7 92.7 ■ 0.587 1800 818*8 112.2 0.711 8700 . 323 tl 183.1 0.780 00 357,7 157.7

Table X.

Teap. « 450*0 P0 = 183.0

t P P ~ Po X 116 ’ 195.8 ,18.2 0.085 178 200.9 17.9 0*184 ' 840 206.5 23.5 0.163 301 210.8 27.8 0*193 481 217.7 34.7 0.241 601 224.7 41;? 0.289 908 831.8 48.3 0.355 1200 ' ' 834,9 51.9 0.360 1800 238.9 55.7 0.386 2700 242.4 59.4 0.412 3600 847.7 64 ;7 0.448 3 hrs 278.8 90,8 0.680 4 hrs 887.0 100.5 0.697 11 hrs 887.0 144.0

Table XI. Analysis of products, (gaseous.) CO CO. 1, Average for unpacked flask 83.7 2.7 13.6 Average for packed flask 87.0 3.0 10.0

An attempt was Bado to carry on the decomposition at

550*0, bu^the reaction proved to be too rapid at that

temperature to be followed, other runs at different temp# -8a-

eraturee wore not made, because it was found that the peculiar behavior of benzyl benzoate merited closer investigations

The result® given in the above tables are shown graphically in Figure* III and 17. X,as ordinate,has been plotted against time.

b. Benzyl benzoate#

Table XII, Temperature = 500°C P 0 - 80.8

t P P-P0 X 128 101.2 20.4 0,107 245 H e . ? 35.9 0.188 305 123.6 42.8 0.224 421 138.0 57.2 0.299 598 157.6 76J8 0.402 717 168.5 87.7 0.468 835 177.1 96.8 . 0.504 960 184.3 103.5 0.542 1080 189,6 108.8 0 #569 1200 195.5 114.7 0.600 1500 809.3 128.5 0.675 1800 818#® 137.8 0.722 2700 236.9 156,1 0.818 GO 271.8 191.0

The results of the above table, together with those of the two following tables have been shown in Figure V, -29-.

Table XIII.

Temperature = 500*0 P0 *» 146.6 m .

t . P P**Pq X 61 159.# 13.2 0.039 121 174.7 28.1 0.084 179 189.8 43.2 0*129 241 205.6 59.0 0+176 . 301 220.7 74.1 0.221 360 235.4 88,8 0.265 420 247.5 100.9 0.301 480 260.7 114.1 0.841 540 272.1 125.5 0.874 899 283.1 136,5 0.407 718 800,3 153.7 0*458 840 315.3 168.7 0*505 : ■ 960 329.8 182.7 0^545 1080 340.8 194.2 0.579 1200 ■ 350.9 204.3 0.610 1500 369 .3 222.7 0.603 1800 885.4 238.8 0.712 2700 415.0 268.4 0.798 00 . 482.1" 335,5

Table XIV $

Temp. » 500*0 P 0 '* 108.7 Peeked flask

t P P-Po X 57 129.4 20.7 0.092 123 153.6 44.9 0.200 183 171.9 63.2 0.281 242 187.6 78.9 0.351 300 200.2 91.5 0.407 360 812.5 103,8 0.462 419 2.9.5 110.8 0.493 X 480 230.6 121.9 0.542 539 838.3 129.6 0.577 600 245.4 136.7 0.608 720 257.4 148.7 0.662 900 271.2 162.5 0.724 1200 '286.8 177.6 0.790 1800 304.5 195.8 0.872 2700 319.0 210.5 0.938 . 09 333.1 224.4 -30-

Table XV.

Temperature « 510*0 P0 » 31 •6 mm.

t P P-Po X 208 53.6 22.1 0.289 308 59.3 27.8 0.364 421 63.9 32.4 0.423 552 69.8 38.3 0.502 861 79.0 47.5 0.628 1125 84.2 52.7 0.690 1740 92.8 61.3 0.803 2640 99.8 68.3 0.893 to 108.0 76.3

Table XVI.

Temperature = 510*0 P0 ■ 147,2

t P X 62 177.0 29.8 0.093 118 19916 52.4 0.163 183 223.1 75.9 0.236 242 241.8 94.6 0.294 299 257.9 110.7 0.345 360 273.6 126.4 0.894 420 286.0 138.8 0.432 483 298.1 150.9 0.468 539 307.3 160.3. 0.500 660 526.0 178.8 0.577 780 339.9 192.7 0.600 900 350.5 203.3 0.633 1020 360.6 213.4 0.664 1140 369 .8 222*0 ; 0.692 1440 383.1 235.9 . 0.735 1800 594.4 247.2 0.770 2700 413.9 266.7 0.831 00 468.3 321.1 . ~#1

Table m i *

Temperature = 510°C. p0 » 152.8

p P-Po X 61 176.5 23.7 0.069 121 801*6 48.4 0.141 182 227.4 74.6 0.218 240 249 .8 97.0 0.283 300 270.9 118.1 0.344 359 287.2 134.4 0.392 420 503.0 150.2 0.438 480 315.9 163.1 0.476 540 326.5 ' 175.7 0.507

599 *#6 0 163.2 0.534

720 * 52.1 199.3 0.581 , 840 366.3 313.5 0.623 963 578.4 225.6 0.668 1080 388.0 235.2 0.686 .... 1200 396.5 243.5 0.110 1500 412*4 889.6 0.756 1800 424*8 272.0 0.795 2700 447.5 294.7 0.860 0© 496.1 343.3

Table XVIII.

B # 510*0 Pm « 53.8 Packed flask.

P P-Po •' x • ■ .

122 76.6 22.8 0*810 178 88.2 54.4 0.517 239 98.5 44.7 0.412 300 107,6 53.5 0.492 / 360 113.4 . 59 .6 0.548' 420 118.8 65.0 0 .597 480 122.7 68.9 0.643 600 129.4 75.6 0*695 720 134.8 81.0 0.745 840 138.4 84.6 0.778 960 140.3 86.5 0.795 1200 14212 88.4 0.822 1800 144.8 91.0 0.836 00 162.6 108.7

The results of Tables XV, XVI, XVII, and XVIII are shown graphically in figure VI. Table Z3Z.

Temperature 520*0. P0 = 36.1 mm.

t P % 64 43.4 13.3 0.152 121 55.7 19.6 0.224 184 84.1 28.0 0.319 240 69.9 55.8 0.385 501 75.4 39.3 0.448 560 79.5 43.4 0.495 481 85,2 49.1 0.560 600 91.4 15.3 0.630 925 100.9 64.8 0.738 1800 106.4 70.3 0.802 1800 111.7 75.6 0.862 2700 116.0 79.9 0.910 00 123.9 87.8

Table XX.

Temperature * 520*0• P0 « 107.3 mm. t p P-Po X 62 129.9 22.6 0.093 123 153.1 45.8' 0.189 183 175.2 67.8 0.280 350 197.7 90.4 0,372 399 209.2 101.9 0.420 359 222.0 114.7 0.473 420 233.2 125.9 0.518 480 243.3 156.0 0,561 .600 259.1 151.8 0.6*7 > 720 269.7 162.4 0.670 840 278.6 171,3 0.707 960 286.7 179.4 0.740 1080 293.0 185.7 0.765 1200 298.0 190.7 0.786 1500 308.1 200.8 0.832 1800 312.2 204.9 0.865 2700 323.4 . 216.1 0.892 00 349.7 242.4

The results of Tables XIX ahd XX are shown graphically in Figure VII, -33-

Table XXI.

The analysis of the gaseous products of decomposition of benzyl benzoate. oo CO. Ha Average in unpacked flask 69.2 17.2 14.0

Average in peeked flask 47.1 37.5 15.4

Interrupted reaction

after five minutes 72.3 . 15.6 12.1

after twenty minutes 69.0 17.6 13.4

There was a strong odor of benzaldehyfte present in the reaction flask after the teo interrupted reaction# reported above, the presence of benzophenone was suspected in the residue left in the flask after reaction, but it could not be positively identified.

c. Benzole aold.

Table XXII.

Temperature « 500°C P0 »= 151.2

t P P-P0 X 120 170.2 19.0 0.129 843 189.2 38.0 0.258 301 196.5 45.3 0.307 360 204.4 53.2 0.360 420 212.0 60.8 0.412 480 220.3 69.1 0.468 540 228.1 75.9 0.521 600 836.0 84.8 0.574 660 248.4 91.2 0.517 720 250.0 98.8 0.668 900 268.0 116.8 0.782 1200 288.0 136.8 0;927 1500 296.5 145.3 0.987 1800 297.0 145.8 0.9*3 2100 2*8 .3 147.1 0.997 2700 8*8.8 147.6 3600 898.8 147.6 Table XXIII.

Analysis of the gaseous products of decomposition of benzoic acid

Carbon dioxide 95*0$

Carbon monoxide 5*0*

d. Benzophenone.

Temperature » 500*0. P0 * 97.8

Pressure increased 5.5 mm. after 16 hours heating. Be-nzcLldehyde..

#/, Po - 7Q.5 Trim. &2. Po = 120.0 •' i 3. Pc = /^y-.cT " > Soo°c. X ^ P0 z /

3o be.7izalde. Hyde..

d'l, PpcMo — !(>*?•.& Pea — fycHo —• lTo 2.7 /?a # l&'f Profr! - Z&y-P J. /^fcyw, - /*A6 /^O = n.2 Pf-ctai = X 1,0 o.l

time, in 7iii*utts. *to Benzyl benzoate.

#/. Po ~ $0.8 O 2. & - 7^6.6 # ’ CLt 5 0 0 ° C. X J 7^ - lot. 7 (.Packed flask) J The arrows dev ofe, iderrticaf points. 1,0 !*■

ti me in in/nates 3o + o Benzyl benzoate.

&l fa - 3/Jf ®

Z Po = I *7.1 O at SIO°C. 3. Po- /S1..9 X x f. fo - «5 3 , y (PtLCksd f-fa 3 Pis l.o represents double point

o.l

tim e in rninufes. HO Benzoic acid. P0 = JJ/-Z at 500°C.

time in TtiivuHs. p-o -It

Btnzyl benzoate,.

*'■ Py 36,/- % \a± 59P°C. X I. Po- 107. 3 , oj LO

3a Interpretation of results.

a. Bonzaiaeby&e.

Befereneea given in the review of the literature show that at elevated temperatures henzaldehyde may undergo the following reactions:

(1) CgBs-OHO . 06** + CO.

(2) 2 0g%-GE0 « (CgHs)g + 2 % + 2 CO.

(S) 2 C&Bg-CEO * CgHs-CHg-O-CO-CgHs. (The Cannizzaro reaetion.)

Any or all of these reactions may he expected to take place in the reaction flaik.. SinSe the formation of benzyl henzoate was reported at temperatures of about 300*550*6» and was not observed at 690°, not at 1150*, it would be expected that reaction (3) would not be very prominent above 500*. This reasoning led to the expectation that reactions (1) and (2) would be the chief reactions taking place at 500* or higher. 8 Kurd and Bennett reported the analysis of the gaseous products of the decomposition of benzaldehyde at 690* to be:

Carbon monoxide 86.7#

Hydrogen 12.9#

Carbon dioxide 0.32#

The average analysis of the gaseous products of the decompos­

ition in a pyrex flask at 500*0. as found in the present

work seen from Table XI to be: Carbon monoxide 83,7%

Carbon dioxide 2.7% . •

Hydrogen 13.6%

The principal difference between the eompoeitlon of the gaseous products at the t m temqperatures is seen to be in the relative amounts of carbon dioxide formed. None of the reactions given above accountsfor the formation of carbon dioxide. It is to be noted from Table XXI, that this gas

Is one of the products of the decomposition of benzyl benzoate. This shows that even at 690°, reaction ( 3) is not entirely absent, and it must take place to quite an appreciable extent at 500°, contrary to expectation.

Since most of the hydrogen arises from reaction (2), and a volume of carbon monoxide equal to twice the volume of hydrogen comes from the same reaction, it is seen from the . above analysis that reaction (1) takes place to about twice the extent of reaction (2).

These considerations are 'supported by the kinetic data obtained. The kinetic data is considered to be de­ pendable, although the demonstration of the duplieability of results was not practical. Duplicate runs can be made only in those rare instances in which two capsules chance to contain the same weight of sample• The regular variation of the time required for completion of one half the total pressure change accompanying the reaction is seen from

Figure III, where X, the fraction of the total pressure

change, is plotted against time measured from the beginning -37-

of the reaction, to vary in a regular manner with the initial pressure of the benzaldehyde. This shows that the kinetic results are consistent with each other.

Note may he made of the fact that, considering the complexity of the reactions involved, the time required for completion of one half the total pressure change, which will he termed the "half time" of the reaction,

is not necessarily the time in which the partial pressure of the henzaldehyde has been reduced to one half the ini­ tial pressure. As will be indicated later, the relation between the two time intervals involved may be quite obscure.

The fact that there is a variation of the half time with a change, of. initial pressure, shows that the reaction

as a whole cannot be unimolecular, as was expected. For a V" ' ■ ' ' ' reaction of higher order than the first, the half time

of the reaction is Inversely proportlomallto the initial pressure of the reacting substance. In the decomposition of bensaldehyde the half time of the pressure change is

directly proportional to the initial pressure, as is

shown by the following tabulations,

*3 half, time. 1. 70.5 5.23 minutes

8. 120.0 4.80 tf

3. 164.5 6.02 *#

4. 195.6 6.58 W This effect is easily explained by considering the simul­ taneous effect of reactions (1), (2), and (3), as given above. Beth'.* (1) and (2) involve an increase in pressure, while (3) results in a decrease. It would be expected that an increase in initial pressure would favor the reaction which takes place with a decrease in pressure; Reaction (3) is, then, speeded up, thus causing an increase in the apparent half time of the reaction. If the ratios of the pairs of pressures, and the ratios of the corresponding half times, are calculated, they are found to be,

Pi/Pg « 0.59 ti/tg = 0.67

P 2/P3 • 0.75 tg/t^ -■0.80

P 3 / P 4 ” 0-84 tg/tA » 0.91 Thus the apparent decrease in the rate of reaction with increase of initial pressure of the benzaldehyde is roughly proportional to the first power of the initial pressure.

This indidatos that the condensation is kinetieally bimoleeular. That reaction (5) is a reversible reaction is shown in the next section under the discussion of the decomposition of benzyl benzoate. Use will have to be made of this fact in the explanation of the behavior of benzaldehyde.

Study of curves (5) and (6) of Figure III, which represent runs made in the packed flask, show that the . . ■ ■ . . decomposition as a whole is negatively catalyzed by an

increase of the surface exposed to the reacting vapors.

This indicates that reaction (3) is considerably -39-

accelerated by the increased surface. From the results represented by curves (5) and (6), the following ecmper-

Isons may be made:

% half time S/Y ratio 1. 164.5 6.02 min. 0.97 2. 151.6 11.00 15.3

3. 195.6 6.58 « 0.97 ' 4. 200.0 8.42 " 13.3

It seems reasonable, therefore, to conclude that the combination of two molecules of benzaldehyde to form ohe molecule of benzyl benzoate is a heterogeneous reaction of apparent second order.

There is another effect of the increase of the surface/rolume ratio which is revealed by the curves (5) and (6) of Figure m . In the packed flask the apparent half time of the reaction is inversely proportional to the initial pressure of the benzaldehyde. If the ratio of the two initial pressures and the corresponding inverse ratio of the two halftimes are calculated, they are found to be 0.758 and 0.756, respectively. This indicates that one of the reactions resulting in an increase in pressure is, in part at least, a bimoleoular wall reaction. By referring to the analysis of the gaseous products of the reaction in the packed flask, it is seen that these consist of,

Carbon monoxide 87.0#

Carbon dioxide 3.0#

Hydrogen 10.0# -40

OoaparlBon with the analysis eonsldered previously, shows that the relative amount of hydrogen has deereased, while the relative amounts of earhon monoxide and carbon dioxide have increased. It would, therefore, seem that reaction (1) is the decomposition which is affected by the increase of the surface/volume ratio of the reaction flask. This indicates that reaction (1) may be classified> as a bimoleo- ular reaction, which is, in part at least, heterogeneous.

The order of reaction (2) is left undetermined. The reaction as represented by the equation shows that two molecules are Involved, but that would not prevent its being kinetlcally unlaoleeular. The fact that it is not accelerated by the Increased surface makes it appear to be a homogeneous reaction.

There have been shown to be three distinct reactions occurring simultaneously at the beginning of the reaction.

Tize decomposition pursue® two distinct courses, and there is present a reversible reaction which tends to reduce the apparent rate of the overall reaction as measured by pressure:;Increases. This is represented graphically in

Figure IX. Curve A represents the course of the pressure change with time, if reaction (2) only took place. Curve B represents reaction (1) in the same manner. Curve 0 repre­ sents the actual increase in pressure due to the formation of benzyl benzoate. The effect of reaction (3) must reach a maximum at some point. From then on, the effect is . - ' ■ . ■ - gradually removed by the reversal of the reaction, caused by the decomposition of the benzaldehyde by reactions (1) -f- PrtiSu it, fttcredte.

— Pressure I'Hcrt*.}*.

F'ijureTJZ. 131 -41'

and (2), Curve D would represent the combined effects of the three reactions, and Is ealoulatedjby adding the ordinates of the the three other curve given. Curve D, then, should represent the experimentally observed course of the pressure change in the thermal decomposition of benzaldehyde. It would be very easy to find curve D if the other three curves were known, but information is not available for the resolution of Curve D into the three component parts.

If the formulation of a differential equation io describe the observed data is attempted, the velocity constants of all the reactions shown to be taking place must be taken into account. If u, v, w, and y arc the amounts of :benzaldehyd&'following the course;: defined'by, (1)

(2), (3), and the reverse of (3), respedtively, it is seen by the law of mass action that,

Sr * Ki x (instantaneous pressure of benzaldehyde).

^ * Kg x (instantaneous pressure of bennaldehyde)?

^ sa Kq x (instantaneous pressure of benzaldehyde)^

^ «= x (instantaneous pressure of benzyl benzoate).

The formation of benzaldehy&e from bensyl benzoate is shown in the next section to be untmolecular. If x repre­ sents the amount of benzaldehyde removed from the sphere of action, then,

S - S " Kl(=-x)2, K2(a-X)\ K3 (o -x )2 -K^(partial pressure of benzyl benzoate). -42-

$he solution of this equation and its application to the observed data is not possible unless z, n, and tho value of the instantaneous pressure of benzyl benzoate can be determined from pressure measurorients,

That the total reaction has a high temperature coef­ ficient is shown by curve (7) of Figure III, Decreasing

the temperature of the reaction by 80° increases the tire required for completion of one heIf the total pressure

change from 6.58 minutes to about 182 minutes, or about twenty eight times. Tho fact that tho curve tends to flatten out after the same time interval as those repre­

senting runs at higher temperature is remarkable• It may be explained in terms of the equilibrium reaction

considered above. Analysis of the products of the reaction

for carbon dioxide shows that the condensation of benzyl

benzoate is less as the temperature increases. Hones,

the formation of benzyl benzprate must be accompanied by

the evolution of energy. The condensation would then be

expected to take place at a greater rate at lower tem­

peratures. This might easily result in the attainment of

equilibrium conditions in the sene time period as at

higher temperatures.

Figure IV shews the effect of the presence of onejof

the reaction products upon the reaction rate. The presence

of an initial pressure of carbon monoxide has tho effect

of increasing the reaction rate. In explanation of this

effect, it may be considered that the carbon monoxide interferes with the adsorption of hemsaldehyde vapors by

the walls of the reaction flask, thus blowing down the rate with whloh the condensation reaction takes place.

Then as the rate of this reaction Is decreased, the pressure increase due to the homogeneous decomposition

of benzaldehyde is not so profoundly affected.

b. Benzyl benzoate.

In the study of benzyl benzoate there is much less

data available for deducing the course of the decomposition. 11 The reaction was studied bo some extent by Hurd and Bennett.

Their work has been mentioned in the section of this paper

reviewing the literature. They suggested that the presence

of benzaldehyde among the decomposition products could be

accounted for by the reversal of the Cannizzaro reaction,

which was shown in the preceding section to take a pro*

minent part in the study of the behavior of benzaldehyde

vapor. They explain the formation of benzoic anhydride and

toluene by assuming a splitting oftwo^molecules of the

ester with recombination to form one molecule of benzole V anhydride and one of dibenzyl ether. Dibenzyl ether was

not reported among the products at the end of the exper­

iment, but was considered to be an intermediate compound

which breaks up as rapidly as formed into benzaldehyde

and toluene. The proposed course of the reaction was as

follows:

0-- cm (1) 06H6-C0-0yCH2-C6I% OgHg-CO CgHg-CHg J , +•i* - # 83 • 0 + _ . © C@%»C0/e~CHg«C6H5 c 6h 5-g o CgE^-GHg

They also suggeated that the benzoic acid may coma from oxidation of the benzaldehyd©% or that it may be formed directly from the ester by thermal means. No mechanism was suggested in either case. Their work was carried out with the liquid in a sealed tube at a temperature of

340-350®C. They reported the development of a high pres­ sure within the tube, but made no further mention of the gaseous products.

In the interpretation of the work at 500* and higher, it is necessary to consider the fate of the products of reaction as given above. At the higher temperatures these products may or may not be intermediate products. The final products lit the ease of benzaldehyde are known from the prededing section. These consist chiefly of carbon monoxide, hydrogen, benzene and diphenyl. The principle gaseous product of the decomposition of benzoic acid is, from the analysis given in Table XXIII, carbon dioxide.

No information is available concerning the behavior of benzoic anhydride.

The analysis of the gaseous products of the decom­ position, in the flask with the low surface/volume ratio, is shown in Table XXI to be,

Carbon monoxide 69,2)6

Carbon dioxide 17.2# Hydrogen 14.0$

For interrupted rune,the gases showed the following analysis at the end of five minutes.

Carbon monoxide 72.3$

Carbon dioxide 15.6$

Hydrogen 14.0$, and at the 6nd of twenty minutes.

Carbon monoxide 69.0$

Carbon dioxide 17.6$

Hydrogen 13.4$.

The three sets of results agree satisfactorily, considering the small amounts of the gases available for analysis. These results show that the formation of gaseous products is uniform throughout the decomposition, and that V/egsfcheider's principle concerning simultaneous reactions may be applied.

V/egscheider’ s principle"*"9 states that the ratio of the amounts of products formed in two side reactions is inde­ pendent of the time, provided the two side reactions are of the same order. The three sets of analysis given

above prove, therefore, that the reaction giving rise to

carbon monoxide occur® simultaneously with the reaction

giving rise to carbon dioxide, and that the two reactions

are of the same order. Additional evidence that the

two reactions are distinct is given by the analysis of

the gases resulting from the decomposition in the packed

flask. The ratio of the monoxide to the dioxide in the

packed flask is 1.26, while in the unpacked flask it is

4.02. The strong odor of benzaldehyde present in the flask when the reaction was Interrupted at the end of five and twenty minutes shows that benzaldehyde is formed as an Intermediate product, and that It does not decompose as rapidly as it is formed.

The course of the portion of the reaction which follows the reverse of the Cannizzaro reaction may he represented as follows:

(2) CgHg-CO•0**CHg-G©Hg • 2 CgEg-CBD

SCgHg-GHD ■ 2 CpH^ + 200 .

" Or B (CgHglg + Eg ♦ 200 • The total pressure change Involved would be from one to four, regardless of the amount of benzaldehyde which followed either of the paths by which it may decompose.

The fact that the formation of carbon monoxide is pic­

tured to eome about by a secondary reaction does not

invalidate the application of Wegseheider1s principle,

since the formation of carbon dioxide may also come from

a secondary reaction which is simultaneous with the

formation of carbon monoxide. The reaction as given

readily accounts for the presence of hydrogen and carbon,

monoxide among the products of the decomposition, but a

discrepancy is seen to exist in the fact that the ratio

of carbon monoxide to hydrogen is different from that

observed in the case of benzaldehyde. Either this is

caused by inaccuracies in the gas analysis, or else there

is a reaction present which produces hydrogen without

giving carbon monoxide. The simplest mechanism whieh could be assigned to the reaction producing carbon dioxide would be,

(3) CgHs-CO^O-Clg-OgHs « CgHg-CHg-OgSg 4 COg.

This would involve a pressure change from one to two.

A combination of reactions (2) and (3) would give a pressure change such that the ratio of final pressure to

initial pressure would lie between two and four. These

ratios calculated for the data in Tables XIII-XXII are

shown below:

Table Temp. S/V ratio Pf/Po ratio m i 500 0.97 3.362 n v 500 0.97 3.388 XV 500 18.3 3.060 XVI 510 0.97 3.430 XVII 510 0.97 3.184 XVIII 510 0.97 3.250 XIX 610 13.3 3.020 XX 5S0 0.97 3.430 XXE 520 0.97 8.878

The average for the open flask is 3.330* and for the

packed flask is 3.040♦ If X is allowed to represent the

fraction of the ester decomposing by course (2), dihdn

1-X will represent the fraction following course three.

Observing that for each molecule of the ester following

course (2), it is seen that four molecules of products are

eventually obtained, fhe total pressure due to the portion

following this course would be 430?o , ah42the total pres­

sure resulting from the part following course (3) would be

2(1-X)P0 . The total pressure at the end of the reaction

would then be,

2?f * 4XP0 + 2(1-X)P0 •48-

from ihence,

Ooneidering the ease of the unpacked flask, Pf » 3.330Po ,

X ® 6,665

If the reactions are as postulated, approximately two thirds

of the ester would decompose according to course (2), and

one third by course (3). If the decomposition of one mol

of the ester is considered, two thirds of it will form

four thirds of a mol of carbon monoxide, and on® third w i n

form one third mol of carbon dioxide# The resulting ratio

CO*.00» would then be 4/3:l/3 or 4:1# The observed ratio

from the analysis of the gases is 69.2:17.2 or 4.02:1.

If similar calculations are carried out for the packed,

flask, it is found that X * 0.52, and 1 -x » 0.48. From this the 00:00a ratio should be 1.04:0.48 or 2.17:1. The

observed ratio from the analysis is 1.26:1, Qiere are

several passible explanations of the discrepancy. The

volume of gases available for analysis from the reaction

in the packed flask was 10.4 oo. This would prevent any

great confidence being placed in the analysis* The increase

in the surface/volume ratio may bring into prominence

another side reaction which gives rise to carbon dioxide.

The fact that the gases contain about the same relative

amounts of hydrogen, regardless of whether the reaction

is carried out in the packed or unpacked flask, suggests

that this reason is the correct one. It may be, of course,

that the mechanisms suggested do not at all give the true -49- eourse of the reaction.

It appears fairly well established that part of the ester deoeaposes by eourse (2). If the suggestion of Hurd and Bennet concerning the formation of toluene and benzole anhydride is considered, it is seen-that two mols of ester eventually give rise to five mols of products, if it is as­ sumed that benzole anhydride decomposes to give one mole­ cule of carbon dioxide and one molecule of benzophenone•

This would give a total change from two mols of ester to five mols of products, or a Pf/P0 ratio of 2.5. This would allow of an over all Pf/P© ratio between 2.5 and

4.0, for the total ehango in the reaction flask. From these considerations calculations may be made to show that the GO/GO * ratio:, should be 3.71 for the unyoked flask, and 2.12 for the packed flask. These results are not so different from those obtained by the first mechanism considered, but are not in as olose agreement with the observed ratio as the first.

It is, therefore, obvious that no definite oonelueions can be reached concerning the exact mechanism of the decomposition. It is seen, however, that the Cannizzaro reastion is reversible, that the earbon dioxide is pro­ duced by a reaction which is catalyzed by the surface of the reaction flask, and that, by wegsoheider*s test, the reactions giving rise to the earbon oxides are ef the same order klnetioally. A consideration of the kinetic data shows at once that the reactions involved are of the first order, since the time for a given fraction of the total pressure change to occur is independent of the initial pressure of the benzyl benzoate. In Figure V, where Z, the fraction of the total pressure change, is plotted against time, it may be seen that the curves representing runs with initial pressures varying from 80.8 to 146.6 mm., are identical for a num­ ber of points, and are very close together at all points.

This is one of the criteria for a first order reaction.

This conclusion is seen to be verified in Figures VI and

VII.

Curve $3) of Figure V, and curve (4) of Figure VI, shows that the apparent rate of the decomposition is . - ' , approximately doubled by increasing the surface/volume ratio. As was pointed out above, the reaction giving rise to carbon dioxide Is catalyzed. The relative amounts of carbon dioxide formed in the unpacked and the packed flasks are 17.8 and 37.5 respectively. The ratio/' is

1:8.14, which is approximately the ratio in which the velocity of reaction is accelerated by increasing the surface/volume ratio by fourteen times. It may be concluded that the effect of the surface in the unpacked flask is negligible. This leads to the conclusion that the reaction giving rise to carbon monoxide is homogeneous, and that the reaction giving rise to carbon dioxide in the unpacked flask is also homogeneous. This fact, considered with the discussion above concerning the analysis of the —Bl*

of the gaseous products of the deeompomltlon, seems to

©onfim the ©pinion that the surface/ volume ratio* In­ crease makes prominent a reaction which gives rise to

carbon dioxide, but which is quite negligible in the unpacked flask.

From the above considerations it nay then be said that benzyl benzoate decomposes by at least two simultaneous homogeneous unlmolecular reactions, followed by the

decomposition of some of the products, as benzaldehyde, benzoic anhydride, and.benzoic acid.

Since either of the reaction mechanisms suggested

could be unimoleoular, no ether conclusions are justified

as to the mechanism, except that 1ft benzole anhydride is

formed, the dissociation and rearrangement of the parts of the molecule must be very rapid, followed by the slow

homogeneous decomposition of benzoic anhydride, .

The differential equation describing the rates of

two simultaneous unimoleoular reactions is,

» k-jja-y) + k2(a-y)

whore y is the amount of reacting substance used up, and

a is the initial concentration of tho reacting substance,

k^and kr, are the velocity constants of the two reactions.

Integrating this equation and evaluating the constant of

integration, the value of K, where % = + kg, may be calculated by substituting experimental data into the

expression,

K " ; T loe53y’ -ss»

If X is made to represent the fraction of the reaction which has taken place at time t, the following relation is true,

y = aX.

Substituting this relation in the expression for the velocity constant,

K = I loga— j- % log

The Tables XII-XXI show X calculated as the fraction of the total pressure change involved. The following tabulations show the values of K as derived from the expression given above, using common logarithms.

1. Temperature » 500*0.

po = 80.8 Po » 146.6 t (sec.) K X 10- t K X 10'

128 3.90 61 : 2.95 245 3.72 121 • 3.23 305 . 3.64 179 5.41 421 3.68 241 3.53 598 3.75 301 3.68 717 3.83 360 3.72 835 3.66 420 5.71 #60 3.54 480 3V791 1080 3.38 540 1800 3.32 599 3,79, 1500 3.25 718 5,71- 1800 3.09 840 M 2 2700 2.74 969 ■■■ ■ W 9 1080 3,49 1200 - Svtl 1500 3 a r 1600 3.01 2700 2.57 2. Temperature = 510*0.

Po « 31.5 147.2 P©- 152.8 t K X 104 t K x lO4 t K x 104

20% 7.30 62 6.93 61 5.08 308 6.40 118 6.53 121 5:53 421 5.70 183 6.40 '■ 182 8;93 • 552 5.50 242 6.24 240 6.03 861 4.92 299 6 .15 300 6.15 1125 4.53 360 6.05 359 6.03 1740 4.06 420 5.86 480 5.98 - ' - 2640 3.68 483 5.68 480 5.87 639 5.60 540 5.70 660 5.66 599 5.65 780 5.12 780 5.12 900 5.84 840 5.05 1020 4.63 963 4.97 1140 4.50 1080 :4.64' 1440 4.01 1200 4.48 1800 3.67 1500 4.08 2700 2.93 1800 5.25 2700 3.16

3. Temperature « 5200C»

P0 » 36.1 PQ * 107.3

t K X 104 t K X 104

64 ; ■ 11; 25 62 6 #4 121 9 .1 7 ■..183. 7.48 184 9.08 183 7.80 240 8.80 250 8.08 301 a; 92 299 7.93 360 8,25 359 7.86 481 . 7.42 420 7.54 600 7:20 480 7.44 925 6:28 600 7.14 1200 6.00 720 6.70 1800 4.77 840 6.36 2700 3 .8 9 960 6.12 1080 5.83 1200 5.58 1500 5.17 1800 4.85 2700 3.58 It is seen that the value of K remains constant over a considerable portion of the deooBposltlon. The rapid, falling off of the the value of K may be caused by the relatively slow decomposition ef the benzeldehyde formed.

It appears that the pressure increase corresponds to a true first order reaction up to the point at which equi­ librium is reached between bensaldehyde and benzyl benzoate

From this point on the pressure changes are considerably less than would be required to maintain the constancy of K.

Because of the complicating consecutive reactions, it is unsafe to draw further conclusions, such as motivation constants^ temperature coefficients, etc,„ from the data available.

c# Miscellaneous experiments.

. Because of the possibility that benzophenone might be one of the end products of the decomposition of

benzaldehyde and benzyl benzoate, anjf experiment was made

to investigate its stability at the temperatures used

in the other experiments. The pressure increased 3,8 mm.

after heating at 500*0. for sixteen hours, with an initial

pressure of 97.3 mm. Thus it was shown to be quite

stable at the temperatures used. On© run was made with benzole a® 14, The results are shown In Table® X n i - X X I H . The data is shown graphically in FigureVIII, in which X, the fraction of the total pressure ®hango,is plotted against time. It is interesting to note that for the major portion of the reaction, X has a linear relationship with t, i.e#, the rate of the decomposition of benzoic acid in the Taper phase is independent of the partial pressure of the vapor. It decom­ poses then as a zero order reaction, and must of necessity be a wall reaction. Analysis of the gaseous products, showing them to be almost entirely carbon dioxide, and the fact that the Pf/P0 ratio is very nearly 2/1, show that the reaction must take the following course, C~%-C00H ■ C6H5 + 002. -ss-

Suamary.

a* Apparatus has been designed whieh is applicable to the dynamic investigation of reaction rates in the vapor phase involving high boiling liquids, in whieh the reaction may be accompanied by a change of pressure. The pressures are measured by means of a glass diaphragm, or

"click" gauge. The sample is introduced into the reaction flask by an electro-magnetic device. All of the apparatus to which the vapors may be exposed may be heated to pre­ vent condensation. The vapors come ix/contact with nothing but glass during the reaction period. The reaction flask is so designed that the gaseous products of the reaction are readily available for analysis.

b. The applicability of the apparatus was demonstrated by a study of the thermal decompositionof benzaldehyde, followed by a shout study of the thermal decomposition of benzyl benzoate. The reactions studied were found to be very complex, and the data difficult of interpretation.

©• Experimental results indicate that the decomposi­ tion of benzaldehyde proceeds according to the equations,

• Cft%-CHO «> CgHg + CO | a heterogeneous bimolecular reaction, and,

2. 2 CgHg-CHO » (C6$4)2 + Hg + 2 CO* a homogeneous reaction whose order could not be determined -w~

beeauae of tho conplioatlng factor of the Cannizzaro reaction,

5. 2 C6H$~CH0 * CeHg-CO-O-eBte^gHg, and its reverse* Evidence was found that the Cannizzaro reaction Is a himolecular wall reaction. The behavior proved to be too cmaple” to allow of a mathematical analysis.

d. Tho decomposition of benzyl benzoate appears to follow the reverse of the Cannizzaro reaction, ... . 1. CgBs^C^^Hg-CgHs = 2 C5H5-CHO, and at least one other reaction which gives rise to carbon dioxide. The subsequent complex decomposition of the benzaldehyde formed prevented the complete mathematical analysis of the results. Both reactions observed are kinetically unimolecular. A slight positive wall effect was observed with the reaction producing carbon dioxide.

Calculated values of K from, the equation,

K log " where X is the fraction of the total pressure increase at time t, were found to remain constant over the greater por­ tion of the reaction.

e. Benzophenone was found to be quite stable at 500*0.

f • Benzoic acid was found to decompose at 500®C aeeord- to the equation, OgEg-COOH * CftHg * C02.

The reaction appears to be heterogeneous ant of zero order. Bibliography.

(1) • Hiaahelwooa and Batehison, Proc. R07. Sod., 1925# A, 111, 380.

(2) Elzwhelmocl aa& Proc. Roy. soo., 1926, A, 115, 221.

(3) Raaaparger, ■ ■ Rev. Sol. Instnmnents, 2 , 738. (1931.)

(4) Hlnahelwood and Hutchison, Proo. Roy* Soo., 1927, A, 111, 245.

(5) Hlnshelwood, Proo. Roy. 800.* 1927, A, 114, 84.

(6 ) Raaaperger and WaddlAgton, JT. A.C.S., 55, 214. (1933.)

(?) Peytral, Bull. 800. oh to., 29, 44 (1921).

(8) Hurd, Pyrolysis of Carbon Compounds, Monograph Series, Ho. 51. Cheaioal Catalog Co. Page 239.

(9) Laohmaimi J.A.C.S. , 46, 720, (1924) .

(10) Haro do Heaptlime, J. Phys. radium, 9, 357-64. (1928). Referneoe taken from C.A. 23: 4408, C.A. '25: 4621. (11) Hurd, loo. 0ItPages 538-9.

(12) Smith and Taylor, J.A.C.S., 46, 1343. (1924).

(13) Soott, standard Methods of Analysis, 4th Ed. Vol. II. Chapters on Gas Analysis.

(14) Hempel’s Gas Analysis, Trans, of 3rd Ger. Ed. by L.M.Dennls. (Macmillan)

(15) Clowes and Coleman, Quantitative Analysis, 11th Ed.(Churchill, London) Chapters on Gas Analysis. (16) Gatteraan, Practical Methods of Organic Chemistry 3rd Ed., Trans. by Sehober and Babasinian, (Macmillan) Pages 899-300.

(17) Organic Synthesis, Vol II. Pages 5*8. Editted by J.B. Conant, (John Wiley and Sons.)

(19) Wegfleheider*e principle is discussed in,

Lerris, A System of physical Chemistry, Vol. I., 1916 Ed.. Page 449. (Longmans and Co.)

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