Competltlve Reactions in the Mechanistic Studyof Catalytic Reactions

5., s. JEWUR; R. SWAMINATHAN & J. C. KURIACOSE

Department of Chemistry, Indian Institute of TechnologyMadras 600036

Received 16 October 1975; accepted 15 December 1975

Acetic acid reacts over ferric oxide surface givingacetone, carbon dioxide and water. The use of corn-petitive reaction technique with the esterification ofacetic acid with alcohols as the competitive reactionto investigate the mechanistic model- Langmutr-Hinshelwood or Rideal-Eley for ketonization isillustrated.

MOST of the bimolecular catalytic reactionsfollow the Langmuir-Hinshelwood mechanism-

where the interaction of two adsorbed species (G-S)takes place giving rise to products (P) as shownbelow:

G G1 1 I I I I

2G+ -5-5- --->- -5-5- --->- -5-S- + P

However, reactions such as the hydrogenationof olefins and the conversion of ortho- and para-hydrogens are known to obey a Rideal-Eley mecha-nism--".

G GI 1 I I I 1

2G+ -5-5- -- -5-5- --->- -5-5- + P

A knowledge of the mechanistic model for hetero-geneously catalysed chemical conversions is veryuseful in the elucidation of the complete mechanismof a particular reaction. .

The technique of competitive kinetics+! hasbeen made use of to deduce the mechanism of thesurface reaction in the ketonization of acetic acidon iron oxide. The present note reports on thelimitations of this technique in determining theorder of the surface reaction. The esterificationof acetic acid by an alcohol has been chosen asthe competing reaction for the ketonization ofacetic acid under identical conditions. Studieson the mutual inhibition of the reaction of aceticacid and alcohol indicate that both the moleculescompete for the same surface of the catalyst. Therate of formation of ester shows a maximum ata definite partial pressure of acid or alcohol.

If the esterification followed a Langmuir-Hinshel-wood mechanism, the rate expression (1) will giveat the maximum for ester formation.

v - k hPAX bMPM- est. (bAPA+bMPM)2

where

••• (1)

bA PMbM = PAwhere band P are the adsorption coefficient andthe partial pressure respectively. On the otherhand, if the Rideal-Eley mechanism is operative

••• (2)

N·dTE,S

CATALYST, 7,5 9

n/VIPEJlATURIj, 375 'r

CONTACT TI/VIE, 0.76 SEe0-8'~ s~~:::•...:.:~tu0..• 0-,~'"~;::~~

.0 L-H NOOEL, bAlbN =1·5

•• L· H /VIOCEL,.w: IbM. 2.25

•o R-E MOOEL OA/ON = 7·5

• R-E NOOEL bAION =2.15

50% 700%CH3COOH

Fig. 1 - Ketonization of isopropanol and Langmuir-Hinshe-lwood (L-R) and Rideal-Eley (R-E) models

CATALYST, 7.59TENPERATURE,31S't

CONTACT ro«, 0.16SEe.•0.8.,

s~.,:2~\<.0,.•..u0 ~;;j:>!oJ.:.;::<{...'"It

~ L -H MODEL, bA/ON.'

A L -H MODEL, OA/O,", = :E

D R-E ,",OOEL OA/b.'/ =,, R-E NOOEL bA/OM, 15

o EXPERINENTAL PLOT

o~ ~ ~ ~ ~100'10 50'/. 100,/.

CH30H CH3COflH

Fig. 2 - Ketonization of methanol and Langmuir-Hinshel-wood (L-H) and Rideal-Eley (R-E) models

for the esterification the rate expression (3) providesthe condition for maximum as

V - k ·bAPA XPM k bMPMXPA- est. b P +b P or est. b P b

A A M M A A+ MPM

bA = [PM]2bM PANow, four theoretical curves can be constructedu~ing .the relative values of the adsorption coeffi-cients In the rate expressions 5 and 6 for ketonization.

V - k '( hPA J2i - keton, b P +b P

A A M ~IandV. - k bAPA2

• - keton, b P +b PA A M M

... (3)

. .. (4)

... (5)

..• (6)

279

INDIAN J. CHEM., VOL. HA, APRIL 1976

A comparison of the experimental results for keto-nization with the theoretical curves permits a choiceof the model for the ketonization reaction.

This method, however, has its limitations. Thereactant involving two of its molecules must beable to undergo a competitive reaction with anothersubstrate. If the ratio of the adsorption coefficientof the original reactant to the competing reactantis less than 3 according to the Langmuir-Hinshelwoodmodel for the esterification, the theoretical plotsfor the two mechanisms do not differ sufficientlyto permit an unambiguous choice.

Isopropyl alcohol and methyl alcohol for~ esterswith, acetic acid and therefore compete with theketonization reaction. The adsorption coefficientof acetic acid relative to isopropanol from theLangmuir-Hinshelwood model for esterification isless than 3 and the results plotted in Fig. 1 revealthat with this combination it is not possible todraw any unambiguous conclusions regarding themechanism of the ketonization reaction. On theother hand the adsorption coefficient of aceticacid with respect to methanol is more than 3 andone can see- from the results plotted in Fig. 2 thatthe ketonization reaction responds unambiguouslyto a Langmuir-Hinshelwood model. Thus one canconclude that the surface reaction of acetic acidon iron oxide follows a Langmuir-Hinshelwoodmechanism.References1. LAIDLER, K. J., Catalysis, Vol. I (Reinhold, New York),

1954, Ch. 4.2. RIDEAL, E. K., Proc. Cambridge Phil. Soc., 35 (1939), 130.3. COUPLER, A. & ELEY, D. D., Discuss. Faraday Soc., 8

(1950), 172.4. KURIACOSE, J. C., J. scient. indo Res., 20 (1961), 82.5. JUNGERS, J. C., BALACEANU, J. C., COUSSEMANT,F.,

ESCHARD, F., GIRAUD, A., HELLIN, M., LEPRINCE,P. &LIMIDO, G. E., Cinetique chimique applique(Societe desEditions Technip, 2 rue de Lubeck, Paris), 1958, 415.

Kinetic Isotope Effect in Chromic AcidOxidation of Acetophenone

A. A. BHALEKAR, R. SHANKER & G. V. BAKOREDepartment of Chemistry, University of Udaipur

Udaipur 313001

Received 24 March 1975; accepted 30 May 1975

The observed kinetic isotope effect (=2·13) in theoxidation of PhCOCH3 and PhCOCH.D by chromicacid is expfafned by assuming a primary kinetic isotopeeffect (= 5·0) and secondary kinetic isotope effect(=1·66).

WATERS and coworkers! in the oxidation ofcyc1ohexanone by chromic aci? have shown

that the reaction involves participation of an enolintermediate. Their conclusions are based on theprimary kinetic isotope effect kH/kD = 5·0. ~nd !hesolvent isotope effect kD.O/kH.o= 4-5. Participationof the enol intermediate has also been demonstratedby Rocek and Riehl'' in the chromic acid oxidationsof isobutyrophenone and 2-chlorocyc1ohexanone. We

280

TABLE 1 - RATE CONSTANTSFOR THE CHROMICACID ANDOXIDATION OF ACETOPHENONE AND ACETOPHENONE-<x-dAT 50°C ([Cr(VI)] =5·5 x 10-3M; [HCIOJ=4'05M; [PhCOCH.]=0·102M; [PhCOCHaD]=0'093M; solvent: 50% AcOH (v/v)}

Substrate 10'kl/

[Ketone]litre mole'?

sec-l

PhCOCH3PhCOCHaD

87·7237-61

8·604·04

*Mean of triplicate runs.

now report in this note our observations on bothprimary and secondary kinetic isotope effects in theoxidation of acetophenone by chromic acid.

The reagents used were either chemically pureor were purified using conventional methods.'Deuterated acetophenone (ex-d) was prepared by themethod of Jones et al.s PMR· analysis showedonly 33% deuteration suggesting the formation ofC6H5COCH2D.

Kinetic measurements - The reaction was followedby quenching the aliquots at different time inter-vals with a slight excess of acidified ferrous ammo-n.iu~ sul~ate (AR grade) s~lution and then estimatingtitrimetrically the ferrous IOns, left unreacted againststandard K2Cr207 using barium diphenylamine sul-phonate as the indicator.

An apparent primary kinetic isotope effect, kH/kr>= 2·13, was calculated from the data given in Table1. In the 'case of acetophenone there will be anstatistical effect in the sense that removal of thethree hydrogen atoms of -CHa group is equallypossible. However, in the case of deuterated ana-logue the rate would include the possibility ofremoval of H as well as D from -CHzD group. Butthe probability of removal, of H is twice that of D:Further because of the presence of C-D bond therate at which C-H fission would take place in the.deuterated sample will be less than that for C-Iffission in ~cetophenone (secondary kinetic isotopeeffect kH/kH). The rate of oxidation of dueteratedacetophenone would, therefore, be equal to 2k~+kD.as shown below: '

3kH

PhCOCHa ky) PhCOCHzOH

, ,/zk~PhCOCH2D >PhCOCHD(OH)

In the case of chromic acid oxidation of cyclohe-xanone, Waters and coworkers- have reported thevalue of primary kinetic isotope effect kH/kD to beequal to 4-5. Assuming kH/kD= 5 in the presentcase also, one can calculate the secondary kineticisotope effect kH/k~ = 1·66 from the relation:

3kH/(2k~+kD) = 8·6/4·04 = 2·13. A secondary kinetic isotope effect of this magni-

tude has also been reported by Be1l4• Thus the observ-ed rate of chromic acid oxidation of PhCOCH2D