3326
CHESTERT. CHMIELAND F. A. LONG [CONTRIBUTION FROM THE
DEPARTMENT O F CHEMISTRY,
Vol. 78
CORNELL UNIVERSITY]
The Hydrolysis of Esters of Some Substituted Benzoic Acids in Strongly Acid Aqueous Solutions1 BY CHESTERT. CHMIEL AND F. -4. LONG RECEIVED FEBRUARY 6, 1956 The hydrolysis rates of esters of several substituted benzoic acids have been found to agree with the Zucker-Hanimctt liypothesis which predicts that rates of acid-catalyzed reactions of uncharged molecules should vary linearly with the concentration of hydrogen ion when there is a water molecule in the transition state but should vary with the ho acidity function if the transition state does not include water. For the following esters the rates vary linearly with CH+: methyl benzoate at 90" in aqueous perchloric acid of from 1 to 4 -If; a-glyceryl monobenzoate and monoanisoate a t 90" in solutions Of from 0.04 t o 6 If aqueous perchloric acid; a-glyceryl esters of benzoic, anisic and 3,1,5-trimethoxybenzoic acids at 50 or (0"in aqueous sulfuric acid of from 1 to 10 M, In contrast, the hydrolysis rate of methyl mesitoate a t 90" in aqueous solutions of from 1 to 6 -11perchloric and sulfuric acids varies linearly with the ho acidity function, not with CH+. Since the unhindered esters undoubtedly hydrolyze by the A-2 mechanism with a rate-determining reaction of the conjugate acid of the ester and a water molecule, proportionalit!- of rate and C H + is the expected result. Methyl mesitoate probably hydrolyzes by the A-1, "unimolecular" mechanism to give an acylium ion; this should lead t o the observed proportionality between rate aud ha.
In several recent investigations of the mechanism of acid-catalyzed hydrolysis reactions, experiments on rates of reaction in concentrated aqueous solutions of mineral acids have been used as a diagnostic tool. The method stems from a proposal by Zucker and Hamrr~ett"~ that for an acid-catalyzed reaction of a neutral molecule to give a transition state which involves only an added proton, the rate will increase in proportion to the HO acidity function. If, however, the reaction involves in addition a water molecule in the transition state, it was proposed that the rate will more nearly vary in proportion to concentration of hydrogen ion. To apply this criterion to reactions catalyzed by aqueous solutions of strong acids it is necessary to make studies a t acid concentrations above about one molar since only then does the acidity as measured by -Ho differ significantly from log CH Since precise calculations of the effect of changing media on rates are not yet possible, the chief justification for the use of the Zucker-Hammett hypothesis is the experimental one that it appears to fit a variety of reactions whose mechanisms are fairly well established by other criteria. From this standpoint one of the particularly important reactions to consider is the acid-catalyzed hydrolysis of esters of carboxylic acids, since an extensive series of studies has led to the fairly definite conclusion that the normal mechanism for this reaction is of the A-2 type4J with the further simplification that in not too concentrated solutions of acid only a negligible amount of the ester is transformed into the conjugate acid. .I.
K'COOR H R'COOR
+H+
--f
+ HzO
H -R'COOR
= 1LI- = R'COOHa+
Ii'COOH2+ = R'COOH ~
-. .
Equilibriuni
__
(Id
+ ROH
Slow
(Ib)
+ H+
Fast
This mechanism leads to the rate law hydrolysis rate = kiCeater= k2C~+CeSt,,
R'COOR R'CO+
(IC)
' 1 1 \l-r)rk supported in p a r t b y a g r a n t from t h e .4tomic Energy Commission. ( 2 ) I,, Zucker and L . P. H a m m e t t , THISJOURNAL, 6 1 , 2791 (1939). (3) I,. P. H a m m e t t , "Physical Organic Chemistry," hIcGrarv-Hill Book Co , S e w York, S . Y., 1940, p. 275. (4) J . n E. D a y arid C . K 7 i i q o I d , 7 r i i i i 5 . I;~lrniliiy .Snr., 37, fiSG t'
K
I'rrds
u
IIllUl~ii.
\' ,
1!4.-J,
1)
;I;;.
R'CO+
+ H,O
=
+
ROH R'COOH?
'
SloW
(Ib'j
Fa51
(lb")
'A'. F.I I c D e v i t . .T. P h y . Colloi*i 829 (1931). , 1381 (1940) ( 7 ) AI. Duboux and A . de.Tousa, H e l i . Lhzftr. A ~ l a 23, (8) K P. R d l . A I. n o w d i n g and H.A . Nohlr. .I. ( ' h ~ wY o r , 3101:
(6,
F. A . Long, F. B. Dunkle and
C h e n i . , 66,
T n R i t l r i , " S t r i i r t i i r r an
