COMMUNICATIONS TO THE EDITOR
June 20, 1964
continues beyond 1.6 -IfHC104 is obscured by the incursion of the two-proton process. A similar situation prevails in the hydrolysis of 2-dimethoxymethylquino h e . The initial decline in kobsd, and hence in f S H +/ftr+, is somewhat steeper and persists into higher acid molarities. One can rationalize these decreases in f s H +/ftr with increasing HC104 molarity in terms of stronger solvation of the transition state than the ground state through solvation by the oxygen atoms of water molecules (cf. ref. 2 and 3 ) . However, the decline in fSH -/ftr+ is not spectacular, especially when viewed on a logarithmic scale. I n effect then, the assumption applied first by Hammett, that a ratio such as f ~ ~ + / +, fsuffers ~~ relatively little medium effect, is reasonably good in the instances of the hydrolyses of the dimethyl acetals of pyridine-2-aldehyde and quinoline-2-aldehyde. It was expected that an 8-t-butyl substituent on the 2-dimethoxymethylquinoline would substantially alter matters. According to molecular models, the N-H proton of the nitrogen conjugate acid ( i . e . , the ground state) is very effectively shielded from solvation. On the other hand, the main sites of developing charge in the transition state are only remotely shielded. On this basis two predictions are made: (1) In any particular medium, f S H - Ks"/ftr+ K s ~ *should t be greater for the 8-t-butyl than the unsubstituted quinoline. ( 2 ) The quantity fSH-/ftr - for the t-butyl derivative should decrease more than the corresponding ratio for the unsubstituted compound as mineral acid molarity is increased. I n other words, in the t-butyl compound, the ground state (i.e., SH+) would have less to lose in free energy of solvation, relative t o the transition state, as the activity of water is decreased. These predictions have been realized in full. Thus kobsd is appreciably greater in any particular medium for the 8-tbutyl derivative (note that the rate results are a t 40' for the t-butyl compound and a t 60' for the unsubstituted one). Furthermore, k o b s d , and hence fsH +/ftr +, declines by a factor of about 12-fold between 0.6 and 6 IIf HC104. I t is of interest t o note t h a t a Hammett plot (somewhat scattered) has a slope of about 0.65 in this instance. TABLEI1 RATECOXSTANTS FOR THE HYDROLYSIS OF 2 - D I M E T H O X Y M E T H Y L Q L J I X O L I X E ( A ) AND 8-t-BUTYL-2-DIMETHOXYMETHYLQUINOLIKE (B) HCIO,,
M
a
0.10 0.58 0.95 1.27 1.60 1.96 2 33 2.i1 3 11 3 54 3.99 4.60 5 09 5 72 6 51 7 60 9 42 Compound insoluble.
----
Compd. A
4.67 3.i2 3.51 ...
3 32 3 21 3.19 3.21 3.33 3 74 4.36 5.74 9 13 18.0 46.5 , . .
...
I O'kobad----Compd. B
a
12.41 10.01 9.13 7 36 , . .
5 25 4.80 4 11 3.44 3.20 1.91 1.60 1.36 1.11 2.03 106.8
2521
For the 8-t-butyl compound the one-proton process predominates over a much wider range of perchloric acid strength; i.e., the rate does not begin to turn up until about 7 M HC104. This is t o be expected since the two-proton hydrolysis should be less effected by the substituent, i.e., solvation of the N H proton should be roughly equal in the ground state and transition state of the two-proton process. While the situation just described is a contrived one it nevertheless allows the conclusion that the assumption that f S H + / f t r - for an A-1 process suffers little change with medium should be applied with care. Unfortunately, the results also demonstrate again that one cannot base mechanistic conclusions on merely the acidity dependence of a rate constant. They further saliently point out t h a t considerably more must be learned about the effect of medium upon the free energies of the ground and transition states of reactions catalyzed in strong acids before the interpretation of the acidity dependence of rate constants can once again be considered to be firmly based. Acknowledgment.-The authors wish to express their appreciation to the National Science Foundation and A$r Force Office of Scientific Research for financial assistance DEPARTMENT OF CHEMISTRY UXIVERSITY OF WASHINGTON SEATTLE5 , WASHINGTOX
ik' M SCHUBERT HOWARD BURKETT ALFREDL SCHY
RECEIVED APRIL 18, 1964
The Possibility of a Cyclic Mechanism for Acid-Catalyzed Ester Hydrolysis
Sir : The observation of Bell' that the rate of hydrolysis of ethyl acetate does not increase with acidity as rapidly as expected for a Hammett indicator2 has raised questions about the pKa of ethyl acetate3 and the involvement of water in the reaction.4 Recently Long has concluded that solvent deuterium isotope effects for ester hydrolysis are best fitted by five or more exchangeable hydrogens in the transition state,j whereas the accepted tetrahedral mechanism has only three such hydrogens6 We now wish to report evidence for a transition state involving protonated ester and two water molecules in a six-membered ring. This transition state has five exchangeable hydrogens, and quantitatively accounts for the water and acidity dependence that we observe. We have studied the rate of hydrolysis of ethyl acetate a t 25' in 11-79% sulfuric acid by the spectrophotometric and dilatometric techniques. The rate reaches a maximum a t 50-60% acid and decreases rapidly a t higher concentrations. To interpret this maximum, it is necessary to know the pK, of ethyl acetate, but none has been reported for an aliphatic ester.7 The concentration of protonated ethyl acetate as a function of sulfuric acid concentration was determined (1) R. P. Bell, A. L. Dowding, and J. A h-oble, J . Chem. Soc.,3106 (1955). (2) M. A Paul a n d F. A Long, Chem. Reu., 67, 1 (1957) (3) A I. Talvik and 1'. A . Palm, Z h . F i z . Khim., 33, 1214 (1959). (4) C . G. Swain and A . S . Rosenberg, J . A m , C h e m . S o c . , 8 3 , 2154 (1961). ( 5 ) P. Salomaa, 1,. L. Schaleger, and F. A . Long, i b i d . , 86, 1 (1964). ( 6 ) M. L . Bender, Chem. R e v . , 6 0 , 53 (1560). (7) E . M. A r n e t t , "Progress in Physical Organic Chemistry," Vol. I , S . G. Cohen, A. Streitwieser, Jr.. and R , W. T a f t , E d , , Interscience Publishers, Inc., N e w York, N . Y., 1563, p 325.
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SLOPE FOR EXCHANGE
1.84
