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C. GARDNRR SWAIN
l7(11. 74
A Transition from Specific Oxonium-ion Catalysis to General Acid Catalysis1 HY C. GARDNER SWAIN 10, 1951
RECEIVED L)ECEMBER
It is suggested that reactions classified as examples of specific oxonium-ion catalysis in the region PH 2-7 may be made to exhibit general acid catalysis experimentally simply by using higher buffer concentrations. One can calculate the approximate buffer concentration required. This suggestion is tested and verified on a typical example of a specific oxonium-ion catalyzed reaction, a displacement reaction on an ethylene oxide. Experimental results exclude the Pedersen mechanism for general acid catalysis (involving the free oxonium ion of the ethylene oxide, Z . C . , the conjugate acid of the substrate, as an intermediate), but agree with a concerted mechanism in which the nucleophilic reagent attacks the carhon atom while an electrophilic reagent (either hydronium ion, acetic acid or cvaterj is protonating the oxygen atom (leaving grcup) of the ethylene oxide. In this mechanism there is no ionic intermetliatc, i.e., the substrate never becomes a charged ion a t any stage.
It is generally accepted’ that certain reactions arc subject to catalysis only by oxonium ions (e.g., hydronium ion in water solution), whereas others arc catalyzed generally by a wide range of un-ionized acid molecules as well. The first class, showing “specific oxonium-ion catalysis,” is considered to include reactions of ethylene oxide, hydrolysis of ethyl ~ r t h o f o r m a t eand , ~ formation and hydrolysis of acetal^.^*^ The second class, showing “general acid catalysis,” includes formation and hydrolysis of esters6 and semicarba~ones,~ hydrolysis of ethyl of ketoness and the hemi~ r t h o a c e t a t eenolization ,~ acetal decomposition illustrated by the niutarotation of glucose.9 TOtest the validity of the experimental basis for this classification we selected a typical example of “specific oxonium-ion catalysis” to see if it could be made to exhibit “general acid catalysis” simply by using a slightly higher buffer concentration. We chose the reaction of iodide ion with a substituted ethylene oxide, viz., epichlorohydrin, as one that could be easily and accurately followed in water solution a t 25’. aqueous 1.2f CH~--J~HCHZCI___f ICH&HCH?Cl ‘ \ / buffer 0 OH
Displacement reactions on ethylene oxides have been reported to be catalyzed by hydronium ion but not by undissociated acids such as acetic acida3For example, doubling the concentration of acetic acid in 0.01 .J1acetic acid-0.02, 0.05 or 0.1 i U sodium acetate buffers had no effect on the second-order rate constant for reaction of acetate ion with epichlorohydrin. The highest concentration of acetic (1) This is paper IX in t h e series “Concerted Displacement Reacrions.” For V I I I , see C . G . Swain a n d J. F. Brow-n, J r , ‘Tms ]OURNAI. 74, 2638 (1952). This work was supported by the Office o f Naval Research under Contract N5ori-07838, Project XR-03-108. ( 3 ) K. J . Laidler, “Chemical Kinetics,” McGraw-€f111 Book C o . . Inc., New York, S , l’., 1050, Chap. 10; I,. P. Hammett, “Physical Organic Chemistry,” McGraiv-Hill Book C o . , I n c . . S c w York S Y , , 19 10, p. 220, 241, 275. c31 J. X. Bronsted. SI. Kilpatrick and 31. Kilpatrick, THISJOIJRNII.. 61, 428 (1929). (41 J. S . BrBnsted and n’. 12, R Wynne-Jones. I ’ r n i i s F a r n d n y Sor , as, 59 (1929). ( 5 ) J . N. Brdnsted and C. Grove, THISJ O U R X A L , Sa, 1394 (1930); A. J . Deyrup, i b i d . , 66, 60 (1934). (6) A. C . Rolfe and C. N , Hinshelwood, Truns. F a r a d a y SOL.,30, 983 (1934). ( 7 ) J. B. Conant and E’. D. Bartlett, T H I S J O U R N A L , 64, 2881 (1!132.) (8)H . bl. Dawson and E. Spivey, J . Chcm. Sac., 2180 (1930). (9) J. N. Bronsted and E. -4. Guggenheim, T H I S J O U R N A L , 49, 2554 t193ij
acid used with each concentration of sodium acetate was 0.02 M . To understand why only “specific oxonium-ion” catalysis was observed, we first calculated the ai)proximate concentration of undissociated acetic acid which would be necessary to double the rate of displacement reactions on epichlorohydrin. The experimental value of c in log r t m c =
c
log ru,jo
where rHo& and Y H ~ Oare~ reactivities of acetic acid and hydronium ion relative to water is 0.44 for mutarotation of glucose a t 18’ (log YHJJ+ = 3.2) and 0.31 for enolization of acetone a t 25’ (log m30T= 0.4js1” From the average value of 0.37 i. 0.0‘7, log YHO.4c can be calculated to be 1.6 + 0.3 for epichlorohydrin (log Y H , O - = 4.3j. Thus even assuming no important effect on Y H O A ~from changing medium or dielectric constant or concentration of acetic acid, the concentration of acetic acid needed to double the rate would be 1.2 111. In view of the variation in c and the possible association of acetic acid in concentrated solution, the necessary niolarity might easily be as high as 4 X. To keep catalysis by hydronium ion below the negligible value of 1% of the total rate, the pH must be a t or above 4.3 - log 0.02 - log 55 = 4.3. ;iccordingly we selected 4 : I acetic acid-sodium acetate buffers (QH 4.3). We expected the reaction to be accelerated about two- to fourfold on changing 1 sodium acetate to -4 from 0.4 111 acetic acid-0.1 A X acetic acid-1 sodium acetate. Results.-The relative initial rates, given in Table I, bear out this prediction. The rate of reaction of iodide ion with epichlorohydrin was accelerated by a factor of 2.2 (cj’. runs 4 and 3 ) by the added acetic acid. TABLE 1 REAC,IIOSO F 0.150 -11 SODIGM IODIDEIVITII 0.129
