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Rate coefficient data are reported for the hydrolysis of benzyl penicillenic acid over pH range 0.5-7 4. The complex rate-pH profile, containing six changes in acidity dependence, is analysed in terms of the dissociation constants and relative reactivities of the tautomeric species present. The yields of the four known reaction products at various pH's provide quantitative support for the mechanistic interpretation.PENICILLENIC acids (I) have a structure tautomeric with that of penicillins. Although U.V. spectral studies demonstrate that the equilibrium concentration in solution is small, in the presence of a reagent such as a mercury salt which disturbs the equilibrium by reacting with the thiol group, the rate of equilibration can be shown to be relatively r a ~i d . ~? ~ Because of their ease of formation and high reactivity with nucleophiles in isolated systems: penicillenic acids have been proposed as intermediates to account for certain reactions of penicillins, such as the binding to proteins in connection with penicillin allergy2 and the formation of penillic acids (II).5 Fragmented studies indicate that the lifetimes of penicillenic acids as active intermediates in solution are relatively short.4 Under a variety of reaction conditions the compounds have been shown to rapidly hydrolyse to penillic acids (11) 5 penamaldic acids (111) ,ly6 penicilloic acids (IV) ,7 and 4-hydroxymethyleneoxazol-5(4H)-one (V) .5 Some confusion as to the stability has arisen due to the formation of more stable disulphides (VI) which have similar U.V. spectra but differ markedly in reactivity both for hydrolysis and reaction with nucleophiles.8 As no comprehensive hydrolysis investigation has previously been undertaken a study has been made of the hydrolysis of 2-benzylpenicillenic acid in the absence of oxidation over a wide range of experimental conditions to determine the factors influencing the rate of hydrolysis and the relative yields of the various reaction products. EXPERIMENTAL Materials.-Benzylpenicillenic acid (I) (R = PhCH,) 322 nm (log E 4.36, lit.,S 4.42)] and 4-(l-carboxyethylaminomethylene) -2-phenyloxazol-5(4H) -one (XI) [&= 349nm (log E 4.50)] were prepared as described previously.8 Benzylpenillic acid (11) (R = PhCH,) and benzylpenicilloic acid (IV) (R = PhCH,) were prepared from penicillin G as described in the literature.gg10 4-Isopropylidene-2-phenyloxazol-5(4H)-one ( X I ) [Amx. 312 nm (log E 4-54), cf. lit.ll Amax. 310 nm (log E 4-52)] was supplied by Dr. J. Preston of these laboratories and D-pencillamine by Koch-Light Laboratories Ltd. Satisfactory n.m.r. and i.r. analyses

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