Effect of Alkyl Group Size on the Mechanism of Acid Hydrolyses of Benzaldehyde Acetals

Hydrolyses of benzaldehyde acetals, PhCH(OR)₂, are specific hydrogen-ion catalyzed when R = methyl, n-butyl, but with secondary and tertiary alkyl derivatives, R = i-propyl, s-butyl, t-butyl, t-amyl, hydrolyses are general-acid catalyzed. The Brønsted α values for both secondary and tertiary alkyl groups are in the range:  α = 0.57−0.61. A simple iterative procedure was developed to estimate the individual rate constants for general-acid catalysis by the diacid and monoacid forms of succinic acid buffer. Plots of log k_obs (at [buffer] = 0 M) against pH are linear for the secondary and tertiary acetals, and plots of log k_H for the H₃O⁺-catalyzed reaction, ¹³C and ¹H chemical shifts, and ¹J_CH coupling constants against the Charton steric parameter, ν, for alkoxy groups are linear. The second-order rate constant, k_H, increases about 100-fold on going from R = Me to R = t-amyl, indicating the significant role of steric effects on reactivity. Steric effects upon ¹³C NMR chemical shifts and coupling constants indicate that increasing the bulk of the alkoxy moiety increases the electron density at the carbon reaction center, which accelerates hydrolysis. Analysis of the Jencks−More−O'Ferrall free energy diagram for the reaction provides support for concerted proton transfer and C−O bond breaking in the transition state for hydrolyses of benzaldehyde acetals with secondary and tertiary alkyl groups in contrast to specific hydrogen catalysis with R = Me and n-Bu. All our results are consistent with rate-determining acid hydrolysis of benzaldehyde dialkyl acetals to hemiacetal intermediates that breakdown rapidly to benzaldehyde.