Spontaneous Cleavage of gem-Diazides: A Comparison of the Effects of α-Azido and Other Electron-Donating Groups on the Kinetic and Thermodynamic Stability of Benzyl and Alkyl Carbocations in Aqueous Solution

The solvolysis reactions of ring-substituted benzylic and 1-propyl gem-diazides in water proceed by a stepwise mechanism through α-azido carbocation intermediates, which are captured by water to give the corresponding aldehyde as the sole detectable product. Rate constant ratios k_az/k_s(M^-1) for partitioning of these carbocations between reaction with azide ion and reaction with water, determined by analysis of azide common ion inhibition of the solvolysis reaction, decrease from 8600 M^-1 for the relatively stable carbocation 4-MeOC₆H₄CH(N₃)⁺ to 2 M^-1 for the highly unstable CH₃CH₂CH(N3)⁺. Rate constants k_s(s^-1) for reaction of these carbocations with solvent water, and equilibrium constants K_az= k_solv/k_az (M) for their formation from the corresponding neutral azide ion adducts (gem-diazides), were calculated from the experimental values of k_az/k_s (M^-1) and k_solv (s^-1) for solvolysis of the gem-diazides, respectively, using k_az= 5 x 10⁹ M^-1 s^-1 for the diffusion-limited reaction of azide ion with α-substituted benzyl carbocations. The polar and resonance Hammett reaction constants are ϱⁿ = ϱ^R = -3.8 for the equilibrium formation of the α-azidobenzyl carbocations from the gem-diazides and ϱⁿ= 1.6 and ϱ^R= 2.6 for their capture by water, respectively. These are slightly larger than the corresponding Hammett constants for the reactions of α-methoxy-α-methylbenzyl carbocations [XC₆H₄CCH₃(OMe)⁺], but considerably smaller than those for the reactions of α-methylbenzyl carbocations [XC₆H₄CH(CH₃)⁺]. This shows that the magnitudes of electron donation from an α-N₃ and an α-MeO group to the cationic benzylic carbon are similar, and that they are much larger than the magnitude of electron donation from an α-methyl group. An α-N₃ group provides ca. 4 kcal/mol less stabilization of the 1-propyl carbocation than does an α-EtO group, relative to the neutral azide ion adducts. There is no simple relationship between rate and equilibrium constants for the formation and reaction of carbocations stabilized by α-EtO, α-N₃ and α-(4-MeOC₆H₄) groups. The α-EtO group provides the greatest thermodynamic stabilization of the 1-propyl carbocation, but this is not expressed as a larger activation barrier for the nucleophilic addition of solvent; rather, the α-EtO-substituted carbocation is also the most reactive toward solvent. Possible explanations for the breakdown of these rate—equilibrium relationships are discussed.