Benzylic carbon, nucleophilic substitution azide ions

Nucleophilic Substitution at Benzyl Derivatives. The sharp break from a stepwise to a concerted mechanism that is observed for nucleophilic substitution of azide ion at X-l-Y (Figs. 2.2 and 2.5) is blurred for nucleophilic substitution at the primary 4-methoxybenzyl derivatives (4-MeO,H)-3-Y. For example, the secondary substrate (4-MeO)-l-Cl reacts exclusively by a stepwise mechanism through the liberated carbocation intermediate (4-MeO)-T, which shows a moderately large selectivity toward azide ion ( az/ s = 100 in 50 50 (v/v) water/ trifluoroethanol). The removal of an a-Me group from (4-MeO)-l-Cl to give (4-MeO,H)-3-Cl increases the barrier to ionization of the substrate in the stepwise reaction relative to that for the concerted bimolecular substitution of azide ion. The result is that both of these mechanisms are observed concurrently for nucleophilic substitution of azide ion at (4-MeO,H)-3-Cl in water/acetone solvents. These concurrent stepwise and concerted nucleophilic substitution reactions of azide ion with (4-MeO,H)-3-Cl show that there is no sharp borderline between mechanisms for substitution at primary benzylic carbon, but instead a region of overlap where both mechanisms are observed. 

For the recombination of a-phenylethyl cations with strong nucleophiles, Richard and Jencks (1984a,b,c) obtained good Y-T correlations with the same r value of 1.15 as observed for the solvolysis (Fig. 35). The p value of -2.7 for the bimolecular substitution reaction of azide ion with 1-phenylethyl derivatives is significantly more positive than the value of p = -5.7 for the solvolysis reaction. This shows that there is a smaller development of positive charge in the transition state for the reaction of azide ion than for solvolysis. It is consistent with a coupled concerted reaction with a transition state in which positive charge development at the benzylic carbon is neutralized by bonding to azide ion. 

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