The Substituent Effect. VIII. Solvolysis of m- and p-Substituted α-Methylbenzyl Chlorides

The rates of solvolysis of twenty-two m- and p-substituted α-methylbenzyl chlorides were determined in 80% aqueous acetone. The relative rates at 45 °C are not correlated linearly with σ0 or σ. The use of Brown’s σ+ improves the fit but the resulting correlation is still concave. An excellent correlation, logk⁄k0=−4.950(σ0+1.147ΔσR+), was obtained by applying the LArSR equation. The r value 1.15 suggests that the π-electronic contribution relative to the inductive contribution of −R para substituents at the transition states differs with reaction, and is more important in the present α-methylbenzyl than Brown’s t-cumyl solvolysis (r=1.00 by definition). A set of Δσ+ and σ+ corresponding to the substituent constants for unit r were calculated. The calculated σ+ values are in precise agreement with Brown’s primary σ+ values. The generality of the LArSR equation and some new substituent constants are illustrated.