Leading to Substitution at a Carbon Atom

Routes Leading to Substitution at a Carbon Atom. In many instances of substitution at a carbon atom it is found that if a homologous series is arranged in the order of increasing chain branching at the seat of reaction, the total rate of reaction passes through a minimum. Thus, for the hydrolysis of alkyl bromides with hydroxide ion, we have the following relative rates of reaction  

With methyl and ethyl bromide the rate of reaction is principally proportional to the concentration of hydroxide ion and of the halide. With isopropyl bromide no simple mathematical relationship exists, and with Jcrf-butyl bromide rate of reaction is dependent only upon the concentration of the halide. At this point in the series, then, the rate of a substitution reaction at a saturat ed carbon atom is completely independent of the concentration of the nucleophilic reagent (hydroxide ion). 

Substitution at a carbon Nucleus, bimolecular, kinctically 2nd ordi r therefore,, S N 2 

Since both paths lead to substitution at Die nucleus of a carbon atom, they are called nucleophilic substitutions. They are differentiated by the symbols and %2. This hypothesis at least correlates the facts of alkaline hydrolysis, since the increasing bulk of the alkyl groups would hinder approach of the hydroxide ion ( Sy2) and stabilization of the car-bonium ion (S yl) would be favored by hyperconjugation. 

---