R group plays vital role on the rate of reaction .
R group may have steric, electronic and neighboring group effect.
Steric hindrance may slow the rate of reaction, as nucleophile adds to the carbon centre in the rate determining step.
In the transition state, the incoming nucleophile(Nuc) and leaving group(LG) are 90° to the R group. Larger R groups can result in increased strain leading to slower reaction rates.
Bulkier R Group increase steric hindrance
A bulkier adjacent group may deflect the trajectory of the nucleophile away from the linear approach.
Sterained Transition state due to adjacent group steric effect
electron withdrawing nature of groups having sp2 carbon(vinyl and phenyl)makes the adjacent carbon more electrophilic and hence reactivity towards nucleophile increases.
Example:
Higher rate for nucleophilic substitution at allyl and benzyl system.
