a. Which reacts faster in an SN² reaction?

b.Which reacts faster in an E1 reaction?

c.Which reacts faster in an SN¹ reaction?

SN² reaction is a nucleophilic substitution reaction where the Nucleophilic attack and bond breaking between carbon and electronegative group happens simultaneously through a 5 coordinated transition state. In this reaction, the rate equation is second order having both the concentration of nucleophile and alkyl bromide.

A reaction mechanism with an experimental compound is given below,

E1 reaction is an elimination reaction where the base attacks the acidic H to generate carbocation and finally an alkene after the breaking of alkyl halide bond. Stability of carbocation is the driving force of E1 reaction.

A reaction mechanism with an experimental compound is given below,

SN¹ reaction is a nucleophilic substitution reaction which form carbocation in 1^st step before the breaking of C-halogen bond. It shows a carbocation like transition state.

A reaction mechanism with an experimental compound is given below,

The less bulky is the alkyl halide, the faster will be the SN² reaction due to the less steric hindrance during ack side attack. The 1^st structure is primary bromide whereas the 2^nd one is secondary bromide. So, the 1^st one undergoes SN2 reaction faster.

E1 reaction is a two-step reaction mechanism where the 1^st step depends on the formation of carbocation breaking the C-X bond (X= halogen). The more polar is the bond, the less energy is needed to break it. For this reason, the 1^st compound undergoes E1 reaction faster as C-I bond is more polar than C-Br bond. Here it is the only rate determining step and both produces tertiary carbocation.

SN¹ reaction also stabilised through a stable carbocation intermediate. The 1^st one is a secondary alkyl bromide whereas the 2^nd one is a tertiary alkyl bromide. So, the 2^nd one will undergo faster reaction as tertiary carbocation is more stable than secondary carbocation.