Question

consider the following for both SN1 and SN2 reaction conditions:

The effect of the structure, i.e. compare:

(a) 1^o , 2^o and 3^o halides. Unhindered 1^o vs. hindered 1^o halides

(b) an allylic halide vs. a 3^o halide

c) simple 3o vs. a complex 3^o halide

Answer 1

SN1 reaction proceeds with the formation of a carbocation intermediate. Hence higher the stability of carbocation, higher is the reactivity.

SN2 reaction proceeds via the back side attack by the nucleophile. Hence less substituted alkyl halides more reactive towards SN2 reaction.

(a): The order of reactivity for SN1 reaction is: 3^o > 2^o > 1^o

Because 3^o alkyl halide forms more stable carbocation due to hyperconjugation and +I effect of the three alkyl groups, where as 1^o alkyl halide is least stable due to hyperconjugation and +I effect of only one alkyl group.

The order of reactivity for SN2 reaction is: 1^o > 2^o > 3^o

Because 1^o is least substituted where as 3^o is most substituted.

Due to back attack, SN2 reaction requires least hindrance possible.

Hence order of reactivity for SN2 reaction is: Unhindered 1^o > hindered 1^o halides

(b): #SN2: Since allylic halide is a primary halide, it is more reactive towards SN2 than a 3^o halide.

#SN1: Both the allylic halide and 3^o halide give SN1 reaction. The reactivity of allylic halide towards SN1 can be either higher or lower than that of the 3^o halide. The reactivity depends on the alkyl groups attached to the double bond in allylic halide.

(c): Complex 3^o halides have more steric hindrance than simple 3^o halides. Hence simple 3^o halides are more reactive towards SN1.