Question

Which of the following is the correct order of decreasing S_N² reactivity?
(a) R₂CHX > R₃CX > RCH₂X
(b) RCHX > R₃CX > R₂CH X
(c) RCH₂X > R₂CHX > R₃CX
(d) R₃CX > R₂CHX > RCH₂X.
(X is a halogen)

Answer 1

(c) : S_N² mechanism occurs as 

 

In S_N² reaction, in the transition state there will be five groups attached to the carbon atom at which reaction occurs. Thus there will be crowding in the transition state, and the bulkier the group, the more the reaction will be hindered sterically. Hence S_N² reaction is favoured by small groups on the carbon atom attached to halogens. So the decreasing order of reactivity of halide is 

RCH₂X > R₂CHX > R₃CX  

(primary) (secondary) (tertiary)