Which of the following is the correct order of decreasing SN2 reactivity? (X is a halogen)
- RCH2X > R2CHX > R3CX
- R3CX > R2CHX > RCH2X
- R2CHX > R3CX > RCH2X
- RCHX > R3CX > R2CHX
In SN2 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, SN2 reaction is favoured by small groups on the carbon atom attached to halogens. So the decreasing order of reactivity of halide is
(primary) > (secondary) > (tertiary)
RCH2X > R2CHX > R3CX
The correct option is A.