S_N2 rate and structure

For reactions that proceed by the S_N2 pathway, the rate of the reaction depends on the nature and concentration of BOTH the nucleophile and the halide. 
 
The rate of a one-step reaction depends on the energy of the transition state because this determines the activation energy for the reaction.  Any factor that increases the energy of the transition state decreases the rate of the reaction.

The energy of the transition state for an S_N2 reaction depends on the structure of the halogenoalkane undergoing substitution. If the carbon bearing the leaving group is more highly substituted by groups other than H, the activation energy is higher and the rate is slower.

More large groups increase the energy of the transition state because of repulsive forces between these in the more crowded transition state.

As shown below the rate of an S_N2 reaction is fastest for primary halides.
 

	>		>
CH3CH2Br  (1°)	>	(CH3)2CHBr  (2°)	>	(CH3)3CBr (3°)
Nucleophile attacks at carbon bearing the green dot.