Steric hindrance

SN2 reactions require a rearward attack on the carbon bonded to the leaving group. If a large number of groups are bonded to the same carbon that bears the leaving group, the nucleophile's attack should be hindered and the rate of the reaction slowed. This phenomenon is called steric hindrance. The larger and bulkier the group(s), the greater the steric hindrance and the slower the rate of reaction. Table shows the effect of steric hindrance on the rate of reaction for a specific, unspecified nucleophile and leaving group. Different nucleophiles and leaving groups would result in different numbers but similar patterns of results. 
TABLE 1 Effects of Steric Hindrance upon Rates of SN2 Reactions 
 

Alkyl Group (ALK) Relative Rate of Substitution 
−CH3(small group) 30
−CH2CH3 (larger group) 1
−CH(CH3)2 (bulky group) 0.03
−C(CH3)3 (very bulky group) 0

SN2 reactions give good yields on 1° (primary) alkyl halides, moderate yields on 2° (secondary) alkyl halides, and poor to no yields on 3° (tertiary) alkyl halides.