Illustrated Glossary of Organic Chemistry

Steric hindrance: The higher energy price (and slower reaction rate) due to the approach of larger atoms or groups in a chemical reaction, compared to a similar reaction involving smaller atoms or groups. In an S_N2 reaction, increasing the number and/or size of alkyl groups on the carbon bearing the leaving group (LG) causes more steric hindrance to backside attack, leading to slower reaction rates:

SN2 reaction rate:		>		>		>>>
Carbon bonded to leaving group:	Methyl carbon		1o carbon		2o carbon		3o carbon
Steric hindrance to backside attack:	Least		Slight		Modest		Severe