SN2 Effect of Substrate

S_N2 Substrate Effects

S_N2 mechanism occurs in one step in which the nucleophile attacks the alkyl halide from the backside of the C-LG bond.  When a nucleophile attacks a methyl or primary halide it does not encounter much steric hindrance (i.e. the Nu can easily attack the carbon without bumping into too many other groups).

On the contrary, if a nucleophile attempted to attack a tertiary halide it would encounter severe hindrance and be pushed away.

• methyl > 1^o > 2^o > 3^o (Sterics are important here, see also β-effect)
• Nucleophile sterics and electronics important.
• Rate=k[Nu][R-LG]
• Stereo chemical inversion (Walden Inversion) resulting from backside attack
• Favors polar aprotic solvents