CH3CH2−O−CH3
CH3−CH3|C−|CH3O−CH3
CH2=CH−CH2−O−CH3
Ph−CH2−O−CH2−CH3
SN2 reactions occur more readily at carbons that are less substituted. It is easier for the nucleophile to attack if there is little substitution around the C-leaving group bond. Therefore, the order of reactivity for alkyl halides is methyl>primary>secondary>>>tertiary.