1st Order Nucleophilic Substitution Reactions, i.e., SN1 reactions3 alkyl halides are essentially inert to substitution by the SN2 mechanism because of steric hindrance at the back side of the a-carbon.
Despite this, 3 alkyl halides do undergo nucleophilic substitution reactions quite rapidly , but by a different mechanism, i.e., the SN1 mechanism.
SN1 = Substitution, Nucleophilic, 1st order (unimolecular).
SN1 reactions obey 1st order kinetics, i.e., Rate = k[RX].
The rate depends upon the concentration of only 1 reactant, the alkyl halide-not the nucleophile
The order of reactivity of substrates for SN1 reactions is the reverse of SN2
3 > 2 > 1 > vinyl > phenyl > Me°
R3C-Br R2HC-Br RH2C-Br CH2=CH-Br -Br H3C-Br increasing rate of SN1 reactions