SN2 Displacement SNi Displacement Cl/ with Inversion with retention rAdde alme a sulfite ester a sufite ester Unlike primary alky l halides, when most tertiary alkyl halides react with simple nucleophiles, the rate at which the substitution reaction proceeds is proportional only to the concentration of the alky I halide, making the reaction first order. This type of first-order, nucleophilic displacement reaction is therefore termed an"S, 1"reaction(substitution, nucleophilic, unimolecular The mechanism for this reaction is best described as stepwise with the involvement of a carbocation intermediate. The rate limiting step for this reaction is the spontaneous breaking of the carbon-halogen bond to form the carbocation, and the reaction with the nucleophile is generally fast. Since only the alky l halide is present in the rate-limiting transition state, only alky l halide concentration will effect the rate SHI Reaction Rate-Limiting Since only one reactant is present in the rate- R-X +Nu limiting transition state the rate ll be R-Nu +independent of the rate of Since carbocations are involved, the reaction is prone to rearrangement, which may involve hydride shifts, alkyl group shifts or skeletal rearrangements.Unlike primary alkyl halides, when most tertiary alkyl halides react with simple nucleophiles, the rate at which the substitution reaction proceeds is proportional only to the concentration of the alkyl halide, making the reaction first order. This type of first-order, nucleophilic displacement reaction is therefore termed an "SN1" reaction (substitution, nucleophilic, unimolecular). The mechanism for this reaction is best described as stepwise with the involvement of a carbocation intermediate. The rate limiting step for this reaction is the spontaneous breaking of the carbon-halogen bond to form the carbocation, and the reaction with the nucleophile is generally fast. Since only the alkyl halide is present in the rate-limiting transition state, only alkyl halide concentration will effect the rate. Since carbocations are involved, the reaction is prone to rearrangement, which may involve hydride shifts, alkyl group shifts or skeletal rearrangements