The SN2 reaction is a one-step bimolecular substitution that occurs between a nucleophile and a molecule with a methyl, primary, or secondary leaving group. The SN1 reaction is a two-step unimolecular substitution between a molecule with a secondary or tertiary leaving group.
The SN2 reaction
The SN1 reaction
Has no intermediate
Has an intermediate
Prefers leaving groups that are not sterically hindered
Prefers leaving groups that are sterically hindered
Reaction rate depends on the concentrations of both the nucleophile and the substrate
Reaction rate depends solely on the concentration of the substrate
Prefers polar aprotic solvents
Prefers polar protic solvents
Produces a racemic mixture
The SN2 reaction prefers leaving groups that aren’t sterically hindered because the substitution occurs through a nucleophilic backside attack; more R-groups means more steric hindrance.
The SN1 reaction prefers leaving groups with more R-groups attached because the rate-determining factor is the carbocation produced by leaving-group dissociation; R-groups stabilize carbocations through hyperconjugation.
To illustrate the mechanisms, let’s use an achiral alkyl halide as our substrate and hydroxide as our nucleophile.
SN2 mechanismIn an SN2 reaction, the nucleophile does a “backside attack” on the leaving group’s carbon, inverting chirality if present. SN2 reactions only have one transition state, and it has partial bonds between the nucleophile and carbon and between the carbon and leaving group. The nucleophile and carbon both have partial negative charges.
In an SN1 reaction, the leaving group dissociates first, and the nucleophile attacks the resulting electrophile (the carbocation). SN1 reactions have two different transition states. The first one (T.S. 1) shows the dissociation of the leaving group with a partial negative on the leaving group and partial positive on the carbon. The second (T.S. 2) shows the bonding of the nucleophile to the carbon with a partial negative on the nucleophile and a partial positive on the carbon.
SN2 reaction diagrams have one single peak because there is only one transition state. The more sterically hindered the leaving group, the greater the energy requirement is to reach the transition state.
SN1 reactions have two transition states and an intermediate between them. Transition state 1 (TS1) is higher energy than TS2 because it leads to the formation of the carbocation. Carbocation formation is the rate-determining step. The less stable the carbocation, the higher in energy both TS1 and the intermediate are.
P.S. Check out my video on how to determine if a mechanism will proceed through SN2, SN1, E2, or E1 using the BIG DADDY FLOWCHART.