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.
Characteristics
The SN2 reaction | The SN1 reaction |
One step | Two steps |
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 |
Inverts stereochemistry | Produces a racemic mixture |
Leaving groupsThe 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.
Mechanisms
To illustrate the mechanisms, let’s use an achiral alkyl halide as our substrate and hydroxide as our nucleophile.
SN2 mechanism
SN2 transition stateIn 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. 
SN1 mechanism
SN1 transition statesIn 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.
Reaction Coordinate
SN2 energy diagram
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 energy diagram
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.
