Substitution Comparison

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 SNreaction 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-groups

Leaving groups


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. 


Mechanisms

To illustrate the mechanisms, let’s use an achiral alkyl halide as our substrate and hydroxide as our nucleophile.

SN2-mechanism

SN2 mechanism


SN2-transition-state

SN2 transition state


In an SNreaction, the nucleophile does a “backside attack” on the leaving group’s carbon, inverting chirality if present. SNreactions 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 mechanism


SN1-transition-states

SN1 transition states


In an SN1 reaction, the leaving group dissociates first, and the nucleophile attacks the resulting electrophile (the carbocation). SNreactions 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 energy diagram


SNreaction 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 energy diagram


SNreactions 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.