Ch. 7 - Substitution ReactionsWorksheetSee all chapters
All Chapters
Ch. 1 - A Review of General Chemistry
Ch. 2 - Molecular Representations
Ch. 3 - Acids and Bases
Ch. 4 - Alkanes and Cycloalkanes
Ch. 5 - Chirality
Ch. 6 - Thermodynamics and Kinetics
Ch. 7 - Substitution Reactions
Ch. 8 - Elimination Reactions
Ch. 9 - Alkenes and Alkynes
Ch. 10 - Addition Reactions
Ch. 11 - Radical Reactions
Ch. 12 - Alcohols, Ethers, Epoxides and Thiols
Ch. 13 - Alcohols and Carbonyl Compounds
Ch. 14 - Synthetic Techniques
Ch. 15 - Analytical Techniques: IR, NMR, Mass Spect
Ch. 16 - Conjugated Systems
Ch. 17 - Aromaticity
Ch. 18 - Reactions of Aromatics: EAS and Beyond
Ch. 19 - Aldehydes and Ketones: Nucleophilic Addition
Ch. 20 - Carboxylic Acid Derivatives: NAS
Ch. 21 - Enolate Chemistry: Reactions at the Alpha-Carbon
Ch. 22 - Condensation Chemistry
Ch. 23 - Amines
Ch. 24 - Carbohydrates
Ch. 25 - Phenols
Ch. 26 - Amino Acids, Peptides, and Proteins
Johnny Betancourt

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-groupsLeaving 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 mechanism

SN2-transition-stateSN2 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-mechanismSN1 mechanism

SN1-transition-statesSN1 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-diagramSN2 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-diagramSN1 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.


Johnny Betancourt

Johnny got his start tutoring Organic in 2006 when he was a Teaching Assistant. He graduated in Chemistry from FIU and finished up his UF Doctor of Pharmacy last year. He now enjoys helping thousands of students crush mechanisms, while moonlighting as a clinical pharmacist on weekends.