Nucleophilic substitution reactions are essential processes in organic chemistry that occur when one nucleophile (A chemical species that donates an electron pair to create bonds is known as a nucleophile. Any molecule or ion that has at least one pi bond or a free pair of electrons can function as a nucleophile.
SN1 and SN2 Reactions
Lewis bases are nucleophiles because they donate electrons.) in a chemical molecule exchange out for another. Based on their processes, these reactions are divided into two categories: SN1 (Substitution Nucleophilic Unimolecular) and SN2 (Substitution Nucleophilic Bimolecular).
SN1 Reaction (Substitution Nucleophilic Unimolecular)
Mechanism
The SN1 reaction is a two-step process:
1. Formation of Carbocation (Rate-Determining Step):
The leaving group (LG) departs, forming a carbocation
This step is slow and determines the reaction rate.
2. Nucleophilic Attack:
The final product is formed when the carbocation is attacked by the nucleophile.
o Racemisation in chiral compounds (Molecules with a non-superimposable mirror image are called chiral compounds. This indicates that even after rotating, translating, or altering its conformation, the molecule cannot be positioned on top of its mirror copy.) can result from an attack from either side since the carbocation is planar.
Kinetics and Rate Law
The rate of reaction depends only on the concentration of the substrate.
Rate = k [Substrate] (first-order reaction).
Characteristics of SN1 Reaction
1. Favored by tertiary carbons: More stable carbocations are formed (tertiary > secondary > primary).
2. Polar protic solvents (e.g., water, alcohols) stabilise the carbocation, enhancing the reaction.
3. Rearrangements possible: The intermediate carbocation may rearrange to form a more stable structure.
4. Product shows racemisation: Due to the planar nature of the carbocation.
Example of Reaction
Hydrolysis of Tertiary Butyl Bromide: C₄H₉Br + H₂O → C₄H₉OH + HBr
SN2 Reaction (Substitution Nucleophilic Bimolecular)
Mechanism
The SN2 reaction occurs in a single concerted step
1. From the other side of the leaving group, the nucleophile assaults the substrate (It serves as the mediator for chemical reactions. Otherwise, this material is usually the chemical reaction’s reactant. It is the chemical element that reacted with the material to transform it into a new product).
2. When the leaving group and the nucleophile are both partially attached, a transition state is created.
3. The product is generated with the structure inverted, and the leaving group is displaced.
Kinetics and Rate Law
The rate of reaction depends on both the substrate and the nucleophile.
Rate = k [Substrate] [Nucleophile] (second-order reaction).
Characteristics of SN2 Reaction
1. Favored by primary carbons: Less steric hindrance allows nucleophilic attack (primary > secondary > tertiary).
2. Polar aprotic solvents: {(Solvents that cannot establish hydrogen bonds with the substrate are known as polar aprotic solvents. They are unable to form hydrogen bonds because they lack hydrogen atoms that are directly attached to electronegative atoms. Acetone, chloroform, dichloromethane, and other aprotic solvents are a few examples.) (e.g., acetone, DMSO)} enhance nucleophilicity and increase the reaction rate.
2. No carbocation intermediate: The reaction proceeds through a transition state.
3. Inversion of Configuration: The stereochemistry of the molecule changes (Walden inversion).
Example Reaction
Reaction of Methyl Bromide with Hydroxide Ion:
CH₃Br + OH⁻ → CH₃OH + Br⁻
Comparison of SN1 and SN2 Reactions
Feature | SN1 Reaction | SN2 Reaction |
Mechanism | Two-step with carbocation | One-step, concerted attack |
Rate Law | First-order (Rate = k[Substrate]) | Second-order (Rate = k[Substrate][Nucleophile]) |
Carbon Type | Favored by tertiary | Favored by primary |
Solvent | Polar protic (H₂O, alcohols) | Polar aprotic (DMSO, acetone) |
Stereochemistry | Racemisation | Inversion (Walden inversion) |
Intermediate | Carbocation formation | Transition state |
Rearrangement | Possible | Not possible |
Summary
Predicting reaction processes in organic synthesis is made easier with SN1 and SN2 reactions. Whereas SN2 is preferred in primary alkyl halides and causes configuration inversion, SN1 is prevalent in tertiary alkyl halides and causes racemisation. The predominant route depends on the solvent, nucleophile strength, and substrate structure.
Nucleophilic substitution processes come in two varieties: SN1 (Substitution Nucleophilic Unimolecular) and SN2 (Substitution Nucleophilic Bimolecular). Whereas SN2 happens in a single step with a direct nucleophilic assault, SN1 has two stages as the reaction moves through a carbocation intermediate.
SN1: Two-step reaction with carbocation intermediate formation.
SN2: One-step reaction where the nucleophile attacks as the leaving group exits.
SN1 is favoured by tertiary alkyl halides because they form stable carbocations.
SN2 is favoured by primary alkyl halides due to minimal steric hindrance.
SN1: First-order reaction, rate depends only on the substrate Rate = k[Substrate].
SN2: Second-order reaction, rate depends on both the substrate and nucleophile Rate = k[Substrate][Nucleophile].
SN1 leads to racemisation because the nucleophile can attack from either side of the planar carbocation.
SN2 leads to inversion of configuration (Walden inversion) because the nucleophile attacks from the opposite side of the leaving group.
SN1 is favored by polar protic solvents (e.g., water, alcohols) that stabilise the carbocation.
SN2 is favored by polar aprotic solvents (e.g., acetone, DMSO) that do not hinder the nucleophile.