Sn1, Sn2, E1, E2 reactions | Organic Chemistry PDF Download

Substitution Reactions

• The substitution reaction occurs when an atom or a functional group replaces another different atom or another different functional group of a compound.
• The presence of nucleophiles tend to favour substitution reactions.

What is a Nucleophile?

• Nucleophiles are nucleus loving. It is an electron-rich species tending to donate electron pairs to electron-deficient species. 
• Nucleophile favours substitution reactions.

Types of Substitution Reactions

Based on reaction conditions, we can classify Substitution Reaction into two types: 

1. Nucleophilic Substitution Unimolecular Reaction 
2. Nucleophilic Substitution Bimolecular Reaction

Nucleophilic Substitution Unimolecular Reaction (S_N1 Reaction)

• As the name suggests, it is a substitution reaction taking place in the presence of a nucleophile.
• Nucleophilic substitution unimolecular reaction (S_N1) obeys first-order kinetics.
• Nucleophilic substitution unimolecular reaction (S_N1) is independent of the strength of nucleophiles.
• Nucleophilic substitution unimolecular reaction (S_N1) proceeds with racemisation, i.e. both retention and inversion of the configuration.
• A polar protic solvent like water and alcohol favours the reaction.
• The reaction rate depends on the concentration of substrate, i.e. alkyl halide.

Rate of Reaction = k[Substrate].
Step 1:
Step 2:
Step 3:

Nucleophilic Substitution Bimolecular Reaction (S_N2 Reaction)

• As the name suggests, it is a substitution reaction taking place in the presence of a nucleophile.
• Nucleophilic substitution bimolecular reaction (S_N2) obeys second-order kinetics.
• Nucleophilic substitution bimolecular reaction (S_N2) is dependent on the strength of nucleophiles.
• Nucleophilic substitution bimolecular reaction (S_N2) proceeds with the inversion of the configuration.
• A polar aprotic solvent like DMF and DMSO favours the reaction.
• The reaction rate depends on the concentration of substrate, i.e. alkyl halide and nucleophile.

Rate of Reaction = k[Substrate][Nucleophile].

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Difference between S_N1 and S_N2 Reaction

Elimination Reaction

An elimination reaction is a type of reaction that is mainly used to transform saturated compounds (organic compounds which contain single carbon-carbon bonds) to unsaturated compounds (compounds that feature double or triple carbon-carbon bonds). 

• It is a primary method for the preparation of alkenes. 
• The presence of base tends to favour elimination reactions.

What is a Base?

• The base is electron-rich species tending to accept protons or donate a pair of electrons to electron-deficient species. 
• Base favours elimination reactions.

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Types of Elimination Reactions

Based on reaction conditions, we can classify Elimination Reaction into two types: 

1. Unimolecular Elimination Reaction 
2. Bimolecular Elimination Reaction

Unimolecular Elimination Reaction (E1 Reaction)

• Unimolecular elimination reaction (E1) follows first-order kinetics.
• Unimolecular elimination reaction (E1) occurs in two steps: ionisation and deprotonation.
• During the ionisation bond between carbon and halogen breaks, an intermediate carbocation is formed.
• During deprotonation, a proton is lost from carbocation.
• The base further forms a pi bond within the molecule.
• The reaction rate depends on the concentration of substrate, i.e. alkyl halide.

Rate of Reaction = k[Substrate].

• It is independent of the strength of the base.
• Unimolecular elimination reactions (E1) are similar to nucleophilic substitution unimolecular reactions (SN1).

Bimolecular Elimination Reaction (E2 Reaction)

• Bimolecular elimination reaction (E2) occurs in the presence of a base.
• Bimolecular elimination reaction (E2) follows second-order kinetics.
• Bimolecular elimination reaction (E2) occurs in a single step.
• The reaction rate depends on the concentration of substrate, i.e. alkyl halide and a base.

Rate of Reaction = k[Substrate][Base].

• It is dependent on the strength of the base.
• Bimolecular elimination reactions (E₂) are similar to nucleophilic substitution bimolecular reactions (S_N2).

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Difference between E1 and E2 Reaction