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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.

Sn1, Sn2, E1, E2 reactions | Organic Chemistry

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 (SN1 Reaction)

  • As the name suggests, it is a substitution reaction taking place in the presence of a nucleophile.
  • Nucleophilic substitution unimolecular reaction (SN1) obeys first-order kinetics.
  • Nucleophilic substitution unimolecular reaction (SN1) is independent of the strength of nucleophiles.
  • Nucleophilic substitution unimolecular reaction (SN1) 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].
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Sn1, Sn2, E1, E2 reactions | Organic Chemistry

Nucleophilic Substitution Bimolecular Reaction (SN2 Reaction)

  • As the name suggests, it is a substitution reaction taking place in the presence of a nucleophile.
  • Nucleophilic substitution bimolecular reaction (SN2) obeys second-order kinetics.
  • Nucleophilic substitution bimolecular reaction (SN2) is dependent on the strength of nucleophiles.
  • Nucleophilic substitution bimolecular reaction (SN2) 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].
Sn1, Sn2, E1, E2 reactions | Organic Chemistry

Difference between SN1 and SN2 Reaction

Sn1, Sn2, E1, E2 reactions | Organic Chemistry

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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.

Sn1, Sn2, E1, E2 reactions | Organic Chemistry

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).

Sn1, Sn2, E1, E2 reactions | Organic Chemistry

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 (SN2).

Sn1, Sn2, E1, E2 reactions | Organic Chemistry

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Difference between E1 and E2 Reaction

Sn1, Sn2, E1, E2 reactions | Organic Chemistry

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