Substitution and Elimination Reactions

Substitution and Elimination ReactionsSubstitution and Elimination reactions are two different kinds of chemical reactions that form a part of Organic Chemistry. These reactions are often studied together but are quite different from one another. These are both unique in their own right. Here is a detailed look at the Substitution and Elimination reactions.

Substitution Reaction

It is a kind of chemical reaction wherein the replacement of one functional group in a chemical is compound by another. This reaction is also referred to as ‘single replacement reaction’ or ‘single displacement reaction’. This reaction is of utmost significance in organic chemistry. It is divided into two categories: Nucleophilic and Electrophilic.

Sn1 Substitution and Elimination Reactions

Sn2 Substitution Reaction Mechanism

Sn2 Substitution and Elimination Reactions

Elimination Reaction

It is a kind of organic reaction wherein two substituents are eliminated from a molecule in one or two-step mechanisms. Most of the elimination reactions result in the loss of at least one hydrogen atom and form double bond. The unsaturation of the molecule increases because of this.

Elimination Reaction

E2 Elimination Reaction Mechanism

E2 elimination reactions

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Difference between Elimination Reaction and Substitution Reaction

The properties as well as mechanism of elimination and substitution reactions are different from each other. Here is a look at the key differences between the two:

Factor Sn1 Sn2
Substrate 3°(requires formation of a relatively stable carbocation Methy>1°>2°(requires unhindered substrate)
Nucleophile Weak Lewis base, neutral molecule,nucleophile may be the solvent(solvolysis) Strong Lewis base, rate favored by high concentration of nucleophile
Solvent Polar Protic(e.g.,alcohols,water) Polar aprotic(e.g.,DMF,DMSO)
Leaving group l>Br>Cl>F for Both Sn1 and Sn2(the weaker the base after the group departs,the better the leaving group

Mechanism of Substitution and Elimination Reactions

Let us first have a look at the mechanism of both these reactions:

  • Elimination Reaction

It is divided into two categories – E1 reactions and E2 reactions. Two-step mechanism is called E1 reaction and one-step mechanism is referred to as E2 reaction.

  • Substitution Reaction

Based on its reaction mechanism, substitution reaction is also divided into two categories – SN1 reaction and SN2 reaction.

Properties of Substitution and Elimination Reactions

Let us now have a look at the properties of both these reactions:

  • Elimination Reaction
  1. E1 reactions: These reactions follow the Zaitsey rule. These are non-stereospecific. A carbocation intermediate forms in the reaction so these are non-concerted reactions. These are unimolecular as their reaction rate is only based on the concentration.
  2. E2 reactions: These reactions are stereospecific. These are known to be biomolecular reactions. The reaction rate here is based on the concentration of base and substrate.

Substitution Reactions

  1. SN1 reaction: These reactions are non-stereospecific. This is because nucleophile may hit molecule from both the sides. A steady carbocation is built in these reactions and thus these are non-concereted. The rate of these reactions is based on the substrate and these are called unimolecular reactions.
  2. SN2 reaction: These are concerted and stereospecific. These reactions take place when nucleophile is more reactive. The rate of this reaction is based on concentration of nucleophile and substrate.

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