Difference Between Elimination and Substitution Reaction

Key Difference – Elimination vs Substitution Reaction
 

Elimination and substitution reactions are two types of chemical reactions mainly found in organic chemistry. The key difference between elimination and substitution reaction can be best explained by using their mechanism. In elimination reaction, rearrangement of previous bonds occurs after the reaction, whereas substitution reaction replaces a leaving group with a nucleophile. These two reactions compete with each other and are influenced by several other factors. Those conditions vary from one reaction to another.

What is an Elimination Reaction?

Elimination reactions are found in Organic Chemistry, and the mechanism involves the removal of two substituents from an organic molecule either in one step or two steps. When the reaction occurs in a single step mechanism, it is known as E2 (bi-molecular reaction) reaction, and when it has a two-step mechanism, it is known as E1 (unimolecular reaction) reaction. In general, most of the elimination reactions involve the loss of at least one hydrogen atom to form the double bond. This increases the unsaturation of the molecule.

Difference Between Elimination and Substitution Reaction

E1 reaction

What is a Substitution Reaction?

Substitution reactions are a type of chemical reactions which involves replacement of one functional group in a chemical compound by another functional group.  Substitution reactions are also known as ‘single displacement reactions’ or ‘single replacement reactions.’ These reactions are very important in Organic Chemistry, and they are mainly classified into two groups, based on the reagents involved in the reaction: electrophilic substitution reaction and nucleophilic substitution reaction. These two types of substitution reactions exist as SN1 reaction and SN2 reaction.

Key Difference - Elimination vs Substitution Reaction

Substitution reaction -Methane Chlorination

What is the difference between Elimination and Substitution Reaction?

Mechanism:

Elimination Reaction: Elimination reactions can be divided into two categories; E1 reactions and E2 reactions. E1 reactions have two steps in the reaction, and E1 reactions have a single step mechanism.

Substitution Reaction: Substitution reactions are divided into two categories based on their reaction mechanism: SN1 reactions andSN2 reactions.

Properties:

Elimination Reaction:

E1 reactions: These reactions are non-stereospecific, and they follow the Zaitsev (Saytseff) rule. A carbocation intermediate is formed in the reaction so that these reactions are non-concerted reactions. They are unimolecular reactions since the reaction rate depends only on the concentration. These reactions do not take place with  primary alkyl halides (leaving groups). Strong acids are able to promote the loss of OH as H2O or OR as HOR if tertiary or conjugated carbocation can be formed as the intermediate.

E2 reactions: These reactions are stereospecific; anti-periplanar geometry is preferred, but synperiplanar geometry is also possible. They are concerted and considered as bimolecular reactions since the reaction rate depends on the concentration of the base and the substrate. These reactions are favored by strong bases.

Substitution Reaction:

SN1 reactions: These reactions are said to be non-stereospecific since the nucleophile can attack the molecule from both sides. A stable carbocation is formed in the reaction and therefore these reactions are non-concereted reactions. The rate of the reaction depends only on the concentration of the substrate, and they are called unimolecular reactions.

SN2 reactions: These reactions are stereospecific and concerted. The rate of the reaction depends on the concentration of both nucleophile and the substrate. These reactions greatly occur, when the nucleophile is more reactive (more anionic or basic).

Definitions:

Stereospecific:

In a chemical reaction, the production of a particular stereomeric form of the product, regardless of the configuration of the reactant.

Concerted reactions:

Concerted reaction is a chemical reaction where all the bonds break and form in a single step.

 

References:
“Elimination Reaction.” Wikipedia. Accessed September 13, 2016. here
“Substitution Reaction.” Wikipedia. Accessed September 13, 2016. here
“Substitution and elimination reactions.” Khan Academy. Accessed September 13, 2016. here
“Elimination Mechanisms Menu.” Chemguide. Accessed September 13, 2016. here
“Nucleophilic Substitution.” McGraw-Hill Education. Accessed September 13, 2016. here
“SN2 , SN1 , E2 , & E1: Substitution and Elimination Reactions.” PDF. here
 
Image Courtesy:
“E1 Elimination Reaction” By V8rik at the English language Wikipedia (CC BY-SA 3.0) via Commons Wikimedia 
“Substitution Reaction” By V8rik at English Wikipedia (CC BY-SA 3.0) via Commons Wikimedia