Imine Formation

Imine and its by-products are vital intermediates in forming nitrogen heterocycles. They are used to synthesise dyes and coordination polymers. They are also known as Schiff Base and are synthesised during condensation reactions of aromatic amines with aldehyde and ketone.

This section will discuss imine, nomenclature of imine, synthesis of imine and applications of the imine.

Table of Content

• What is an Imine?
• Nomenclature of Imine
• Synthesis of Imine
• Applications of Imine
• Frequently Asked Questions – FAQs

What is an Imine?

An imine is a functional group containing a carbon-nitrogen double bond. Carbon is attached to two groups or hydrogen atoms. A nitrogen atom can be attached to a hydrogen atom or an organic group. The imine is called a Schiff base if the nitrogen atom is bonded with the alkyl group. The first imine compound was discovered in 1883 by a German chemist Albert Ladenburg.

Nomenclature of Imine

• The imine is a compound having an R₂C=NR bond. They are also referred to as aldehydes or ketones by replacing an oxygen atom with an NR group.
• When an R=H group replaces the oxygen atom, the compound is a primary imine.
• When an R is a hydrocarbyl, the compound is a secondary imine.
• When R3 is an OH group, the compound is an oxime.
• When R3 is an NH₂ group, the compound is a hydrazone.
• When C is attached to both a hydrocarbyl and a Hydrogen atom in primary imine, it is a primary aldimine.
• When C is attached to both a hydrocarbyl and a Hydrogen atom in secondary imine, it is a secondary aldimine.
• When C is attached to two hydrocarbyls in primary imine, it is a primary ketimine.
• When C is attached to two hydrocarbyls in a secondary imine, it is a primary ketimine.
• Aziridine and its derivatives are also referred to as imine.
• Nomenclature of aldimines is done by removing e from the radical and adding -imine.

Example: Ethanimine.

• Nomenclature of carbonyl compounds whose keto group is replaced by an imino group is done by adding the word imine as a suffix.

Example: Acetone imine.

• N-Sulfinyl imine contains the sulfinyl group on the nitrogen atom.
• An iminium cation refers to a compound with four nitrogen bonds giving it a positive charge.

Synthesis of Imine

Reaction with Primary Amines

Aldehydes and Ketones react with ammonia or primary amines to yield imine or imine by-products. During imine formation, the carbonyl oxygen is terminated, and the nitrogen of the primary amine replaces the carbonyl oxygen to form the imine C=N bond.

It is also known as Schiff bases.

It is also called a condensation reaction because water is eliminated during the reaction.

Mechanism of Imine Formation

Step 1: Nucleophilic Addition

A nucleophile attacks the carbonyl group to form an iminium ion derivative.

Step 2: Proton transfer

A proton is transferred from an iminium ion to the electron-rich oxygen atom.

Step 3: Protonation of OH

Lone pairs of oxygen attract the proton to yield an oxonium ion derivative.

Step 4: Removal of water (nucleophile elimination)

Nitrogen transfers its lone pairs towards the carbon atom, leading to water expulsion, forming an iminium ion derivative.

Step 5: Deprotonation

Proton is removed from the iminium ion derivative; stability is retained.

Applications of Imine

• Imine and its by-products are vital intermediates in forming nitrogen heterocycles.
• They are used to synthesise dyes and coordination polymers.
• They are used in pharmaceutical industries.
• They are used to synthesise organic compounds.
• A solution of rosaniline and sulfurous acid in the water, commonly known as Schiff reagent, is used to test aldehydes, and ketones are imine.