Amines

Introduction to Amines – Compounds Containing Nitrogen

Amines are one of the most important classes of organic compounds which can be derived when we replace one or more hydrogen atoms of ammonia molecule with an alkyl group.

Table of contents

• What are Amines?
• Recommended video for amines
• Structure of Amine
• Types of Amines
• Preparation of Amines
• Basicity of Amines
• Uses of Amines
• FAQs

What are Amines?

An amine is generally a functional group with a nitrogen atom having a lone pair. Amines resemble ammonia structurally where nitrogen can bond up to 3 hydrogen atoms. It is also characterized by various properties that are based on carbon connectivity.

Compounds of nitrogen connected to a carbonyl group are called as amides, they have a structure R–CO–NR′R″ and vary in properties with amines.

Amines are organic compounds that contain nitrogen atoms with a lone pair. Basically, they are derived from ammonia (NH₃) in which one or more hydrogen atoms is replaced by an alkyl or aryl group, and so they are known as alkylamines and arylamines respectively.

Recommended Video

Amine Structure

Nitrogen has 5 valence electrons and so is trivalent with a lone pair. As per VSEPR theory, nitrogen present in amines is sp³ hybridized and due to the presence of lone a pair, it is pyramidal instead of tetrahedral shape which is a general structure for most sp³ hybridized molecules. Each of the three sp3 hybridized orbitals of nitrogen overlap with orbitals of hydrogen or carbon depending upon the configuration of amines. Due to the presence of a lone pair, the C-N-H angle in amines is less than 109 degrees which is a characteristic angle of tetrahedral geometry. The angle of amines is near about 107 degrees.

Occurrence of Amines

Naturally, amines occur in proteins, vitamins, hormones, etc. and they are also prepared synthetically to make polymers, drugs, and dyes.

Types of Amines

On the basis of how the hydrogen atoms are replaced by an ammonia molecule, amines can be divided into 4 types.

1. Primary AminesWhen one of the hydrogen atoms of the ammonia molecule is replaced by an alkyl or aryl group.Eg: Methylamine CH₃NH₂, Aniline C₆H₅NH₂
2. Secondary AminesTwo organic substituents replace the hydrogen atoms of the ammonia molecule forming an amine.Eg: Dimethylamine (CH₃)2NH, Diphenylamine  (C₆H₅)₂NH
3. Tertiary AminesWhen all 3 of the hydrogen atoms are replaced by an organic substituent, it could be an aryl or aromatic group.Eg: Trimethylamine N(CH₃)₃, Ethylenediaminetetraacetic acid (EDTA)
4. Cyclic AminesThese are secondary or tertiary amines in an aromatic ring structure. Eg: Piperidine (CH₂)₅NH, Aziridines C₂H₅N

Preparation of Amines

Some processes for preparing primary amines are mentioned below.

Preparation of Primary Amines

1. Making of amines from halogenoalkanes

This process will be carried out in a sealed tube. Here haloalkanes will be heated with the concentrated solution of ammonia in ethanol. The mixture cannot be heated under the reflux as ammonia would move out in the form of gas from a container.

2. Reduction of nitriles

We can get primary amines when nitriles are reduced with lithium aluminium hydride. This method is mainly used for the preparation of amines which contain one carbon atom more than the starting amine.

3. Gabriel phthalimide synthesis

We can get primary amines easily by Gabriel synthesis. In this process, on the treatment of phthalimide with ethanolic potassium hydroxide, we get potassium salts of phthalimide. When this is further heated with alkyl halide followed by alkaline hydrolysis then primary amine is produced. We cannot prepare aromatic primary amines because aryl halides do not undergo nucleophilic substitution with the anion which is formed by phthalimide.

Basicity of Amines

Similar to ammonia, primary & secondary amines have protic hydrogens and thus they showcase a degree of acidity. Whereas tertiary amines have no protic hydrogen and thus do not possess a degree of acidity.

pK_a value for primary & secondary amines is about 38, which makes them a real weak acid. Whereas if we take the pK_b, it is about 4. This makes the amines much more basic than acidic. Thus, an aqueous solution of an amine is strongly alkaline.

Uses of Amines

Amines have a widespread application in our daily lives. Some uses of amines are listed below:

• It is used in water purification, medicine manufacturing and development of insecticides and pesticides.
• It is involved in the production of amino acids which is the building block of proteins in living beings. Many varieties of vitamins are also made by amines.
• Serotonin is an important amine that functions as one of the primary neurotransmitters. It controls the feelings of hunger and is critical for the speed with which the brain operates in general.
• Pain-relieving medicines such as Morphine and Demerol which are also known as analgesics are made from amines.

From the above discussion, students would have gained some understanding of the basic concepts of amines but to know more about amines, chemical and physical properties of amines, register with BYJU’S.