Introduction and Structure of Amines

Introduction to amines

Amines are groups of nitrogen atoms, each possessing a single pair of electrons. An amine is a structural analogue of ammonia, in which nitrogen can bind up to three hydrogen atoms with the help of hydrogen bonds. It also has a variety of features that stem from its carbon-based structure.

Amides are nitrogen-carbonyl compounds that contain the formula R–CO–NR′R′′ and differ from amines in terms of their characteristics. A lone pair of nitrogen atoms is all that is required to make an amine. Alkylamines and arylamines are chemical compounds produced from ammonia (NH3) by substituting an alkyl or an aryl group for one or more of the hydrogen atoms.

The alkyl or aryl groups are found on nitrogen atoms in the primary (1°), secondary (2°) or tertiary (3°) structure of amines, depending on how many alkyl groups are present on the nitrogen. Amines occur in nature in proteins, vitamins, hormones and so on. Amines can also be synthesised. Polymers, drugs and dyes all use amines,

Structure of amines

Nitrogen is trivalent with a lone pair because it possesses five valence electrons. Nitrogen in amines is sp3 hybridised according to VSEPR theory. Because of the existence of a lone pair, it has a pyramidal shape rather than a tetrahedral shape common to most sp3 hybridised compounds. Depending on the structure of amine, each of nitrogen’s s three sp3 hybridised orbitals overlaps an orbital of hydrogen or carbon. Amines’ C-N-H angle is less than 109 degrees, which is a typical angle of tetrahedral geometry, as a lone pair is present. Amines have a rotational axis of 107°.

Alkylamines

The nitrogen centres in alkylamines are tetrahedral. The C-N-C and C-N-H bond angle is 109° in this instance. The C-N distance is smaller than the C-C distance. The nitrogen centre of the amines can also hold four replacements to produce lone pairs, giving them chirality.

Aromatic amines

In aromatic amines (anilines), nitrogen virtually possesses a planar structure. This is because the aryl substituent is mixed in with the lone pair. The C-N frequency range is narrower. As with C-C and C-C for aniline, C-N is the same as C-C for aniline.

Types of amines

Amines are classified depending on how many carbon atoms are directly attached to the nitrogen atom. The alkyl or aryl groups are found on nitrogen atoms in primary (1°), secondary (2°) or tertiary (3°) amines, depending on how many alkyl groups are present on the nitrogen. Amines can be split into four groups based on the way an ammonia molecule replaces the hydrogen atoms, namely,

• Primary amine structure ( 1°)

When an alkyl or aryl group is added to an ammonia molecule in place of one of the hydrogen atoms, a primary structure of amine is formed.

Example.: Methylamine CH3NH2

• Secondary amine structure ( 2°)

A secondary structure of amine is formed when two organic substituents replace the hydrogen atoms in ammonia.

Example: Dimethylamine (CH3)2NH

• The tertiary structure of amine ( 3°)

An aryl or aromatic group can be formed when all three hydrogen atoms are replaced with organic substituents; this is known as the tertiary structure of amine.

Example: Trimethylamine N(CH3)3

• Cyclic amine

These cyclic amines are aromatic amines with secondary or tertiary amine structures.

Example: Piperidine (CH2)5NH

Basicity of amines

Primary and secondary amines, like ammonia, have protic hydrogens, which makes them slightly acidic. There is no acidity in the protic hydrogen of tertiary amines since they do not contain protic hydrogen.

Primary and secondary amines have a pKa value of 38, making them extremely weak acids. The pKb, on the other hand, is around 4. Amines become substantially more basic rather than acidic as a result of this. Consequently, the alkalinity of amine solutions in water is extremely high.

How to distinguish between the primary, secondary and tertiary structure of amines?

For the most part, the Hinsberg test is used to identify primary, secondary and tertiary amines. A Hinsberg reagent reacts with an amine in the presence of an aqueous alkali. As a result, this is the Hinsberg test in its purest form.

There are a few things to note following the reaction:

N-ethylbenzenesulphonyl amide and benzene sulfonyl chloride are the most commonly used Hinsberg reagents for the reaction of primary amines with benzene sulfonyl chloride. The hydrogen that is connected to the nitrogen molecule makes this acidic. As a result, alkali dissolves this solution.

Amines are commonly found in various products, including pharmaceuticals, cosmetics and even pictures. From rocket propellants to pesticides, a wide range of substances can be made from amines. It is utilised to manufacture synthetic fibres used to make Kevlar, a vital component in bulletproof vests and helmets. Amines are used in many industries, in addition to their traditional chemical uses.

Preparation of amines

• • Conversion of halogenoalkane to amines

An enclosed tube is used for this procedure. Haloalkanes are heated with a concentrated solution of ammonia in ethanol. For the mixture to be heated under the reflux, ammonia would have to be released from the container as gas.

• • Nitriles are reduced

When lithium aluminium hydride is used to reduce nitriles, we can obtain primary amines. For amines that have one more carbon atom than the beginning amine, this technique is most commonly used.

• Synthesis of phthalimide by Gabriel synthesis

By using Gabriel synthesis, we may easily obtain primary amines. We get potassium salts of phthalimide after treating phthalimide with ethanolic potassium hydroxide. After additional heating with an alkyl halide and alkaline hydrolysis, primary amines are formed. Due to the lack of nucleophilic substitution, we cannot synthesise primary aromatic amines from aryl halides.

Properties of amines

The properties of amines are as follows:

• It is gaseous in nature, the lower aliphatic amines that have a fishy odour.
• Primitive amines have three to four carbons, while higher amines are solids at room temperature.
• Open storage of arylamines, such as aniline, results in their oxidation, turning them colourless.
• Lower aliphatic amines can make hydrogen bonds with water molecules, making them water-soluble.
• In order to reduce water solubility, increasing the hydrophobic alkyl part decreases the molecular weight of the amine.
• The amines are easily dissolved in organic solvents like ethanol and benzene.

Uses of amines

Amines are found in a wide variety of products that we use regularly. The following are some of the amines’ applications:

• They are used to purify water and to manufacture medicines and pesticides.
• Proteins are made up of amino acids, and this enzyme is involved in their creation. Amines are also used to produce a wide range of vitamins as well.
• As one of the principal neurotransmitters, serotonin is an important amine. It regulates the sensation of hunger and is essential to the brain’s general speed.
• As the name suggests, analgesics such as Morphine and Demerol are manufactured from amines.

Conclusion

Amines are organic compounds in which an alkyl or even an aryl group replaces one or more hydrogen atoms from the ammonia molecule. Carbon atoms are tightly bound to nitrogen atoms in this structure. Amines are known for their distinctive features, such as their distinct smells. They smell like decaying eggs or fish. When compared to aromatic amines, aliphatic amines have stronger ammonia connections but are less dense. Rubber, dyes, medicines, synthetic resins and fibres are among the most common industrial uses. A number of assays are used to identify the primary, secondary, and tertiary structure of amines– for instance, the Hinsberg reaction.