A Level Chemistry Revision: Organic Chemistry – Amines

A level organic chemistry includes the study of amines. This involves learning about the different types of amines, their structure, and their physical properties. To help you prepare for your exam, we’ve put together an overview of the key points you need to revise.

Amines: an introduction

Amines are organic compounds derived from ammonia. Released through the decomposition of amino acids, amines are common by-products of natural biological processes. However, they can also be produced artificially through various chemical processes.

Amines have a wide range of industrial and commercial applications, which include synthesising medicines, manufacturing dyes, and producing epoxy resin curing agents. They’re also used to remove hydrogen sulphide and carbon dioxide from natural gases during the refining process.

Knitted fabric being dyed red in a factory
Amines are used in the manufacture of dye.

What are amines and what are their structures?

Amines have a nitrogen functional group that has a lone pair of electrons. The nitrogen gives amines basic or alkaline properties. It also makes them nucleophilic or, in other words, they’re attracted to the hydrogen ions of acids.

An amine can be classed as primary, secondary or tertiary, depending on the number of R groups attached to the nitrogen. See the illustration below.

An illustration of primary, secondary and tertiary amines

The R groups can either take the form of a straight-chain alkyl group or an aromatic/cyclic aryl group. As you can see, the attached substituent can be one, two or three. This allows for varieties of amine compounds. 

Amino acids and biogenic amines are two of the most important amines. Amino acids are the building blocks of proteins, which comprise the various structural and biochemical functions of organisms. Meanwhile, biogenic amines function as neurotransmitters like serotonin and dopamine.

Amines can be divided into three categories based on their structure or the number of substituents attached to the amine group. The compounds can also be further subclassified as either aliphatic or aromatic. Aliphatic amines contain alkyl substituents while aromatic amines contain benzene ring substituents.

  • Primary (1°) amines These contain one alkyl or aromatic substituent. Most amino acids are classified as primary amines.
  • Secondary (2°) amines They contain two organic substituents, which can either be alkyl, aryl or both.
  • Tertiary (3°) amines These types of amines contain three organic substituents. Common examples include ethylenediaminetetraacetic acid and trimethylamine.

How are amines prepared?

An amine is produced by replacing one, two or three of the hydrogen atoms in ammonia with an organic substituent. The two main categories of chemical reactions used to synthesise amines are nucleophilic substitution and nitrile reduction.

Halogenoalkane reactions

The reaction between halogenoalkane and ammonia is an example of a nucleophilic substitution reaction. A hydrogen is replaced by an alkyl or aryl substituent, while the hydrogen combines with a halogen. The generalised balanced reaction is illustrated below. Similar results can be produced through the alkylation reaction of ammonia.

An illustration of halogenoalkane reactions

Nitrile reduction reactions

Nitriles can be reduced into amines through the hydrogenation process. The process requires a reducing agent that breaks the triple bonds and donates extra electrons. The generalised balanced chemical reaction is shown below. As you can see, lithium aluminium hydride is used as a reducing agent.

Nitrile reduction reactions

What are the physical properties of amines?

The bonds between nitrogen and hydrogen in amines are polar, which allows them to form hydrogen bonds. As a result, amines have weaker bonds than alcohols and therefore lower boiling points.

Many amines are soluble in water, but as the alkyl or aryl substituents get larger, their water solubility decreases. Aromatic amines have comparably lower water solubility than aliphatic amines.

Amine pH

Just like their parent molecule ammonia, amines are classified as basic or alkaline when dissolved in water. They are either proton (hydrogen ion) acceptors or electron-pair donors, as defined by the Bronsted-Lowry and Lewis theories respectively.

Even tertiary amines, in which all of the hydrogens have been replaced by an organic substituent, still behave as a base because of the lone electron pair of nitrogen. 

How do amines chemically react?

Amines can react with other organic or inorganic compounds in several ways. The products will depend on the types of reactants. Amines commonly undergo alkylation, acylation and sulfonation. They can also combine with metal ions as ligands to form coordination complexes. Below is a generalised illustration of the alkylation of amines by alkyl halides.

Alkylation of amines by alkyl halides

Meanwhile, acyl chlorides and acid anhydrides can react with either primary or secondary amines to produce amides. The generalised chemical reaction is shown below.

Acyl chlorides and acid anhydrides reaction with primary and secondary amines

When amines react with sulfonyl chlorides, organic sulphur compounds called sulfonamide are formed. A similar reaction known as the Hinsberg reaction is used to test the presence of primary, secondary or tertiary amines.

Amine applications

Amines have important biological roles, especially when it comes to amino acids and neurotransmitters. Some of the most important amine neurotransmitters are epinephrine, dopamine, serotonin, and histamine. The brain will not function normally if there is an imbalance or deficiency in one or more of these neurotransmitters.

In addition to their biological applications, amines have several industrial uses, such as in manufacturing dyes and synthesising drugs. About 42% of drugs are either synthesised from amines or contain amine groups. Blister packs and white bottles of pills 

Some of the common drugs that are synthesised from amines or contain amine functional groups include:

  • Ephedrine and phenylephrine decongestants
  • Thioridazine antipsychotic drug
  • Amitriptyline, imipramine, and clomipramine antidepressants
  • Opiate analgesics such as morphine, codeine, and heroin

Amines also play important roles in the petroleum industry, where they’re used to remove impurities like hydrogen sulphide from natural gases.

To view more revision guides, visit our A Level Chemistry Resources Hub.

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