Amines, Amino Acids and Proteins (OCR A-Level Chemistry A): Revision Notes

Amines

What are amines?

Amines are organic compounds based on the ammonia structure ($\text{NH}_3$), where one or more hydrogen atoms have been substituted with carbon chains or rings. They contain nitrogen as a key functional group and are important compounds both in synthetic chemistry and biological systems.

There are two main types of amines based on what is bonded to the nitrogen atom:

Classification of amines

Amines are categorized into three types based on how many alkyl or aryl groups are bonded to the nitrogen atom. This classification system divides amines into primary, secondary, and tertiary amines.

Primary amines have one alkyl or aryl group attached to the nitrogen, with the remaining two bonds connected to hydrogen atoms. The general structure can be written as $\text{RNH}_2$. Examples include:

• Aliphatic: methylamine ($\text{CH}_3\text{CH}_2\text{CH}_2\text{NH}_2$)
• Aromatic: phenylamine ($\text{C}_6\text{H}_5\text{NH}_2$)

Secondary amines have two alkyl or aryl groups bonded to the nitrogen, with one remaining hydrogen. The general structure is $\text{R}_2\text{NH}$ or $(\text{R})_2\text{NH}$. Examples include:

• Aliphatic: diethylamine ($(\text{C}_2\text{H}_5)_2\text{NH}$)
• Aromatic: N-methylaniline ($\text{C}_6\text{H}_5\text{NH}(\text{CH}_3)$)

Tertiary amines have three alkyl or aryl groups attached to the nitrogen, with no hydrogen atoms bonded to it. The general structure is $\text{R}_3\text{N}$. Examples include:

• Aliphatic: trimethylamine ($(\text{CH}_3)_3\text{N}$)
• Aromatic: N,N-dimethylaniline ($\text{C}_6\text{H}_5\text{N}(\text{CH}_3)_2$)

Amines in nature

Amines are widespread in biological systems and many have significant effects on human physiology.

Naming amines

The nomenclature system for amines depends on whether they are primary, secondary, or tertiary, and also on the structure of the carbon chain or ring.

Primary amines

For a primary amine where the $-\text{NH}_2$ group is at the end of the carbon chain, add the suffix -amine to the name of the alkyl chain.

When the amine group is positioned on a carbon other than carbon-1, use the prefix amino- along with a number to indicate the position of the $-\text{NH}_2$ group on the carbon chain.

Secondary and tertiary amines

When secondary or tertiary amines contain the same alkyl groups repeated, the prefixes di- or tri- are used to indicate the number of identical groups attached to the nitrogen.

When two or more different groups are bonded to the nitrogen atom, the compound is named as an N-substituted derivative of the larger group.

Reactions of amines

Amines as bases

Amines behave as bases in chemical reactions because the lone pair of electrons on the nitrogen atom can accept a proton ($\text{H}^+$). This is similar to how ammonia acts as a base.

When an amine accepts a proton, a dative covalent bond (also called a coordinate bond) forms between the lone pair on nitrogen and the proton. This creates a positively charged ion called an ammonium ion.

The general reaction can be written as:

$\text{R-NH}_2 + \text{H}^+ \rightarrow \text{R-NH}_3^+$

Salt formation

Since amines are bases, they neutralize acids to form salts. This reaction is analogous to the neutralization of acids by ammonia.

Preparation of amines

Preparation of aliphatic amines

Formation of primary amines

Ammonia has a lone pair of electrons on the nitrogen atom, which allows it to function as a nucleophile in substitution reactions with haloalkanes. The nucleophilic substitution produces an ammonium salt. When aqueous alkali (sodium hydroxide) is added to this salt, it releases the free amine.

The formation of propylamine from 1-chloropropane and ammonia follows this two-step process:

Formation of secondary and tertiary amines

The reaction described above cannot produce a pure primary amine. The product still contains a lone pair of electrons on the nitrogen atom, which means it can react further with more haloalkane molecules to form secondary and then tertiary amines. Each reaction produces another ammonium salt.

Preparation of aromatic amines

Aromatic amines, such as phenylamine ($\text{C}_6\text{H}_5\text{NH}_2$), are prepared through the reduction of nitrobenzene ($\text{C}_6\text{H}_5\text{NO}_2$).

The reduction process involves heating nitrobenzene under reflux with tin metal and concentrated hydrochloric acid. This converts the nitro group ($-\text{NO}_2$) into an ammonium salt (phenylammonium chloride). Excess sodium hydroxide is then added to release the free aromatic amine, phenylamine. Tin and hydrochloric acid together act as the reducing agent.

The overall equation can be written as:

$\text{C}_6\text{H}_5\text{NO}_2 + 6[\text{H}] \xrightarrow[\text{2. excess NaOH(aq)}]{\text{1. Sn/conc HCl}} \text{C}_6\text{H}_5\text{NH}_2 + 2\text{H}_2\text{O}$

Where $[H]$ represents the reducing agent providing hydrogen atoms.