Primary amines

Primary amines are organic derivatives of ammonia (NH₃) where one hydrogen atom has been replaced by an alkyl group (like ethyl, -C₂H₅) or an aryl group (like phenyl, -C₆H₅). This gives them the general formula R-NH₂. The key feature is the amino group (-NH₂), which contains a nitrogen atom with a lone pair of electrons. This lone pair is the source of their characteristic chemical properties, particularly their basicity and nucleophilicity. For simple amines, we name them by adding '-amine' to the alkyl group name, e.g., ethylamine. For more complex molecules, the -NH₂ group is treated as a substituent and named 'amino', with its position indicated by a number, e.g., 2-aminopentane.

There are several key routes to synthesise aliphatic primary amines, which are common exam questions. The choice of method depends on the starting material and whether the carbon chain needs to be extended.

1. From Halogenoalkanes: Heating a halogenoalkane with an excess of hot, ethanolic ammonia causes nucleophilic substitution. The ammonia acts as the nucleophile. Using excess ammonia is vital to minimise the formation of secondary and tertiary amines, as the primary amine product is also a nucleophile.
2. Reduction of Nitriles: This is a two-step process that extends the carbon chain by one carbon. First, a halogenoalkane is reacted with potassium cyanide (KCN) in ethanol to form a nitrile (R-CN). The nitrile is then reduced to a primary amine using a strong reducing agent like LiAlH₄ in dry ether, or by catalytic hydrogenation (H₂/Ni).
3. Reduction of Amides: Primary amides can be reduced directly to primary amines using a powerful reducing agent like lithium tetrahydridoaluminate (LiAlH₄) in dry ether.

Phenylamine (also known as aniline) is the simplest aromatic amine. It is a vital starting material for producing dyes, pharmaceuticals, and polymers. The standard laboratory and industrial preparation involves the reduction of nitrobenzene. This is achieved by heating nitrobenzene with tin (Sn) and concentrated hydrochloric acid (HCl). The tin and acid react to produce hydrogen in situ, which reduces the nitro group (-NO₂) to an amino group (-NH₂). Because the reaction is carried out in acidic conditions, the phenylamine product is protonated to form the phenylammonium ion (C₆H₅NH₃⁺). To obtain the free phenylamine, a strong base such as sodium hydroxide (NaOH) is added at the end of the reaction to deprotonate the salt.

Amines are weak bases. Their basicity stems from the lone pair of electrons on the nitrogen atom, which can accept a proton (H⁺) from an acid, forming a dative covalent bond. The strength of an amine as a base depends on the availability of this lone pair. The more available the lone pair (i.e., the higher its electron density), the more readily it can accept a proton, and the stronger the base. We can compare the basicity of ethylamine (an aliphatic amine), ammonia, and phenylamine (an aromatic amine):

• Ethylamine is a stronger base than ammonia. The ethyl group is electron-donating (it has a positive inductive effect). It 'pushes' electron density onto the nitrogen atom, making the lone pair more available and attractive to a proton.
• Phenylamine is a weaker base than ammonia. The lone pair on the nitrogen atom overlaps with the delocalised pi-electron system of the benzene ring. This delocalisation spreads the lone pair's electron density into the ring, making it less available to accept a proton.

Phenylamine reacts readily with aqueous bromine (bromine water) at room temperature. Unlike benzene, which requires a halogen carrier catalyst (like FeBr₃) for bromination, phenylamine is much more reactive. This is because the -NH₂ group is a powerful activating group. The lone pair of electrons on the nitrogen atom is delocalised into the benzene ring, significantly increasing the electron density of the ring. This makes the ring highly susceptible to attack by electrophiles like the Br⁺ in Br₂(aq). The reaction is rapid and extensive, resulting in the substitution of three hydrogen atoms on the ring at positions 2, 4, and 6. The product, 2,4,6-tribromophenylamine, is a white solid that precipitates out of the solution. This reaction is often used as a test for phenylamine.

This is a two-stage process that is unique to primary aromatic amines like phenylamine and is used to create brightly coloured azo dyes.

Stage 1: Diazotisation Phenylamine is reacted with nitrous acid (HNO₂) to form a benzenediazonium salt. A critical condition is that the temperature must be kept below 10°C (ideally 0-5°C). This is because the diazonium salt is unstable and will decompose at higher temperatures. Since nitrous acid is also unstable, it is generated in situ (in the reaction mixture) by reacting sodium nitrite (NaNO₂) with a strong acid, usually cold, dilute hydrochloric acid.

Stage 2: Coupling Reaction The cold solution of the benzenediazonium salt is then immediately reacted with a coupling agent. A common coupling agent is phenol, dissolved in sodium hydroxide solution (to form the more reactive phenoxide ion). The diazonium ion (C₆H₅N₂⁺) acts as an electrophile and attacks the electron-rich phenol ring, forming an azo compound. This reaction is called a coupling reaction. The product contains an azo group (-N=N-) which links the two benzene rings. This creates a large, stable, delocalised electron system which absorbs visible light, causing the compound to be coloured. The product from coupling with phenol is an orange precipitate.