Revise Phenylamine and azo compounds for Cambridge 9701 (34.2)

Phenylamine ($C_6H_5NH_2$) is prepared by the reduction of nitrobenzene ($C_6H_5NO_2$). This is a common synthetic route in organic chemistry. The reaction is typically carried out using a reducing agent mixture of tin metal and concentrated hydrochloric acid, heated under reflux. The acidic conditions initially produce the phenylammonium ion, $C_6H_5NH_3^+$. A strong alkali, such as sodium hydroxide, must be added afterwards to deprotonate this ion and liberate the free phenylamine base.

$C_6H_5NO_2 + 6[H] \xrightarrow{Sn/conc. HCl, heat} C_6H_5NH_2 + 2H_2O$

Phenylamine is an aromatic amine and its basicity is significantly different from aliphatic amines like ethylamine. It is a much weaker base than both ammonia and ethylamine. This is because the lone pair of electrons on the nitrogen atom is partially delocalised into the delocalised $\pi$-system of the benzene ring. This overlap reduces the electron density on the nitrogen, making the lone pair less available to accept a proton. In contrast, the alkyl group in ethylamine has a positive inductive effect, pushing electron density onto the nitrogen and making it a stronger base than ammonia.

One of the most important reactions of phenylamine is its conversion to a diazonium salt. This process is called diazotisation. Phenylamine is reacted with nitrous acid ($HNO_2$) at a temperature kept strictly below 10°C (ideally 0–5°C). Since nitrous acid is unstable, it is generated in situ by reacting sodium nitrite ($NaNO_2$) with a cold, dilute strong acid like HCl. The product is benzenediazonium chloride, $C_6H_5N_2^+Cl^-$. The diazonium ion is highly reactive and serves as a versatile intermediate in synthesis.

$C_6H_5NH_2 + NaNO_2 + 2HCl \xrightarrow{0-5^\circ C} C_6H_5N_2^+Cl^- + NaCl + 2H_2O$

The benzenediazonium ion is a weak electrophile. It can react with highly activated aromatic rings, such as phenols or other aromatic amines, in an electrophilic substitution reaction known as a coupling reaction. For coupling with phenol, the reaction is carried out in cold, alkaline conditions. The alkaline conditions are necessary to deprotonate the phenol to form the more reactive phenoxide ion ($C_6H_5O^-$), which is a better nucleophile. The diazonium ion then attacks the benzene ring of the phenoxide ion, usually at the 4-position, to form a stable azo compound.

$C_6H_5N_2^+Cl^- + C_6H_5OH \xrightarrow{NaOH(aq), <10^\circ C} C_6H_5-N=N-C_6H_4OH + HCl$

The product, 4-hydroxyphenylazobenzene, is an example of an azo dye. Azo compounds contain the $-N=N-$ functional group, known as the azo group. This group acts as a chromophore, the part of the molecule that absorbs light. When the azo group links two benzene rings, it creates a large, stable, conjugated system of delocalised electrons. This extended delocalisation lowers the energy gap between the highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO). As a result, the molecule absorbs light in the visible region of the electromagnetic spectrum, and we perceive the complementary colour. In this case, the product is an orange solid.