Amines, basicity and aromatic nitrogen chemistry

What you’ll learn

• Explain amine basicity using the nitrogen lone pair.
• Compare aliphatic amines, ammonia and phenylamine.
• Recognise substitution, acylation and dye-forming transformations.

Understand the idea

Amines are derivatives of ammonia in which carbon groups replace hydrogen atoms. A primary amine has one carbon group attached to nitrogen, regardless of how substituted that carbon is. The nitrogen lone pair accepts a proton, making amines bases, and can attack an electron-deficient carbon, making them nucleophiles. Small amines interact strongly with water through hydrogen bonding, but water solubility decreases as the hydrophobic carbon portion grows.

For the usual primary-amine comparison in water, an alkyl group donates electron density towards nitrogen and makes a primary aliphatic amine more basic than ammonia. In phenylamine, the lone pair is partly delocalised into the aromatic ring and is less available to accept a proton, so phenylamine is less basic. This comparison does not establish a universal order for all substituted amines because solvation and steric effects also matter.

At the reaction-scheme level, halogenoalkanes can undergo substitution by ammonia, nitriles can be reduced to primary amines, and aromatic nitro groups can be reduced to aromatic amines. Further alkylation can create mixtures, while acylation forms an amide. Aromatic amines can be converted into diazonium ions, whose coupling with an activated aromatic ring produces an azo linkage in a dye. These scheme relationships describe functional-group changes rather than laboratory preparation instructions.

Key formulae

$$\mathrm{RNH_2+H_2O\rightleftharpoons RNH_3^++OH^-}$$

$$\mathrm{RNH_2+H^+\longrightarrow RNH_3^+}$$

$$\mathrm{RCN+4[H]\longrightarrow RCH_2NH_2}$$

$$\mathrm{ArNO_2+6[H]\longrightarrow ArNH_2+2H_2O}$$

Worked example

Order butan-1-amine, ammonia and phenylamine by increasing basicity in water, and explain the trend.

1. Phenylamine has its nitrogen lone pair partly delocalised into the ring, reducing its availability for protonation.
2. Ammonia has a localised lone pair without the electron-donating butyl group.
3. The butyl group increases electron density at nitrogen in butan-1-amine.

Answer: Phenylamine < ammonia < butan-1-amine for this comparison.