Carbonyls, aromatics and amines · Grade 12
Carbonyl compounds contain the C=O group: aldehydes have it at the end of the chain, ketones inside it. Benzene is a ring of six carbon atoms whose π electrons are delocalised, which makes it unusually stable. Amines are organic bases related to ammonia, and amino acids carry both an amine group and a carboxylic acid group.
Key facts
- In benzene, C₆H₆, each carbon uses one electron in a ring of delocalised π electrons above and below the plane. All six C–C bonds are the same length, and the enthalpy of hydrogenation (−208 kJ/mol) is much less exothermic than expected for three C=C bonds (3 × −120 = −360 kJ/mol).
- Benzene reacts by electrophilic substitution, which keeps the delocalised ring. Nitration uses concentrated HNO₃ and H₂SO₄ (electrophile NO₂⁺); Friedel–Crafts acylation uses an acyl chloride and AlCl₃ (electrophile RCO⁺).
- Primary alcohols are oxidised by acidified dichromate(VI) to aldehydes (distil off) and then carboxylic acids (reflux); secondary alcohols give ketones; tertiary alcohols do not react. The orange dichromate turns green.
- Aldehydes give a silver mirror with Tollens’ reagent and a brick-red precipitate with Fehling’s solution; ketones do not. Both give an orange precipitate with 2,4-DNPH.
- NaBH₄ reduces aldehydes to primary alcohols and ketones to secondary alcohols. HCN (from KCN and acid) adds across C=O by nucleophilic addition, giving a hydroxynitrile.
- Amines are bases: the lone pair on nitrogen accepts H⁺. Ethylamine is a stronger base than ammonia; phenylamine is weaker, because its lone pair is delocalised into the ring.
- Amino acids are zwitterions (–NH₃⁺ and –COO⁻) at their isoelectric point. They join by condensation to form peptide bonds, –CONH–, losing water.