Carbonyls, aromatics and amines

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.

Worked examples

Example 1

The enthalpy of hydrogenation of cyclohexene is −120 kJ/mol and of benzene is −208 kJ/mol. How much more stable is benzene than a structure with three separate C=C bonds?

  1. Three separate C=C bonds would give three times the value for cyclohexene. 3 × (−120) = −360 kJ/mol
  2. The real value is far less exothermic. −208 − (−360) = 152 kJ/mol
  3. Benzene is 152 kJ/mol more stable: this is the delocalisation energy.

Answer: 152 kJ/mol

Example 2

How could you tell ⟦propanal⟧ from ⟦propanone⟧ using Tollens’ reagent?

  1. Warm each with Tollens’ reagent (silver ions in ammonia) in a water bath.
  2. ⟦Propanal⟧, an aldehyde, is oxidised to ⟦propanoic acid⟧; the silver ions are reduced to silver. Ag⁺ + e⁻ → Ag
  3. ⟦Propanone⟧, a ketone, is not oxidised, so nothing happens.

Answer: ⟦Propanal⟧ gives a silver mirror; ⟦propanone⟧ shows no change.

Example 3

Glycine (Mr = 75) and alanine (Mr = 89) join to form a dipeptide. Find its relative formula mass.

  1. The –COOH of one amino acid reacts with the –NH₂ of the other, forming –CONH– and releasing water.
  2. Subtract one water molecule. 75 + 89 − 18 = 146

Answer: 146