Part 2: Nitrile conversion and aromatic C–C synthesis

Carry the new carbon through reduction or hydrolysis and distinguish aromatic alkylation from acylation.

The nitrile carbon becomes CH₂NH₂

Reduction with hydrogen and a nickel catalyst converts a nitrile into a primary amine. RCN + 4[H] → RCH₂NH₂, or RCN + 2H₂ → RCH₂NH₂. The nitrile carbon remains bonded to the original R group and becomes the carbon attached to NH₂.

Propanenitrile, CH₃CH₂CN, therefore gives propylamine, CH₃CH₂CH₂NH₂. It does not give ethylamine. Compare direct bromoethane/ammonia substitution, which retains two carbons, with bromoethane/cyanide followed by reduction, which gives a three-carbon amine.

In a multifunctional molecule, consider other groups that may also react under catalytic hydrogenation conditions. An examination may supply selectivity information; use it rather than assuming every functional group is untouched.

The nitrile carbon becomes COOH

Heating a nitrile with dilute aqueous acid under reflux hydrolyses it to a carboxylic acid. Nitrogen ends in an ammonium ion in the acidic solution. One balanced ionic equation is RCN + 2H₂O + H⁺ → RCOOH + NH₄⁺.

Check the atoms: the two oxygens come from two waters; four H atoms and one extra H⁺ account for the acid OH and NH₄⁺. Writing free NH₃ as the sole nitrogen product in strongly acidic solution ignores its protonation.

For a hydroxynitrile, hydrolysis converts CN to COOH while the neighbouring OH remains under the intended route. Carbon count does not change in this conversion; the chain extension occurred in the earlier cyanide step.

Friedel–Crafts gives a different way to add carbons

Benzene reacts with a haloalkane in the presence of a suitable anhydrous halogen carrier such as AlCl₃ to give an alkylbenzene. Benzene plus chloroethane can give ethylbenzene, with HCl as the overall by-product. The two-carbon ethyl group attaches to the ring.

With an acyl chloride and an appropriate halogen carrier, benzene forms an aromatic ketone. Benzene plus propanoyl chloride gives C₆H₅COCH₂CH₃. The carbonyl carbon is directly attached to the ring, and all three carbons of the acyl fragment are retained.

The two routes differ by the incoming functional group. Acylation preserves C=O, whereas alkylation does not introduce one. Both replace ring H and form a C–C bond, retaining aromaticity overall. Do not confuse acylation of the ring with ester formation at phenol’s oxygen.

Worked quantitative route: account for each stage

An original two-stage route starts with 0.0800 mol bromoethane. Nitrile formation gives a 75.0% isolated yield, so 0.0800 × 0.750 = 0.0600 mol propanenitrile is available. Its reduction gives an 80.0% yield, so the final amount is 0.0600 × 0.800 = 0.0480 mol propylamine.

The overall yield is 0.750 × 0.800 × 100 = 60.0%, not the arithmetic mean of 75% and 80%. With Mr 59.0, final amine mass is 2.83 g. Check the 1:1 organic mole ratio at each stage before multiplying yields.

For any proposed sequence, make four checks: the carbon skeleton reaches the target; each reagent gives the drawn intermediate; all functional groups are considered; and any final acid/base work-up gives the requested chemical form.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources