Part 1: Extending a carbon skeleton with cyanide

Distinguish substitution from addition and use atom tracking to plan a one-carbon extension.

Functional-group conversion does not always change chain length

Oxidising an alcohol to an acid changes a functional group but normally retains the carbon skeleton in the taught route. Making a larger molecule requires forming a new carbon–carbon bond. Cyanide routes and Friedel–Crafts reactions provide tools for doing this.

Before selecting reagents, compare the number and arrangement of carbons in starting material and target. Mark which target carbon will be supplied by a new reagent. This prevents choosing a route that changes NH₂ or OH correctly but gives the wrong chain length.

The carbon in CN⁻ is a real skeletal carbon. It is retained when a nitrile is reduced to an amine or hydrolysed to a carboxylic acid. Treating “CN” as a label outside the carbon count causes systematic one-carbon errors.

Nucleophilic substitution adds the nitrile carbon

A haloalkane reacts with cyanide ions from an appropriate cyanide salt in ethanol on heating. The cyanide carbon attacks the δ+ carbon bonded to the halogen. A curly arrow starts at the carbon lone pair of CN⁻ and ends at that electrophilic carbon; a second arrow starts at the C–halogen bond and ends on the halogen.

The product is a nitrile, R–C≡N, plus a halide ion. The C≡N bond remains intact and the new bond joins two carbon atoms. The reaction is nucleophilic substitution because cyanide replaces the halogen.

For bromoethane: CH₃CH₂Br + CN⁻ → CH₃CH₂CN + Br⁻. The product is propanenitrile, with three carbons. A mechanism should use the reacting CN⁻ ion rather than drawing an electron-pair arrow from an undifferentiated KCN formula.

Nucleophilic addition keeps oxygen as OH

HCN addition to an aldehyde or ketone also forms a new C–C bond. CN⁻ attacks the carbonyl carbon while the C=O π electrons move to O; protonation of the alkoxide gives a hydroxynitrile. No leaving halide is involved.

For ethanal, CH₃CHO + HCN → CH₃CH(OH)CN. The product has three carbons and an OH group on the original carbonyl carbon. The mechanism is addition because atoms add across the C=O bond without substitution of a group from that carbon.

Compare this with bromoethane substitution. Both gain one carbon, but the carbonyl route also retains oxygen as OH. Choosing between them depends on whether the target needs that hydroxy group. Keep cyanide’s serious toxicity in mind; these are examination transformations, not unsupervised preparation instructions.

Worked planning example: two three-carbon acids

To make propanoic acid from a two-carbon haloalkane, use bromoethane → propanenitrile → propanoic acid. The new nitrile carbon becomes the acid’s carboxyl carbon.

To make 2-hydroxypropanoic acid from a two-carbon carbonyl compound, use ethanal → CH₃CH(OH)CN → CH₃CH(OH)COOH. Hydrolysis changes CN into COOH while retaining the OH produced by addition.

Write the intermediate structure before naming reagents. If the desired acid has an OH on carbon 2, an ordinary haloalkane-to-nitrile route will not introduce it by itself. If the target has no OH, carbonyl addition may introduce an unnecessary group.

Quick checks

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

Sources