6.2.4 Carbon–Carbon Bond Formation

When a synthesis needs more carbon atoms than the starting material, look for a C–C bond-forming step. In this part of OCR, the main moves are CN⁻ substitution of a haloalkane, HCN addition to a carbonyl compound, and Friedel–Crafts alkylation or acylation of an aromatic ring.

Count carbons before and after the step. Both cyanide routes add the carbon atom of CN⁻ to the skeleton, so the organic product has one extra carbon.

Heat the haloalkane with KCN in ethanol. CN⁻ replaces the halide, giving a nitrile and extending the carbon chain by one carbon.

Take bromoethane: CH₃CH₂Br → CH₃CH₂CN. Keep ethanol in the conditions: OCR examiner reports have specifically picked up answers that incorrectly add acid or use aqueous cyanide for this conversion.

CN⁻ is the nucleophile. Start the curly arrow at the carbon end of CN⁻ and point it to the δ⁺ carbon bonded to the halogen. At the same time, move the C–X bonding pair onto X.

The product contains a new C–C bond and X⁻ leaves. For CH₃CH₂Br, the organic product is CH₃CH₂CN. A full curly arrow always tracks an electron pair, so both arrows must start where those electrons are before the step.

Aldehydes and ketones react with HCN to form hydroxynitriles. OCR commonly supplies the HCN through NaCN(aq)/H⁺(aq).

The carbonyl carbon stays in the molecule and the carbon from CN⁻ becomes an extra carbon in the product. Ethanal, CH₃CHO, therefore gives CH₃CH(OH)CN, which has three carbons.

Start with the polar C=O bond. The curly arrow goes from the carbon end of CN⁻ to the δ⁺ carbonyl carbon, while the C=O π pair moves onto oxygen.

That gives an O⁻ intermediate with the new C–C bond already formed. Protonate O⁻ to make the –OH group. Check the finished product has both –OH and –CN on the former carbonyl carbon.

Reduce the nitrile with H₂ and a Ni catalyst. The –C≡N carbon is retained and becomes –CH₂NH₂, so the product is a primary amine.

Reducing propanenitrile, CH₃CH₂CN, gives CH₃CH₂CH₂NH₂. Keep the full carbon skeleton when you draw the product; the nitrile carbon does not disappear during reduction.

Heat the nitrile under reflux with dilute aqueous acid, for example HCl(aq). The –C≡N carbon becomes the carboxyl carbon of –COOH.

CH₃CH₂CN therefore gives CH₃CH₂COOH, not ethanoic acid. In acidic solution the nitrogen ends up as NH₄⁺; for synthesis questions, the key organic change is nitrile → carboxylic acid with the same number of carbons.

For Friedel–Crafts alkylation, react the aromatic compound with a haloalkane and a halogen carrier such as AlCl₃. The alkyl group becomes attached directly to the ring, creating a new C–C bond.

Benzene with CH₃Cl/AlCl₃ gives methylbenzene. When the haloalkane is unfamiliar, keep its carbon skeleton intact as you attach the alkyl group to the aromatic ring.

For Friedel–Crafts acylation, use an acyl chloride with a halogen carrier such as AlCl₃. The group attached to the ring is RCO–, so the product is an aromatic ketone.

Benzene with CH₃COCl/AlCl₃ gives C₆H₅COCH₃. Keep the C=O in the group you attach; removing it changes the reaction into alkylation.