Carbon–carbon bond formation — OCR A-Level Chemistry

OCR H432 section 6.2.4 covers two routes that build new carbon–carbon bonds and so lengthen a carbon chain. The first uses cyanide: a haloalkane reacts with KCN in aqueous ethanol under reflux to give a nitrile, adding one carbon to the chain; reducing the nitrile (for example with LiAlH₄ in dry ether, or H₂ with a nickel catalyst) gives a primary amine, and hydrolysing the nitrile with dilute acid gives a carboxylic acid, each with the extended chain. The second is Friedel–Crafts acylation: an aromatic ring reacts with an acyl chloride in the presence of anhydrous AlCl₃ catalyst to give a ketone, attaching the acyl group's carbons to the ring. Count carbons carefully: the nitrile route adds exactly one carbon, whereas Friedel–Crafts adds the whole acyl group.

Carbon–carbon bond formation is the key step that lengthens a carbon chain during synthesis. In this specification the reactions to know are the Friedel–Crafts reactions of benzene with a halogenoalkane or an acyl chloride in the presence of an AlCl₃ catalyst, and the addition of a carbonyl compound to a cyanide ion followed by acid hydrolysis. In each case a new C–C bond joins two fragments, so the product has more carbon atoms than the starting material. For example, benzene reacting with CH₃CH₂Cl and AlCl₃ gives ethylbenzene, whose side chain adds two carbons to the ring. A carbonyl compound reacting with HCN/KCN gives a hydroxynitrile, and acid hydrolysis converts the nitrile group into a carboxylic acid, adding one carbon. Count carbons before and after to confirm the chain has grown.

This statement introduces the formation of a nitrile group, written C–C≡N, in which a carbon atom of the skeleton is bonded to a carbon that is triply bonded to nitrogen. The nitrile carbon counts as one added carbon, so making a nitrile is a one-carbon chain-lengthening step. The statement is a heading whose sub-clauses list the specific reactions; learners should be ready to recognise the reagents and conditions for each route and to write the organic product with the C–C≡N group attached to the correct carbon. Because the sub-clauses are not reproduced here, treat the heading as the scope boundary and rely on the reactions taught in the course rather than inventing routes.

This item tests whether you can extend the nucleophilic substitution chemistry of haloalkanes (met in 4.2.2 c) to a new nucleophile, the cyanide ion. When a haloalkane is warmed with a solution containing CN⁻ in ethanol, the cyanide ion acts as a nucleophile and attacks the electron-deficient carbon atom that carries the halogen. The carbon–halogen bond breaks and a new carbon–carbon bond forms, so the carbon chain lengthens by one carbon atom and a nitrile is produced. For a primary haloalkane such as bromoethane, the organic product is propanenitrile. The mechanism is nucleophilic substitution: the lone pair on the carbon of CN⁻ attacks the δ⁺ carbon, the C–X bond breaks heterolytically and the halide ion leaves. Ethanol is the solvent that brings the ionic cyanide into contact with the organic haloalkane.

This item extends the carbonyl chemistry of 6.1.2 b–c to reaction with hydrogen cyanide. The C=O bond is polar, so the carbonyl carbon is electron-deficient (δ⁺) and the oxygen is δ⁻. In HCN, the cyanide ion CN⁻ provides a lone pair and acts as a nucleophile. The mechanism is nucleophilic addition: CN⁻ attacks the δ⁺ carbonyl carbon, the π bond of C=O breaks and the electrons move onto the oxygen, giving a negatively charged intermediate. A proton then transfers to that oxygen, producing a hydroxynitrile (a cyanohydrin) with a new carbon–carbon bond. For ethanal the product is 2-hydroxypropanenitrile. Because a new chiral centre can be created at the carbon bearing OH and CN, a racemic mixture may form.

This sub-topic follows on from the formation of a hydroxynitrile in (b): a carbonyl compound reacts with HCN (or with cyanide ions followed by acid) to add a –C≡N group and lengthen the carbon chain by one carbon. Part (c) asks you to use that nitrile as a synthetic intermediate. The key reactions are hydrolysis of the nitrile to a carboxylic acid (heating with dilute aqueous acid, or with aqueous alkali followed by acidification) and reduction of the nitrile to a primary amine (for example H₂ with a nickel catalyst). You must be able to name the product, identify the functional group change and explain why the nitrile route is valuable for carbon–carbon bond formation.

This statement covers the reduction of a nitrile to an amine. The nitrile, formed by addition of HCN to a carbonyl compound, contains a –C≡N group. Reducing it with hydrogen and a nickel catalyst converts the triple bond and adds hydrogen so that the carbon becomes bonded to an –NH₂ group, giving a primary amine. You should be able to write the equation or displayed formula, name the amine product and state the reagent and catalyst. Recognise that the carbon skeleton is unchanged apart from the functional group, so the amine has the same number of carbons as the nitrile.

A nitrile group, –C≡N, can be converted into a carboxylic acid group, –COOH, by acid hydrolysis. The nitrile is heated with dilute aqueous acid, such as dilute hydrochloric or sulfuric acid. The acid is a reagent here, not a regenerated catalyst: it is consumed as the nitrile nitrogen is released as an ammonium ion, NH₄⁺. For example, heating ethanenitrile with dilute acid gives ethanoic acid and ammonium ions. The carbon skeleton is unchanged, so the number of carbon atoms in the acid equals the number in the nitrile. This step is used after cyanide addition to a carbonyl compound, where the hydroxynitrile is hydrolysed to a carboxylic acid that has one more carbon than the original carbonyl compound.

This statement covers Friedel–Crafts chemistry, in which a benzene ring gains a new carbon–carbon bond to a substituent. In alkylation a haloalkane such as chloroethane reacts with benzene, and in acylation an acyl chloride such as ethanoyl chloride reacts with benzene. Neither reaction proceeds usefully without a halogen carrier, a Lewis acid catalyst such as aluminium chloride or iron(III) chloride, which accepts a chloride ion from the haloalkane or acyl chloride to generate a reactive electrophile. The electrophile attacks the electron-rich ring, and loss of a proton restores aromaticity, giving an alkylbenzene or a ketone respectively. Alkylation attaches an alkyl group; acylation attaches an acyl group, forming an aromatic ketone such as phenylethanone.