Part 2: Hydroxynitriles, stereochemistry and carbon-chain extension

Write the cyanide-addition mechanism, name its products and account for the extra carbon and possible optical isomers.

The nitrile carbon becomes part of the product

Carbonyl compounds react with KCN followed by dilute acid to form hydroxynitriles. The overall stoichiometry can be written using HCN, adding H and CN across the C=O group. The product contains both an OH group and a C≡N group; the original carbonyl oxygen becomes the OH oxygen. One carbon atom is added to the molecule.

For naming, the nitrile carbon is carbon 1 of the parent chain. Propanal forms CH₃CH₂CH(OH)CN, 2-hydroxybutanenitrile. Propanone forms (CH₃)₂C(OH)CN, 2-hydroxy-2-methylpropanenitrile. Never drop the nitrile carbon when counting or naming.

CN⁻ attacks through its carbon

The cyanide ion is the nucleophile. Its carbon lone pair attacks the δ⁺ carbonyl carbon while the C=O π bond moves onto oxygen. The resulting alkoxide is protonated in the dilute-acid stage. Drawing the new bond from cyanide nitrogen would give a different connectivity and the wrong product.

Keep the full charge on CN⁻ distinct from the partial charge on the carbonyl. After addition, the nitrile group is neutral and the negative charge is on oxygen. The carbonyl carbon changes from trigonal planar to tetrahedral. A nucleophilic addition has no leaving group expelled from that carbon.

Distinguish a reagent from an equation shorthand

KCN supplies cyanide ions; dilute acid supplies the proton for the product-forming stage. HCN is a weak acid, so its aqueous solution contains relatively little CN⁻. The expression “HCN addition” is useful for the overall equation but does not mean the initial arrow starts from a neutral HCN hydrogen.

Cyanide salts and HCN are highly toxic, and acidification can release volatile HCN. This is reaction knowledge for the course, not an unsupervised preparation: use only the authorised laboratory method and controls. More acid is not automatically better because protonating CN⁻ lowers the nucleophile concentration.

Check four groups after attack

Ethanal and many other aldehydes form a new chiral carbon; methanal is the exception because its product has two H atoms. An unsymmetrical ketone may form a chiral centre, whereas a symmetrical ketone such as propanone does not. In achiral conditions, equally likely attack on the two faces of a planar carbonyl produces a racemate when that new centre is chiral.

An already chiral reactant or chiral catalyst can make the two faces non-equivalent; equal amounts are not a universal result for every possible carbonyl substrate. For the simple single-centre AQA examples, link planarity, equal attack and product chirality explicitly.

Worked example: maximum hydroxynitrile mass

Constructed example: 4.35 g of propanal, Mᵣ = 58.0, reacts with excess cyanide/proton source. The product CH₃CH₂CH(OH)CN has formula C₄H₇NO and Mᵣ = 85.0. n(propanal) = 4.35/58.0 = 0.0750 mol. The 1:1 ratio gives a theoretical product mass of 0.0750 × 85.0 = 6.375 g. If 4.78 g is isolated, percentage yield = 4.78/6.375 × 100 = 75.0% to three significant figures.

The ideal overall HCN-addition equation has 100% atom economy because it has one product. That is not a claim of 100% yield, no hazardous reagents or no process waste: actual reagent preparation, work-up and solvent use are additional considerations.

Quick checks

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

Sources