Butanal, CH3CH2CH2CHO, reacts with hydrogen cyanide, HCN, in the presence of a trace of potassium cyanide, KCN, to form an addition product. (a) Name the type of mechanism by which HCN adds across the carbonyl group of butanal, and identify the species that attacks the carbonyl…

Answer: ((a)) Nucleophilic addition; the cyanide ion, CN^-, is the attacking species (nucleophile). ((b)) The lone pair on the carbon of CN^- attacks the (delta+) carbonyl carbon, forming a new C-C bond; the C=O pi electrons move fully onto the oxygen, giving a tetrahedral, negatively charged alkoxide intermediate; this intermediate then removes a proton from a further HCN molecule,… ((c)) CH3CH2CH2CH(OH)CN, 2-hydroxypentanenitrile; it has one more carbon than butanal because the carbon of the incoming CN group becomes bonded directly to the former carbonyl carbon, adding a new carbon to the chain.. Part (a): Type of mechanism and the attacking species Butanal's carbonyl carbon is electron-poor (δ+), because oxygen is more electronegative than carbon and pulls electron density away from it in the C=O bond. This makes the carbonyl carb…

AS Chemistry 9701: Nucleophilic Addition Mechanism of HCN with an Aldehyde

Part (a): Type of mechanism and the attacking species

Butanal’s carbonyl carbon is electron-poor (δ+), because oxygen is more electronegative than carbon and pulls electron density away from it in the $\text{C=O}$ bond. This makes the carbonyl carbon susceptible to attack by an electron-rich species, a nucleophile. A trace amount of $\text{CN}^-$ ion is present, either added directly (as KCN) or formed because HCN is a weak acid that partially dissociates, $\text{HCN} \rightleftharpoons \text{H}^+ + \text{CN}^-$ . It is this $\text{CN}^-$ ion, not the neutral HCN molecule, that acts as the nucleophile and attacks first. The overall mechanism is therefore nucleophilic addition.

Part (b): The mechanism, step by step

The carbon atom of the $\text{CN}^-$ ion uses its lone pair of electrons to attack the δ+ carbonyl carbon of butanal, forming a new carbon-carbon bond.
As this bond forms, the two electrons in the $\text{C=O}$ pi bond move fully onto the oxygen atom, breaking the double bond.
This gives a tetrahedral, negatively charged alkoxide intermediate: the former carbonyl carbon is now bonded to four groups, an oxygen atom carrying the negative charge, the $\text{CN}$ group, a hydrogen atom, and the propyl chain.
The negatively charged oxygen of this intermediate then removes a proton from a further molecule of HCN, which neutralises the charge (giving the neutral -OH group of the final product) and regenerates a $\text{CN}^-$ ion, allowing the small amount of catalyst to continue reacting.

Part (c): The product and why it has an extra carbon atom

Overall: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO} + \text{HCN} \rightarrow \text{CH}_3\text{CH}_2\text{CH}_2\text{CH(OH)CN}$ The product is 2-hydroxypentanenitrile, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH(OH)CN}$ . Butanal has four carbon atoms, but the product has five. This is because the nucleophile that attacks in step 1 is the carbon atom of the $\text{CN}^-$ ion (a completely new carbon, not one already present in butanal) and this carbon becomes covalently bonded to the former carbonyl carbon. So every nucleophilic addition of HCN to a carbonyl compound extends the carbon chain by exactly one carbon atom, while also introducing both a new -OH group and a new nitrile group in a single step.

Final answers

(a) Nucleophilic addition; the attacking species is the $\text{CN}^-$ ion.
(b) $\text{CN}^-$ attacks the δ+ carbonyl carbon, the $\text{C=O}$ electrons move onto oxygen to give a tetrahedral alkoxide intermediate, which is then protonated (by HCN) to give the neutral product and regenerate $\text{CN}^-$ .
(c) 2-hydroxypentanenitrile, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH(OH)CN}$ , one more carbon than butanal because the carbon of the $\text{CN}$ group joins the chain.