Carbonyl Compounds: Question 8

Worked solution

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

The carbonyl carbon of butanone is electron-poor (δ+), because the more electronegative oxygen atom of the $\text{C=O}$ bond pulls electron density towards itself. A trace of $\text{CN}^-$ ion (from the KCN, or from partial dissociation of HCN itself) acts as a nucleophile, using a lone pair of electrons on its carbon atom to attack this δ+ carbonyl carbon. This is the same nucleophilic addition mechanism seen with an aldehyde: $\text{CN}^-$ attacks first, forming a tetrahedral alkoxide intermediate that is then protonated (by a further HCN molecule) to give the neutral product and regenerate $\text{CN}^-$.

Part (b): The product of the addition

$\text{CN}^-$ adds to the carbonyl carbon of butanone, and the oxygen (after protonation) becomes an -OH group on that same carbon: $\text{CH}_3\text{COCH}_2\text{CH}_3 + \text{HCN} \rightarrow \text{CH}_3\text{CH}_2\text{C(OH)(CH}_3\text{)CN}$ The former carbonyl carbon is now bonded to four different groups: an -OH group, a $\text{CN}$ group, a $\text{CH}_3$ group, and a $\text{CH}_2\text{CH}_3$ group. Naming the longest chain from the nitrile carbon gives a four-carbon chain (a butanenitrile), with a hydroxy substituent and a methyl substituent both on carbon 2. The product is 2-hydroxy-2-methylbutanenitrile, $\text{CH}_3\text{CH}_2\text{C(OH)(CH}_3\text{)CN}$.

Part (c): Why butanone reacts more slowly than butanal

Butanal’s carbonyl carbon is bonded to one alkyl group (a propyl chain) and one hydrogen atom. Butanone’s carbonyl carbon, by contrast, is bonded to two alkyl groups (a methyl group and an ethyl group), for two reasons this slows the reaction:

• Electronic effect: alkyl groups are electron-donating by the inductive effect. With two alkyl groups feeding electron density into the carbonyl carbon (instead of just one, as in the aldehyde), the δ+ charge on butanone’s carbonyl carbon is smaller, so it attracts the $\text{CN}^-$ nucleophile less strongly.
• Steric effect: the two bulky alkyl groups either side of butanone’s carbonyl carbon physically hinder the approach of the incoming $\text{CN}^-$ ion, whereas butanal’s smaller hydrogen atom leaves the carbonyl carbon more open to attack.

Both effects act in the same direction, so nucleophilic addition of HCN to butanone proceeds more slowly than to butanal.

Final answers

• (a) Nucleophilic addition; the attacking species is the $\text{CN}^-$ ion.
• (b) 2-hydroxy-2-methylbutanenitrile, $\text{CH}_3\text{CH}_2\text{C(OH)(CH}_3\text{)CN}$.
• (c) Butanone’s carbonyl carbon carries two electron-donating, sterically bulky alkyl groups (versus one alkyl group and one hydrogen in butanal), which both reduce its δ+ charge and hinder the approach of $\text{CN}^-$, so butanone reacts more slowly.