CAIE A-Level Chemistry 17.1.3 HCN Addition to Carbonyl Compounds

Practise drawing nucleophilic addition of HCN to aldehydes and ketones with dipoles, charges, lone pairs and arrows.

Exam points

draw CN− attacking the δ+ carbonyl carbon while the C=O electron pair moves to oxygen
show the tetrahedral O− intermediate with the new C–CN bond and all relevant charges
protonate O− using HCN, drawing the arrow from the oxygen lone pair to H to regenerate CN−

[Maximum number: 3]

$4\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCHO}$ is used in the synthesis of some antibiotics.

X, Y and Z are all isomers of $\left(\mathrm{CH}_{3}\right)_{3} \mathrm{CCHO}$ .

A summary of some of the reactions and properties of X, Y and Z is shown in the table.

Table for Question 17.1.3—Reactions of hydrogen cyanide question 1 — CAIE A-Level Chemistry AS

X contains a carbonyl group. X reacts with HCN , in the presence of a small amount of NaCN , to form $\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CN}$ as shown.

\mathbf{X}+\mathrm{HCN} \rightarrow\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{C}(\mathrm{OH}) \mathrm{CN}

Draw the mechanism of the reaction of X with HCN .
- Draw the structure of X and the intermediate.
- Include all charges, partial charges, lone pairs and curly arrows.

Figure for Question 17.1.3—Reactions of hydrogen cyanide question 1 — CAIE A-Level Chemistry AS

Figure for Answer to Question 17.1.3—Reactions of hydrogen cyanide question 1 — CAIE A-Level Chemistry AS

M1: Correct structure of X and correct dipole on C=O

M2: curly arrow from C=O bond to O AND intermediate with CN attached and -ve charge on the O

M3: curly arrow from lone pair on $\mathrm{CN}^{-}$ to C(=O) in X AND curly arrow from lone pair in the intermediate to $\mathrm{H}^{+}$

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17.1.3—Reactions of hydrogen cyanide question 1