Compound J is butanal, CH3CH2CH2CHO. Compound K is butan-2-one, CH3COCH2CH3. Both are treated separately with sodium tetrahydridoborate(III), NaBH4. (a) State the type of reaction that NaBH4 brings about at a carbonyl group, and identify the species that first attacks the carbon…

Answer: ((a)) Reduction; the hydride ion, H^-, acts as a nucleophile and attacks the (delta+) carbonyl carbon. ((b)) CH3CH2CH2CH2OH, butan-1-ol. A primary alcohol. ((c)) CH3CH(OH)CH2CH3, butan-2-ol. A secondary alcohol.. Part (a): The type of reaction and the attacking species NaBH4 is a source of hydride ions, H^-. In solution it releases H^-, which behaves as a nucleophile: it uses its lone pair of electrons to attack the electron-poor (δ+) carbon of the…

AS Chemistry 9701: Reduction of an Aldehyde and a Ketone by NaBH4

Compound J is butanal, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO}$ . Compound K is butan-2-one, $\text{CH}_3\text{COCH}_2\text{CH}_3$ . Both are treated separately with sodium tetrahydridoborate(III), $\text{NaBH}_4$ .

(a) State the type of reaction that $\text{NaBH}_4$ brings about at a carbonyl group, and identify the species that first attacks the carbonyl carbon. [2]

(b) Give the structural formula and name of the organic product formed when J is reduced by $\text{NaBH}_4$ , and state whether it is a primary or secondary alcohol. [2]

(c) Give the structural formula and name of the organic product formed when K is reduced by $\text{NaBH}_4$ , and state whether it is a primary or secondary alcohol. [2]

Part (a): The type of reaction and the attacking species

$\text{NaBH}_4$ is a source of hydride ions, $\text{H}^-$ . In solution it releases $\text{H}^-$ , which behaves as a nucleophile: it uses its lone pair of electrons to attack the electron-poor (δ+) carbon of the carbonyl group, since oxygen’s higher electronegativity pulls electron density away from carbon in the $\text{C=O}$ bond. This is a reduction, effectively adding two hydrogen atoms across the double bond ( $\text{C=O} \rightarrow \text{CH-OH}$ ), it belongs to the same general family of reaction, nucleophilic addition, as the addition of HCN to a carbonyl group.

Part (b): Reduction of butanal (J)

Butanal is an aldehyde. Reduction of an aldehyde with $\text{NaBH}_4$ converts the $\text{-CHO}$ group into a $\text{-CH}_2\text{OH}$ group, giving a primary alcohol: $\text{CH}_3\text{CH}_2\text{CH}_2\text{CHO} \xrightarrow{\text{NaBH}_4} \text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$ The product is butan-1-ol, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$ , a primary alcohol (the carbon bearing the -OH group is attached to only one other carbon atom).

Part (c): Reduction of butan-2-one (K)

Butan-2-one is a ketone. Reduction of a ketone with $\text{NaBH}_4$ converts the $\text{C=O}$ group into a $\text{CH-OH}$ group, giving a secondary alcohol: $\text{CH}_3\text{COCH}_2\text{CH}_3 \xrightarrow{\text{NaBH}_4} \text{CH}_3\text{CH(OH)}\text{CH}_2\text{CH}_3$ The product is butan-2-ol, $\text{CH}_3\text{CH(OH)}\text{CH}_2\text{CH}_3$ , a secondary alcohol (the carbon bearing the -OH group is attached to two other carbon atoms).

Final answers

(a) Reduction; the hydride ion, $\text{H}^-$ , is the attacking nucleophile.
(b) Butan-1-ol, $\text{CH}_3\text{CH}_2\text{CH}_2\text{CH}_2\text{OH}$ , a primary alcohol.
(c) Butan-2-ol, $\text{CH}_3\text{CH(OH)}\text{CH}_2\text{CH}_3$ , a secondary alcohol.