Question 1

Why is KCN used with a weak acid instead of just using HCN gas?

Accepted Answer

HCN is a highly toxic, volatile liquid. Using an aqueous solution of KCN (which provides the CN⁻ nucleophile) and then adding a weak acid is much safer to handle in a lab. It also ensures a high concentration of the cyanide nucleophile, as HCN itself is a weak acid and only partially dissociates.

Question 2

Why don't ketones get oxidised by Tollens' or Fehling's reagents?

Accepted Answer

The oxidation of a ketone would require the breaking of a strong carbon-carbon (C-C) bond adjacent to the carbonyl group. This requires a significant amount of energy, so ketones are resistant to mild oxidising agents. Aldehydes have a C-H bond on the carbonyl carbon, which is much weaker and easier to break, allowing for ready oxidation.

Question 3

What is the difference between NaBH₄ and LiAlH₄ as reducing agents?

Accepted Answer

Both are sources of the hydride ion (:H⁻) and reduce aldehydes and ketones. However, LiAlH₄ (lithium aluminium hydride) is a much stronger and more reactive reducing agent. It reacts violently with water and protic solvents, so it must be used in a dry ether solvent. NaBH₄ is milder and safer, stable in aqueous or alcoholic solutions. For A-Level, you primarily need to know about NaBH₄ for reducing carbonyls.

Aldehydes and ketones

What you need to know

Carbonyl Chemistry: Attack and Test