aldehydes and ketones [O2] PAPER 2

Question 1

aldehydes readily oxidise to…

Answer 1

carboxylic acids

Question 2

name and outline the mechanism, state the reagent, and the conditions for the reduction of aldehydes to primary alcohols / ketones to secondary alcohols.

Answer 2

• nucleophilic addition
• NaBH4
• aqueous

Question 3

name and outline the mechanism, state the reagents, and the conditions for the reaction of carbonyl compounds to produce a hydroxynitrile.

Answer 3

• nucleophilic addition
• KCN and dilute acid
• aqueous

Question 4

state the hazard associated with using KCN

Answer 4

toxic

Question 5

suggest a reason, other than safety, why KCN is used instead of HCN

Answer 5

HCN is weak (so, [CN–] is low when using HCN)

Question 6

Aqueous NaBH4 reduces aldehydes but does not reduce alkenes.

Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methylbut-1-ene.

Answer 6

• the nucleophile (H– ion) , is attracted to the δ+ C (of the C=O in 2-methylbutanal)
• the nucleophile (H– ion) is repelled by the electron rich C=C

Question 7

overall equation of a carbonyl compound reacting with HCN?

Answer 7

carbonyl compound + HCN → hydroxynitrile

Question 8

overall reduction reaction of an aldehyde/ketone?

Answer 8

aldehyde/ketone + 2[H] → 1°/2° alcohol

Question 9

why can nucleophilic addition reactions of KCN with aldehydes/ketones followed by dilute acid, produce a mixture of enantiomers?

Answer 9

• planar carbonyl group
• equal chance of nucleophilic attack from above or below the plane

Question 10

Answer 10

• CH3 has a positive inductive effect, so C in C=O is less δ+
• so, higher activation energy

Question 11

Explain why NaBH4 reduces 2-methylbutanal but has no reaction with 2-methylbut-1-ene.

Answer 11

• nucleophile is attracted to δ+ C (of the C=O in 2-methylbutanal)
• electron rich C=C (in 2-methylbut-1-ene) repels the nucleophile