Aldehydes + ketones EQ's

Question 1

The table contains data on propanone and ethanoic acid. Explain, by reference to the data and any intermolecular forces involved, the difference in the boiling temperatures.

Answer 1

• Similar molar masses :- similar London forces.
• They both form permanent dipole-dipole forces
• only ethanoic acid forms hydrogen bonding
• which is stronger so requires more energy to break

Question 2

What test is used to identify a carbonyl compound?

Answer 2

2,4-dinitrophenylhydrazine (2,4-DNPH)

Question 3

Describe a chemical test, and its result, to distinguish between pentan-2-one, CH3CH2CH2COCH3, and pentan-3-one, CH3CH2COCH2CH3.

Answer 3

• add a solution of iodine and alkali
• only pentan-2-one give a pale yellow precipitate

Question 4

How are carbonyls reduced?

Answer 4

Aldehydes are reduced to primary alcohols and ketones are reduced to secondary alcohols. This is done with lithium tetrahydridoaluminate / lithium aluminium hydride, LiAlH4, in dry ether. The LiAlH4 generates a hydride ion nucleophile, :H-
The hydride ion reduces the carbonyl group in an aldehyde or a ketone

Question 5

The product of this reaction, 2-hydroxy-2-methylpentanenitrile, has a chiral centre.
Explain why a racemic mixture of 2-hydroxy-2-methylpentanenitrile is formed in this
reaction.

Answer 5

petan-2-one/ ketone is planar about carbonyl carbon so the CN- nucleophile attacks either side.