3.3.8 Aldehydes and ketones

Question 1

Whats the difference between aldehydes and ketones?

Answer 1

Both aldehydes and ketones are carbonyl compounds (contain the C=O functional group). The difference is the position of the carbonyl group.

Aldehydes:
* Have the carbonyl group at the end of the carbon chain
* End in the suffix -al

Ketones:
* Have the carbonyl group in the middle of the carbon chain
* End in the suffix -one, and often have a number to show which carbon the carbonyl group is on

Question 2

Explain what happens to Tollen’s reagent when it is added to an aldehyde?

Answer 2

When heated in a test tube with an aldehyde, the Ag⁺ ions are reduced to Ag atoms and a silver mirror form a few minutes later

Question 3

Explain what happens to Tollen’s reagent when it is added to a ketone?

Answer 3

Ketone can’t be oxidised by Tollen’s reagent, so with ketones there is no reaction or colour change

Question 4

Explain what happens to Fehlings solution when it is added to an aldehyde?

Answer 4

When heated with an aldehyde, the copper(II) ions (in Fehling’s) are reduced and form a brick red precipitate of copper(I) oxide

Fehlings solution is blue before it is reduced

Question 5

Explain what happens to Fehling’s solution when it is added to a ketone?

Answer 5

Ketones don’t react with Fehling’s solution, so no precipitate is formed

Question 6

Explain how aldehydes can be reduced back into alcohols

Answer 6

Using the reducing agent, NaNH₄, dissolved in water with methanol, maybe indicated as [H] in equations.

Reducing an aldehyde to a primary alcohol:

RCHO + 2[H] -> RCH₂OH

1. An :H^- ion from the reducing agent attacks the δ+ carbon
2. A pair of electrons from the C=O double bond is attracted onto the δ- oxygen
3. The pair of electrons on the water is attracted to and bonds to an H⁺ form a water molecule (or weak acid)
4. This forms a primary alcohol

This is a nucleophillic addition reaction

Question 7

Explain how ketones can be reduced back into alcohols

Answer 7

Using the reducing agent, NaBH₄, dissolved in water with methanol, maybe indicated as [H] in equations.

Reducing a ketone to a primary alcohol:

RCOR’ + 2[H] -> RCHOHR’

1. An :H^- ion from the reducing agent attacks the δ+ carbon
2. A pair of electrons from the C=O double bond is attracted onto the δ- oxygen
3. The pair of electrons on the water is attracted to and bonds to an H⁺ form a water molecule (or weak acid)
4. This forms a primary alcohol

This is a nucleophillic addition reaction

Question 8

Explain how KCN will react with carbonyl compounds by nucleophillic addition to produce hydroxynitriles

Answer 8

Potassium cyanide dissociates in water to form K⁺ and CN^- ions: KCN -> K⁺ + CN^-

1. The CN^- groups attacks the partially positive carbon atom (from the polar carbonyl group) and donates a pair of electrons. A pair of electons from the double bond transfer to the oxygen
2. H⁺ ions add to the oxygen to form the hydroxyl group (OH). Acidified KCN is usually used so there’s a source of of H⁺ ions
3. The overall reaction for an aldehyde is: RCHO_(aq) + KCN_(aq) –H⁺_(aq)–> RCH(OH)CN_(aq) + K⁺_(aq)
4. The overall reaction for a ketone is: RCOR’_(aq) + KCN_(aq) –H⁺_(aq)–> RCR’(OH)CN_(aq) + KCN_(aq)
5. If you start with an unsymetrical ketone or any aldehyde (except methanal) you will produce a mixture of enantiomers

Question 9

Why is KCN dangerous to work with?

Answer 9

Potassium cyanide is an irritant and is also extremely dangerous if ingested (eaten) or inhaled. It can react with moisture to produce hydrogen cyanide, a highly toxic gas.

Question 10

What precautions need to be taken when using KCN

Answer 10

• Wear gloves
• Safety goggles
• Wear a lab coat
• Perform the experiment in a fume cupboard