Carbonyls
Topic 8a – Carbonyl compounds
Revision Notes
1. Introduction
• Aldehydes and ketones are carbonyl compounds • They contain the carbonyl group C=O • The functional group in aldehydes is –CHO on the end of a chain e.g. ethanal CH3CHO • The functional group in ketones is C=O not at the end of a chain e.g. propanone CH3COCH3
2. AS Recap
• Primary alcohols are oxidised by acidified potassium dichromate. An aldehyde is produced first and this can be further oxidised to a carboxylic acid. To get the aldehyde, it must be distilled off as it is formed. To get the acid, heat under reflux. • Secondary alcohols are oxidised to ketones by acidified K2Cr2O7. Colour change is orange to green. • The C=O bond in aldehydes, ketones, carboxylic acids and esters can be identified by infrared spectroscopy. It produces a large peak around 1700 cm-1.
3. Reduction of Aldehydes and Ketones
Reduction, here, means addition of hydrogen.
a) Reduction using NaBH4
A specific reducing agent for aldehydes and ketones is sodium borohydride, NaBH4. In equations the reducing agent is represented by [H].
Aldehydes are reduced to primary alcohols by NaBH4 e.g.
CH3CHO + 2[H] ( CH3CH2OH
Ketones are reduced to secondary alcohols by NaBH4 e.g.
CH3COCH3 + 2[H] ( CH3CH(OH)CH3
Other points to note about this reaction are: • It is an addition reaction (there is only one product) • The mechanism is called nucleophilic addition • The nucleophile is H- which is provided by NaBH4
[pic]
b) Comparison with hydrogen gas
NaBH4 will reduce C=O double bonds but it will not reduce C=C double bonds
e.g. CH2=CH-CHO + 2[H] ( CH2=CH-CH2OH
To reduce both C=O and C=C use H2 with Ni catalyst
e.g. CH2=CH-CHO +2H2 ( CH3CH2CH2OH
4. Reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH)
•
Revision Notes
1. Introduction
• Aldehydes and ketones are carbonyl compounds • They contain the carbonyl group C=O • The functional group in aldehydes is –CHO on the end of a chain e.g. ethanal CH3CHO • The functional group in ketones is C=O not at the end of a chain e.g. propanone CH3COCH3
2. AS Recap
• Primary alcohols are oxidised by acidified potassium dichromate. An aldehyde is produced first and this can be further oxidised to a carboxylic acid. To get the aldehyde, it must be distilled off as it is formed. To get the acid, heat under reflux. • Secondary alcohols are oxidised to ketones by acidified K2Cr2O7. Colour change is orange to green. • The C=O bond in aldehydes, ketones, carboxylic acids and esters can be identified by infrared spectroscopy. It produces a large peak around 1700 cm-1.
3. Reduction of Aldehydes and Ketones
Reduction, here, means addition of hydrogen.
a) Reduction using NaBH4
A specific reducing agent for aldehydes and ketones is sodium borohydride, NaBH4. In equations the reducing agent is represented by [H].
Aldehydes are reduced to primary alcohols by NaBH4 e.g.
CH3CHO + 2[H] ( CH3CH2OH
Ketones are reduced to secondary alcohols by NaBH4 e.g.
CH3COCH3 + 2[H] ( CH3CH(OH)CH3
Other points to note about this reaction are: • It is an addition reaction (there is only one product) • The mechanism is called nucleophilic addition • The nucleophile is H- which is provided by NaBH4
[pic]
b) Comparison with hydrogen gas
NaBH4 will reduce C=O double bonds but it will not reduce C=C double bonds
e.g. CH2=CH-CHO + 2[H] ( CH2=CH-CH2OH
To reduce both C=O and C=C use H2 with Ni catalyst
e.g. CH2=CH-CHO +2H2 ( CH3CH2CH2OH
4. Reaction with 2,4-dinitrophenylhydrazine (2,4-DNPH)
•