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Carbonyls

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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)


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