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Aldehydes & Ketones

Organic

A2

Aldehydes and ketones

AQA Content

Write overall equations for reduction reactions using [H] as the reductant
Outline the nucleophilic addition mechanism for reduction reactions with NaBH4 (the nucleophile should be shown as H–)
Write overall equations for the formation of hydroxynitriles using HCN
Outline the nucleophilic addition mechanism for the reaction with KCN followed by dilute acid
Explain why nucleophilic addition reactions of KCN, followed by dilute acid, can produce a mixture of enantiomers.

Specification Notes

Aldehydes are readily oxidised to carboxylic acids.
Chemical tests to distinguish between aldehydes and ketones including Fehling’s solution and Tollens’ reagent.
Aldehydes can be reduced to primary alcohols, and ketones to secondary alcohols, using NaBH4 in aqueous solution. These reduction reactions are examples of nucleophilic addition.
The nucleophilic addition reactions of carbonyl compounds with KCN, followed by dilute acid, to produce hydroxynitriles.
Aldehydes and unsymmetrical ketones form mixtures of enantiomers when they react with KCN followed by dilute acid.
The hazards of using KCN.

Notes

Aldehydes and ketones are types of organic compounds that contain the carbonyl functional group, C=O, which is why they are also known as carbonyls. The main difference between aldehydes and ketones is the groups bonded to the carbon of the carbonyl group.


In an aldehyde, the carbonyl group is always situated at the end of the chain. When naming aldehydes, the carbonyl carbon is always number 1 on the chain and the '1' is not included in the name. The simplest aldehyde is methanal, HCHO, with the only carbon being that of the carbonyl group.

In a ketone, the carbonyl group is always situated in the chain itself (not at the end). The simplest ketone is propan-2-one, CH3COCH3, as you need an alkyl group on either side of the carbonyl carbon in a ketone.

propanone
Aldehydes and ketones can be prepared by oxidizing primary and secondary alcohols. During the oxidation of a primary alcohol to an aldehyde, the apparatus must be set up to distill off the aldehyde as it is produced. Further oxidation of primary alcohols can then take place, with aldehydes able to be easily oxidized to form carboxylic acids.


Ketones, however, are very resistant to oxidation and no further oxidation reaction will take place with secondary alcohols. The oxidizing agent used for all of the oxidation reactions is acidified potassium dichromate (K2Cr2O7 with sulfuric acid, H2SO4)

 

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