An aldehyde contains –CHO, with a hydrogen bonded to the carbonyl carbon. A ketone has its carbonyl carbon bonded to two carbon groups. Aldehyde names end in al and the aldehyde carbon is carbon 1. Ketone names end in one, with a locant specifying the carbonyl position. Count the carbonyl carbon in the parent chain.
Primary alcohol oxidation gives aldehydes; secondary alcohol oxidation gives ketones. Aldehydes readily oxidise to carboxylic acids in acidic conditions or carboxylate ions in alkaline conditions. Ordinary ketones resist these mild oxidants. Tollens reagent can give silver metal with an aldehyde, while acidified dichromate changes from orange to green. Test results classify functional groups within the stated reaction conditions; they do not establish an entire unknown structure.
Example 1
Name CH3CH2CHO and give its acidic oxidation product.
The three-carbon aldehyde is propanal. Oxidation gives CH3CH2COOH, propanoic acid.
Example 2
Distinguish CH3COCH3 from CH3CH2CHO using Tollens reagent.
Propanal is an aldehyde and can reduce Tollens reagent to a silver mirror. Propanone is a ketone and gives no corresponding positive reaction under the stated mild conditions.
Watch out:Carbonyl groups also occur in ketones, acids, esters and amides; the atoms attached to the carbonyl carbon determine the class.
Aldehydes, ketones and distinguishing reactions practice | Year 12 Chemistry | Ascent Arcade