Preparation of Aldehydes and Ketones

1. By Oxidation of Alcohols

The most direct way to reach a carbonyl compound is to oxidise the corresponding alcohol. A primary alcohol, oxidised carefully, stops at the aldehyde; a secondary alcohol, oxidised under the same kind of conditions, gives a ketone (this chemistry of alcohols was studied in Unit 7 of Class XII). Because a primary alcohol can easily be oxidised straight through to a carboxylic acid, a mild, selective oxidant is normally chosen so the reaction halts cleanly at the aldehyde stage; a secondary alcohol cannot be oxidised any further than the ketone, since that carbon has no more hydrogen left to lose.

2. By Dehydrogenation of Alcohols

Instead of adding oxygen, this method removes hydrogen. Alcohol vapours are passed over a heated heavy-metal catalyst (silver or copper), and a molecule of hydrogen is stripped from the alcohol. A primary alcohol dehydrogenated this way gives an aldehyde, and a secondary alcohol gives a ketone — the same outcome as oxidation, reached by a different mechanism (this was also studied in Unit 7 of Class XII). Because it works on alcohol vapour passed over a solid catalyst bed, it suits volatile alcohols particularly well and is used industrially.

3. From Hydrocarbons

Two routes reach a carbonyl compound starting directly from a hydrocarbon rather than from an alcohol.

Ozonolysis of alkenes. An alkene is first treated with ozone and the resulting ozonide is then cleaved reductively with zinc dust and water. This snips the carbon–carbon double bond in two, and depending on how the alkene was substituted at that double bond, the products are aldehydes, ketones, or one of each (this ozonolysis chemistry was studied in Unit 9 of Class XI).

$\text{R}_2\text{C=CR}_2 \xrightarrow{\text{O}_3} \text{ozonide} \xrightarrow{\text{Zn, H}_2\text{O}} \text{R}_2\text{C=O} + \text{O=CR}_2$ …