Preparation of Aldehydes and Ketones

Aldehydes and ketones can each be reached by several independent routes, chosen according to what starting material is conveniently available.

From alcohols (oxidation). A primary alcohol, oxidised carefully, gives an aldehyde; a secondary alcohol, oxidised, gives a ketone (tertiary alcohols resist oxidation entirely, since there is no hydrogen on the carbinol carbon to remove). The difficulty with a primary alcohol is that ordinary oxidants such as acidified $\text{KMnO}_4$ or $\text{K}_2\text{Cr}_2\text{O}_7$ do not stop at the aldehyde -- the aldehyde itself is even more easily oxidised than the alcohol was, so the reaction runs on to the carboxylic acid. Milder, selective reagents are therefore used when the aldehyde itself is wanted: pyridinium chlorochromate (PCC), a mild $\text{Cr(VI)}$ reagent used in anhydrous dichloromethane, oxidises a primary alcohol cleanly to the aldehyde and stops there, because it works in the absence of water (an aqueous medium is what allows the aldehyde to hydrate and be oxidised further). Alternatively, passing the vapour of a primary or secondary alcohol over hot copper at about $573\ \text{K}$ dehydrogenates it (removes $\text{H}_2$ rather than adding oxygen) to the aldehyde or ketone respectively.

From acid chlorides. An acid chloride, $\text{RCOCl}$, is reduced selectively to an aldehyde by hydrogen over a palladium catalyst that has been partly poisoned with barium sulphate (the Rosenmund reduction); the poisoned catalyst is deliberately made less active so that the reaction stops at the aldehyde stage instead of continuing to the alcohol. For ketones, an acid chloride can instead be treated with a dialkylcadmium reagent, $\text{R}'_2\text{Cd}$ (itself made from a Grignard reagent and $\text{CdCl}_2$), which reacts selectively with the acid chloride and not with other carbonyl groups that might be present in the same molecule. …