Preparation of Aldehydes

1. From Acyl Chloride (Acid Chloride) — the Rosenmund Reduction

An acyl (acid) chloride can be hydrogenated directly to the corresponding aldehyde using hydrogen gas over a palladium catalyst that has been partially deactivated by depositing it on barium sulphate. This partial poisoning of the catalyst is deliberate: an ordinary, fully active palladium catalyst would keep reducing the product all the way to the alcohol, whereas the poisoned catalyst is just active enough to stop cleanly at the aldehyde. This reaction is named the Rosenmund reduction.

Benzoyl chloride is reduced by this method to benzaldehyde.

2. From Nitriles and Esters

A nitrile can be reduced with stannous chloride in the presence of hydrochloric acid; this first gives an imine, which is then hydrolysed to the aldehyde. This sequence is called the Stephen reaction.

Alternatively, a nitrile can be reduced selectively with diisobutylaluminium hydride (DIBAL-H) to the imine stage, which is then hydrolysed to the aldehyde. "Selectively" matters here too: the reduction is controlled so it stops at the imine rather than running on to the amine.

Esters can be reduced the same way — DIBAL-H reduces an ester to the aldehyde stage, stopping short of the alcohol that a stronger reducing agent would give.

3. From Hydrocarbons — Aromatic Aldehydes

Aromatic aldehydes, such as benzaldehyde and its ring-substituted derivatives, are built up from aromatic hydrocarbons by three distinct routes.

(i) Oxidation of methylbenzene. A strong oxidising agent applied to toluene would normally carry the oxidation all the way to benzoic acid, so this route instead converts the methyl group into an intermediate that resists further oxidation, then hydrolyses that intermediate back to the aldehyde. Two reagents are used for this:

• Chromyl chloride ($\text{CrO}_2\text{Cl}_2$) oxidises the methyl group of toluene, in carbon disulphide as solvent, to a chromium complex; hydrolysing this complex with aqueous acid gives benzaldehyde. This sequence is the Etard reaction.

• Chromic oxide ($\text{CrO}_3$) in acetic anhydride converts toluene (or a substituted toluene), at 273–283 K, into benzylidene diacetate; hydrolysing this diacetate with aqueous acid on warming gives benzaldehyde.

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