How are aldehydes prepared in the laboratory?

Laboratory Preparation of Aldehydes

1. From Primary Alcohols
   Primary alcohols can be oxidized with mild oxidizing agents like PCC or DMP. Care is taken to avoid strong oxidants because they convert aldehydes further into acids.
2. From Hydrocarbons
   Hydrocarbons such as toluene can be converted into aldehydes by controlled oxidation or through side-chain halogenation followed by hydrolysis.
3. From Acyl Chlorides (Rosenmund Reduction)
   Acyl chlorides are reduced with hydrogen gas in the presence of a poisoned palladium catalyst (palladium on barium sulfate) to give aldehydes.
4. From Nitriles (Stephen Reaction)
   Nitriles are partially reduced using stannous chloride and hydrochloric acid, followed by hydrolysis, to form aldehydes.
5. From Esters (DIBAL-H Reduction)
   Esters can be reduced with diisobutylaluminium hydride (DIBAL-H) at low temperatures to yield aldehydes.
6. From Alkenes (Hydroformylation)
   Alkenes react with carbon monoxide and hydrogen in the presence of a cobalt or rhodium catalyst to form aldehydes.
7. From Alkynes
   Terminal alkynes undergo hydroboration–oxidation using bulky borane reagents, producing aldehydes through anti-Markovnikov addition.

In short: aldehydes are prepared in the lab mainly by oxidation of primary alcohols, reduction of acyl chlorides, nitriles, or esters, and by addition reactions of alkenes and alkynes.