# Preparation of Aldehydes

Aldehydes are prepared by oxidation of primary alcohols with PCC, Swern oxidation, or Dess-Martin periodinane. Aldehydes are also prepared via the ozonolysis of alkenes that have one or more vinylic hydrogens (hydrogens attached to the double bond). In addition, aldehydes are prepared via hydroboration-oxidation hydration of terminal alkynes.

Oxidation of an alcohol involves the loss of one or more hydrogens from the carbon attached to the hydroxyl group. An alcohol is a hydrocarbon containing a hydroxyl functional group (${\rm{-}{OH}}$). A primary alcohol is an alcohol with a hydroxyl group attached to a carbon that also has two hydrogen atoms attached to it. That is, the carbon has only one R group other than the alcohol. The loss of one of the alpha hydrogens results in a ${\rm{C{=}O}}$ double bond to form an aldehyde (${\rm{R{-}CHO}}$). The loss of both by further oxidation would form a carboxylic acid (${\rm{R{-}COOH}}$).

Pyridinium chlorochromate (PCC) is used to oxidize a primary alcohol to an aldehyde. Because the reaction is not run in water, the oxidation of the alcohol stops at the aldehyde and is not further oxidized to a carboxylic acid. Oxidation is a reaction that involves the removal of an electron from an atom.
Swern oxidation is a mild method of converting a primary alcohol into an aldehyde without the use of chromium reagents. Swern oxidation involves the addition of dimethyl sulfoxide ((CH3)2SO) and oxalyl chloride (C2O2Cl2) followed by triethylamine (Et3N). Swern oxidation of a primary alcohol creates an aldehyde because, like PCC, the reaction is not run in water and stops at the aldehyde and is not further oxidized to a carboxylic acid.
Dess-Martin periodinane (DMP) is an aromatic reagent that contains a cyclic ring with a pentavalent (five-bonded) iodine. DMP is used to oxidize a primary alcohol to an aldehyde. Like PCC and Swern, this reaction stops at the aldehyde and does not oxidize to the carboxylic acid.
Ozonolysis is the cleavage of an alkene, alkyne, or azo group with ozone (O3). Azo groups (${\rm{R{-}N{=}N{-}R'}}$) contain nitrogen-nitrogen double bonds. The double bond of the alkene is cleaved, and the molecule is divided into two smaller molecules. Aldehydes are prepared via the ozonolysis of alkenes that are not tetrasubstituted. There must be a hydrogen directly attached to the alkene for ozonolysis of alkenes to produce aldehydes. In the first step of the reaction, ozone is added to the double bond to form an ozonide. The second step involves reduction of the ozonide with dimethyl sulfide, cleaving the molecule, to form the two smaller molecules. Reduction is a reaction that involves the addition of an electron to an atom.