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 (−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 C=O double bond to form an aldehyde (R−CHO). The loss of both by further oxidation would form a carboxylic acid (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.
Oxidation of a primary alcohol with PCC yields an aldehyde.
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.
Swern oxidation of a primary alcohol creates an aldehyde.
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.
Dess-Martin oxidation of a primary alcohol creates an aldehyde.
Ozonolysis is the cleavage of an alkene, alkyne, or azo group with ozone (O3). Azo groups (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.
Ozonolysis of an alkene creates two smaller molecules. A trisubstituted alkene will undergo ozonolysis to yield a ketone and an aldehyde. An ozonide intermediate is formed during the ozonolysis reaction.
Terminal alkynes are used to prepare aldehydes with hydroboration-oxidation hydration. It is an electrophilic addition reaction. Alkynes undergo hydroboration-oxidation to first form enols, which tautomerize immediately to the aldehyde. Keto-enol tautomerism is a chemical equilibrium between a ketone or aldehyde and an enol. A tautomer is one of two or more constitutional isomers that differ only in the placement of a single proton. The reaction is an anti-Markovnikov addition to the alkyne. Anti-Markovnikov additions occur where the functional group that adds to an alkene will add to the least stable position instead of the most stable position. The addition of BH3 is stereoselective syn addition. A syn addition is when both groups are added to the same side of the double bond in an addition to an alkene or alkyne.
Hydration
Hydration of terminal alkynes yields an aldehyde and is anti-Markovnikov.
