Aldehydes can be prepared via a number of pathways. Some of the common methods are explained here.
The oxidation of primary alcohols
Primary alcohols can be oxidized by mild oxidizing agents, such as pyridinium chlorochromate (PCC), to yield aldehydes.
The use of strong oxidizing agents would lead to the fully‐oxidized product (carboxylic acid).
The reduction of acyl chlorides, esters, and nitriles
Reduction by mild reducing agents converts acyl chlorides, esters, and nitrites into aldehydes. The reducing agents of choice are usually lithium tri‐tert‐butoxy aluminum hydride (LATB—H) and diisobutylaluminum hydride (DIBAL—H). Following are the structures for these compounds:
Acyl chloride reduction
Acyl chlorides can be reduced by reacting them with lithium tri‐tert‐butoxyaluminum hydride at −78°C.
Following is a typical reduction of an aromatic compound employing this reagent.
The mechanism for acid chloride reduction proceeds via a hydride‐ion transfer from the reducing agent to the acid chloride. The following steps summarize this mechanism.
1. An acid‐base reaction occurs between an electron pair on the oxygen of the carbonyl group and the aluminum atom of the LATB—H.
2. A hydride ion is transferred to the carbonyl carbon with a corresponding movement of the π electrons of the carbonyl group to the carbonyl oxygen.
3. A chloride ion is liberated with anchiomeric assistance from an electron pair on the carbonyl oxygen.
4. The aluminum complex is hydrolyzed by the addition of water to form the aldehyde.
Ester and nitrile reduction
You can use diisobutylaluminum hydride to reduce both esters and nitriles to aldehydes. Typical examples are the reduction of ethyl ethanoate (ethyl acetate) and ethanenitrile (acetonitrile) to ethanal (acetaldehyde).
The mechanism for both of these reactions is very similar to the mechanism for the reduction of acyl chlorides by LATB—H. The first step is an acid‐base reaction between an unshared electron pair on oxygen or nitrogen with the aluminum atom of the DIBAL—H. The second step is the transfer of a hydride ion from the DIBAL—H to the carbon atom of the carbonyl or nitrile group. The last step is the hydrolysis of the aluminum complex to form the aldehyde.
Ozonolysis of alkenes
Alkenes in which the carbon(s) of the double bond possess one or more hydrogen atoms react with ozone (O 3) to generate aldehydes. The reaction of propene with ozone to form acetaldehyde and formaldehyde illustrates this method of preparation.
Hydroboration of terminal alkynes
Terminal alkynes react rapidly with borane to produce an intermediate compound that is easily oxidized to an aldehyde. For example, you can produce pentenal by reacting pentyne with borane and oxidizing the resulting intermediate with aqueous hydrogen peroxide.