Esters

Esters undergo aminolysis, hydrolysis, and alcoholysis.  They can also be reduced to aldehydes and alcohols. 

Hydrolysis of Esters

Saponification

Saponification is the base-catalyzed hydrolysis of esters.  Sapon is latin for soap, so saponification means "Soap making".

Under these conditions the carboxylic acid is converted to the carboxylate.

Mechanism

Hydroxide ions are good nucleophiles and attack the carbonyl carbon forming a tetrahedral intermediate.  The alkoxide leaves resulting in the formation of a carboxylic acid.  In the basic environment, the carboxylic acid is deprotonated by the alkoxide or a hydroxide ion to yield the carboxylate.

Soap is prepared by the hydrolysis of triacylglycerides (fat).  This produces glycerol and soap.

Soap has a hydrophilic head and a hydrophobic tail.  They form micellar structures around grease and dirt (hydrophobic).   They dissolve grease into water with their hydrophilic heads.

Acid-Catalyzed Hydrolysis

Esters are also hydrolyzed directly to carboxylic acids under acidic aqueous conditions.

Aminolysis of Esters

Reactive esters will undergo aminolysis producing amides when the conditions are correct.  They tend to react slowly unless a rather reactive ester is used.

For example, ethyl formate will react with an amine to produce and amide.

Reduction of Esters 

Esters can be reduced with LiAlH4 to form alcohols.  The organic group on the ester O atom also becomes an alcohol.

The reduction proceeds through the corresponding intermediate aldehyde which is rapidly reduced to the alcohol.  Recall that aldehydes are readily reduced to alcohols with LiAlH₄ so the aldehyde can not be obtained using LiAlH₄.

To obtain the aldehyde one can use DiBALH.

For example, the reduction of ethyl benzoate with DiBALH followed by an acid water workup yields the aldehyde.

Alcoholysis of Esters (Transesterification)

Esters will also undergo alcoholysis to produce other esters.  This is also referred to as transesterification.

Biodiesel can be produced from new or used triacylglycerides (fat like soybean oil, sunflower oil) in a transesterification process.  Treating these triacylglycerides with methanolunder acidic or basic conditions results in the formation of biodiesel.  Biodiesel are methyl esters of long chain carboxylic acids.  Sometimes ethanol is used instead.  Glycerol is produced as a byproduct which must be removed before the biodiesel can be used.