Organic Chemistry - Esters

Unit 4: Organic Chemistry

Esters

R-COO-R'

PHYSICAL PROPERTIES

• Mostly liquid at room temp.
• Insoluble in water (despite their polarity) because they cant form H-bonds with H2O.

CHEMICAL STUFF

Preparation

• Esters are prepared by warming an alcohol and a carboxylic acid under reflux with a few drops of conc. H2SO4 catalyst. This process is called esterification. IT IS A REVERSIBLE RXN.
  • C2H5OH + CH3COOH <-> CH3COOCH2CH3 + CH3CH2OH
• Esters can also be obtained by reacting an alcohol with an acid chloride (a.k.a acyl chloride) at room temp. This way obtains a higher yield because the reaction is not reversible!
  • CH3CH2OH + CH3COCl -> CH3COOCH2CH3 + HCl

Hydrolysis

• Esters are hydrolysed by heating under reflux with either an aqueous acid catalyst or an aqueous alkali catalyst.

Acid-catalysed hydrolysis:

• CH3COOCH2CH3 + H2O <-> CH3COOH + CH3CH2OH
• This is a reversible reaction!!!!

Base-catalysed hydrolysis:

• CH3COOCH2CH3 + NaOH -> CH3COONa + CH3CH2OH
• Here, the salt of the carboxylic acid is formed.
• This reaction has a higher yield than the acid-catalysed reaction (it is not reversible).
• If the carboxylic acid is required, the products are cooled and excess dilute strong acid (i.e. HCl or H2SO4) is added.
• It is this reaction which produces soaps, a process known as saponification. A fat or oil (in other words, a natural ester) is heated with NaOH, to give a carboxylate salt and an alcohol!

Other reactions…