Esters

What an ester is

• An ester is an organic compound formed from the reaction of a carboxylic acid with an alcohol
• The functional group is the ester linkage, written as −COO− and drawn as a carbon with a double bond to one oxygen and a single bond to a separate −O− that continues into the rest of the molecule
• General structure: R–COO–R′, where R is the alkyl group of the carboxylic acid (minus its acidic H) and R′ is the alkyl group of the alcohol (minus its hydroxyl H)
• Physical properties:
  • Small esters are colourless, sweet-smelling oily liquids
  • They are insoluble or only slightly soluble in water but mix freely with organic solvents
  • Many natural fruit aromas are esters: ethyl butanoate (pineapple), 3-methylbutyl ethanoate (banana), octyl ethanoate (orange)
  • Esters are widely used as food flavourings and as the scent base of many perfumes

Esterification: making an ester

• The reaction between a carboxylic acid and an alcohol, in the presence of a concentrated sulfuric acid catalyst, is called esterification
• Word equation:

carboxylic acid + alcohol → ester + water

• The catalyst is concentrated sulfuric acid (H₂SO₄) — it speeds the reaction up and helps drive water out of the mixture
• The −OH of the acid combines with the −H of the alcohol to leave behind a molecule of water; the remaining fragments join through the ester linkage

Example — methyl propanoate from propanoic acid and methanol

CH₃CH₂COOH(l) + CH₃OH(l) → CH₃CH₂COOCH₃(l) + H₂O(l)

propanoic acid + methanol → methyl propanoate + water

• One molecule of acid plus one molecule of alcohol gives one molecule of ester plus one molecule of water
• The reaction is reversible — leaving the ester in contact with the water and the catalyst eventually re-makes some of the starting materials. This is why esters are distilled off (boiled out) as they form during a school preparation (section 2)

Naming an ester

• An ester's name has two words, one for each parent molecule:
  • First word — derived from the alcohol that supplied the −OR′ group. Drop the "-ol" ending of the alcohol and replace it with "-yl" (e.g. methanol → methyl, ethanol → ethyl, propan-1-ol → propyl)
  • Second word — derived from the carboxylic acid that supplied the −COO− group. Drop the "-oic acid" ending of the acid and replace it with "-oate" (e.g. methanoic acid → methanoate, ethanoic acid → ethanoate, propanoic acid → propanoate)
• The first word comes from the alcohol, the second from the acid

Predicting an ester structure from its name

• Take pentyl butanoate as a worked example:
  • "pentyl" → from pentan-1-ol, so the −OR′ side has 5 carbons (C₅H₁₁)
  • "butanoate" → from butanoic acid, so the −COO− side has 4 carbons in total (including the carboxyl carbon — so 3 in the alkyl part and one in the C=O)
  • Combine: butanoate side is C₃H₇−COO−, pentyl side is −C₅H₁₁
  • Full formula: C₃H₇COOC₅H₁₁
• The same procedure works in reverse — given an ester's structure, identify the −COO− linkage, split the molecule at the O–C single bond, and read the two parent molecules from each fragment