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(g) Esters

Syllabus
2024
Topic
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Level
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Learning objectives

438C Ester functional groupKnow that esters contain the –COO– functional group.439C Formation of ethyl ethanoateKnow that ethyl ethanoate is the ester produced when ethanol and ethanoic acid react in the presence of an acid catalyst440C Ethyl ethanoate structuresUnderstand how to write the structural and displayed formulae of ethyl ethanoate441C Ester structures and namesUnderstand how to write the structural and displayed formulae of an ester, given the name or formula of the alcohol and carboxylic acid from which it is formed and vice versa442C Properties and uses of estersKnow that esters are volatile compounds with distinctive smells and are used as food flavourings and in perfumes443C Ester preparation practicalPractical: prepare a sample of an ester such as ethyl ethanoate

Recognise the ester functional group

Esters contain the functional group −COO−\ce{-COO-}.

The ester linkage is −C(=O)−OX−\ce{-C(=O)-O-}: one carbon is double-bonded to oxygen and single-bonded to a second oxygen, which continues to another carbon group.

The complete group includes both oxygen atoms. Unlike −COOH\ce{-COOH} in a carboxylic acid, an ester linkage has carbon attached beyond the single-bonded oxygen rather than an O−H\ce{O-H} bond.

Form ethyl ethanoate from ethanol and ethanoic acid

Ethanol reacts with ethanoic acid in the presence of an acid catalyst to form the ester ethyl ethanoate and water.

\ce{CH3CH2OH + CH3COOH <=> CH3COOCH2CH3 + H2O}

Reactant Contribution to the ester name
ethanol ethyl
ethanoic acid ethanoate

The acid acts as a catalyst: it increases the reaction rate without being used up. Concentrated sulfuric acid is a suitable catalyst in the preparation.

Water is the other product; this is not the hydration of ethene. The alcohol supplies the first part of the ester name and the acid supplies the “anoate” part.

Write the structure of ethyl ethanoate

Ethyl ethanoate has structural formula CHX3COOCHX2CHX3\ce{CH3COOCH2CH3} and connectivity CHX3−C(=O)−O−CHX2−CHX3\ce{CH3-C(=O)-O-CH2-CH3}.

Part Atoms in ethyl ethanoate
ethanoate side CHX3−C(=O)X−\ce{CH3-C(=O)-}
ester linkage −C(=O)−OX−\ce{-C(=O)-O-}
ethyl side −O−CHX2−CHX3\ce{-O-CH2-CH3}

For a displayed formula, show every atom and bond: draw C=O\ce{C=O}, attach that carbon to CHX3\ce{CH3} and to O, then attach the O to CHX2CHX3\ce{CH2CH3}. Check carbon has four bonds, oxygen two and hydrogen one.

Do not write CHX3CHX2COOCHX3\ce{CH3CH2COOCH3}: that connectivity is methyl propanoate. In ethyl ethanoate, the ethyl group is attached after the single-bonded oxygen.

Link ester names, structures and reactants

An ester name has two parts: the alcohol supplies the first alkyl name, and the carboxylic acid supplies the second name ending in “anoate”.

Alcohol Acid Ester Structural formula
methanol methanoic acid methyl methanoate HCOOCHX3\ce{HCOOCH3}
methanol propanoic acid methyl propanoate CHX3CHX2COOCHX3\ce{CH3CH2COOCH3}
ethanol methanoic acid ethyl methanoate HCOOCHX2CHX3\ce{HCOOCH2CH3}
ethanol propanoic acid ethyl propanoate CHX3CHX2COOCHX2CHX3\ce{CH3CH2COOCH2CH3}

To work backwards from RCOORX′\ce{RCOOR'}, split the structure at the single C−O\ce{C-O} bond: RCOOX−\ce{RCOO-} identifies the acid and −RX′\ce{-R'} identifies the alcohol. Restore −OH\ce{-OH} to each reactant: RCOOH\ce{RCOOH} and RX′OH\ce{R'OH}.

Do not count all carbons as one chain. The group after oxygen is named first, while the chain containing the carbonyl carbon becomes the “anoate” part.

Connect ester properties to their uses

Esters are volatile compounds with distinctive smells, and they are used as food flavourings and in perfumes.

Property Consequence Use
volatile molecules evaporate readily and reach the nose perfumes
distinctive smells particular esters can reproduce recognisable aromas food flavourings and perfumes

A distinctive sweet or fruity smell can provide evidence that an ester formed in a small-scale preparation. In a laboratory, detect odours only by the instructed safe wafting method, never by inhaling directly.

Volatile means evaporates readily; it does not mean chemically reactive. Do not claim every ester smells pleasant—the syllabus requires distinctive smells and the named uses.

Prepare an ester safely

A small sample of ethyl ethanoate can be prepared by warming ethanol with ethanoic acid and an acid catalyst in a water bath.

Stage Action and purpose
1 place small quantities of ethanol and ethanoic acid in a test tube
2 carefully add the instructed acid catalyst
3 stand the tube in a hot water bath to warm the mixture and increase the reaction rate
4 after heating, allow the mixture to cool and identify ester formation by its distinctive smell using the instructed safe wafting method

Use a water bath rather than a direct Bunsen flame because ethanol, ethyl ethanoate and the mixture are flammable and could ignite.

The acid is a catalyst, not the ester reactant. Do not heat the flammable mixture directly, smell it closely, or invent unverified separation stages beyond the supplied procedure.