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Carboxylic acids and esters
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Carboxylic acids are weak acids found in vinegar. They react with alcohols to make sweet-smelling esters.
Carboxylic acids
Functional group: -COOH. Methanoic HCOOH, ethanoic CH3COOH, propanoic C2H5COOH, butanoic C3H7COOH.
Vinegar is an aqueous solution of ethanoic acid.
With metals: salt + hydrogen. ethanoic acid + magnesium → magnesium ethanoate + hydrogen
With carbonates: salt + water + carbon dioxide. ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide
Vinegar is an aqueous solution of ethanoic acid.
With metals: salt + hydrogen. ethanoic acid + magnesium → magnesium ethanoate + hydrogen
With carbonates: salt + water + carbon dioxide. ethanoic acid + sodium carbonate → sodium ethanoate + water + carbon dioxide
Esters
Functional group: -COO-.
An alcohol and a carboxylic acid react (with a concentrated sulfuric acid catalyst) to form an ester and water:
ethanol + ethanoic acid ⇌ ethyl ethanoate + water
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Naming: the alcohol part first (ending -yl), then the acid part (ending -oate). Methanol + propanoic acid gives methyl propanoate.
An alcohol and a carboxylic acid react (with a concentrated sulfuric acid catalyst) to form an ester and water:
ethanol + ethanoic acid ⇌ ethyl ethanoate + water
CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O
Naming: the alcohol part first (ending -yl), then the acid part (ending -oate). Methanol + propanoic acid gives methyl propanoate.
Properties and the practical
Esters are volatile (evaporate easily) and have distinctive, often fruity, smells. They are used in flavourings and perfumes.
Practical: warm ethanol and ethanoic acid with a few drops of concentrated sulfuric acid in a water bath. Pour the mixture into sodium carbonate solution to neutralise the acids, then carefully waft to smell the ester.
Practical: warm ethanol and ethanoic acid with a few drops of concentrated sulfuric acid in a water bath. Pour the mixture into sodium carbonate solution to neutralise the acids, then carefully waft to smell the ester.
Name the ester made from propanol and ethanoic acid.
- Alcohol part first: propanol gives propyl.
- Acid part second: ethanoic acid gives ethanoate.
Answer: Propyl ethanoate
Carboxylic acids contain -COOH and react with metals (salt + hydrogen) and carbonates (salt + water + carbon dioxide). Vinegar is aqueous ethanoic acid. Alcohol + carboxylic acid gives an ester (-COO-) and water, with a sulfuric acid catalyst. Ethyl ethanoate is CH3COOC2H5. Esters are volatile and smell fruity.
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