Carboxylic acids: the -COOH\text{-COOH}-COOH group sets their names and reactions
Definition
Carboxylic acid
An organic compound containing the functional group -COOH\text{-COOH}-COOH.
- A carboxylic acid is an organic compound with the carboxyl functional group -COOH\text{-COOH}-COOH.
- Carboxylic acids form a homologous series, so they share this group and react in similar ways.
- Each member differs from the next by a CH2\text{CH}_2CH2 unit.
- Names use meth-, eth-, prop- and but- followed by -anoic acid.
- Methanoic acid has one carbon: HCOOH\text{HCOOH}HCOOH.
- Ethanoic acid has two carbons: CH3COOH\text{CH}_3\text{COOH}CH3COOH.
- Propanoic acid has three carbons: CH3CH2COOH\text{CH}_3\text{CH}_2\text{COOH}CH3CH2COOH.
- Butanoic acid has four carbons: CH3CH2CH2COOH\text{CH}_3\text{CH}_2\text{CH}_2\text{COOH}CH3CH2CH2COOH.
- The carbon in the -COOH\text{-COOH}-COOH group is counted as part of the chain.
Common Mistake
- When counting carbons, include the carbon in -COOH\text{-COOH}-COOH, not just those written before it.
- Do not treat the -OH\text{-OH}-OH inside -COOH\text{-COOH}-COOH as an alcohol group; it sits on a carbon double-bonded to oxygen.
Carboxylic acids form acidic solutions and fizz with carbonates
Definition
Carbonate
A compound that contains the carbonate ion, CO32−\text{CO}_3^{2-}CO32−.
- Each of the first four carboxylic acids dissolves in water to give an acidic solution with a pH below 7.
- A carboxylic acid reacts with a metal carbonate to give a salt, water and carbon dioxide.
- The carbon dioxide bubbles off, so you see fizzing, also called effervescence.
- The word equation is carboxylic acid + carbonate →\rightarrow→ salt + water + carbon dioxide.
- The salt's name ends in -anoate, for example sodium ethanoate from ethanoic acid.
Key Idea
- A carbonate reaction is recognised by fizzing, because carbon dioxide is released.
- The three products are always a salt, water and carbon dioxide.
Carboxylic acids react with alcohols to make esters
Definition
Catalyst
A substance that increases reaction rate without being used up by providing an alternative pathway with lower activation energy.
- A carboxylic acid reacts with an alcohol to make an ester and water.
- This reaction is called esterification.
- The mixture is warmed with concentrated sulfuric acid, which acts as a catalyst.
- The word equation is carboxylic acid + alcohol →\rightarrow→ ester + water.
- Ethanoic acid and ethanol make ethyl ethanoate and water.
- In “ethyl ethanoate”, ethyl comes from ethanol and ethanoate comes from ethanoic acid.
Example
- Warm ethanoic acid and ethanol with a sulfuric acid catalyst.
- The word equation is ethanoic acid + ethanol →\rightarrow→ ethyl ethanoate + water.
- Ethyl ethanoate is the only ester name you need to know.
Exam technique
- Name the products and give the observation; you are not expected to write balanced equations for these reactions.
- Match the salt or ester name to the acid, so an ethanoic acid reaction gives an ethanoate.
Self review
- What is the functional group of a carboxylic acid?
- Name the first four carboxylic acids.
- What forms when a carboxylic acid dissolves in water?
- What are the products and the observation when a carboxylic acid reacts with a carbonate?
- What forms when ethanoic acid reacts with ethanol?
