All subjects · Chemistry · WCH14 (Unit 4)

Carboxylic acids

Carboxylic acids contain the -COOH group. They are weak acids, they hydrogen bond strongly, and they can be turned into salts, alcohols, acyl chlorides and esters.

Naming and properties (spec 15.9, 15.10)

Suffix -oic acid; the COOH carbon is carbon 1: ethanoic acid CH3COOH, 2-methylpropanoic acid (CH3)2CHCOOH. Carboxylic acids have high boiling points because they form hydrogen bonds (often pairs of molecules, called dimers). Small acids are soluble in water because they hydrogen bond with water molecules; solubility falls as the hydrocarbon chain grows.

Preparation (spec 15.11)

Oxidation of a primary alcohol or an aldehyde: heat under reflux with excess acidified potassium dichromate(VI).
Hydrolysis of a nitrile: heat under reflux with dilute hydrochloric acid: CH3CH2CN + 2H2O + H+ -> CH3CH2COOH + NH4+

Reactions (spec 15.12)

LiAlH4 in dry ether: reduced to a primary alcohol: CH3COOH + 4[H] -> CH3CH2OH + H2O
Bases: form salts. With NaOH: CH3COOH + NaOH -> CH3COONa + H2O. With carbonates or hydrogencarbonates, fizzing CO2 is the test for -COOH: CH3COOH + NaHCO3 -> CH3COONa + H2O + CO2
PCl5: forms an acyl chloride, with steamy fumes of HCl: CH3COOH + PCl5 -> CH3COCl + POCl3 + HCl
Alcohols, with a few drops of concentrated sulfuric acid, warm: form an ester and water (reversible): CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O

Worked example

Give reagents and conditions to convert propanenitrile into propanoic acid, and then into propan-1-ol.

Nitrile to acid: heat under reflux with dilute HCl (hydrolysis).
Acid to alcohol: LiAlH4 in dry ether, then dilute acid (reduction).

Answer: Reflux with dilute HCl; then LiAlH4 in dry ether

Key idea: -COOH acids hydrogen bond strongly (high boiling points, small ones soluble). Made by oxidising alcohols or aldehydes, or hydrolysing nitriles. React with LiAlH4 (alcohol), bases (salts, CO2 with carbonates), PCl5 (acyl chloride) and alcohols with H2SO4 (esters).

Check yourself

What is the name of CH3CH2CH2COOH?
- Propanoic acid
- Butanoic acid
- Butanal
- Butan-1-ol
Show answer
Butanoic acid. Four carbons including the COOH carbon.
Why does ethanoic acid have a higher boiling point than propanal, although their Mr values are similar?
- Ethanoic acid has more London forces only
- Propanal is ionic
- Ethanoic acid is a gas at room temperature
- Ethanoic acid molecules form hydrogen bonds with each other
Show answer
Ethanoic acid molecules form hydrogen bonds with each other. -COOH has an O-H group and forms hydrogen-bonded dimers.
What is the organic product when ethanoic acid reacts with PCl5?
- Chloroethane
- Ethanoyl chloride
- Ethyl ethanoate
- Ethanal
Show answer
Ethanoyl chloride. PCl5 replaces the -OH of the acid with -Cl.
What does LiAlH4 in dry ether convert propanoic acid into?
- Propanal only
- Propan-2-ol
- Propan-1-ol
- Propane
Show answer
Propan-1-ol. Carboxylic acids are reduced to primary alcohols.
Which reaction produces a carboxylic acid?
- Heating a nitrile under reflux with dilute hydrochloric acid
- Reducing an aldehyde with LiAlH4
- Oxidising a tertiary alcohol
- Reacting an alkene with steam
Show answer
Heating a nitrile under reflux with dilute hydrochloric acid. Acid hydrolysis converts -CN into -COOH.
0.0200 mol of ethanoic acid reacts with excess sodium hydrogencarbonate. What volume of CO2 forms at room temperature and pressure, in cm³? (molar volume 24.0 dm³ mol^-1)
Show answer
480 cm³. 1 : 1 ratio, so 0.0200 mol CO2; V = 0.0200 x 24 000 = 480 cm³.
What forms when propanoic acid is warmed with methanol and a few drops of concentrated sulfuric acid?
- Methyl propanoate and water
- Propyl methanoate and water
- Propanoyl chloride
- Propan-1-ol and methanoic acid
Show answer
Methyl propanoate and water. The alcohol part is named first: methyl; the acid part becomes -oate: propanoate.

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Last updated 7 November 2026. Spotted a mistake? Tell us.