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Alkanoic (Carboxylic) Acids

Key Concepts

  1. Number the longest carbon chain starting with the carbon atom of the COOH functional group.

  2. Name the carbon chain.

  3. Add the suffix -oic acid
  • Properties of carboxylic (alkanoic) acids:
  1. Carboxylic (alkanoic) acids are polar molecules.

  2. Carboxylic (alkanoic) acids are weak acids.
  • Reactions of carboxylic (alkanoic) acids:
  1. react with bases in neutralisation reactions

  2. react with carbonates to produce salt, carbon dioxide gas and water

  3. react with active metals to produce hydrogen gas

  4. react with alkanols (alcohols) in esterification reactions

Examples of Carboxylic (Alkanoic) Acids

formula longest
carbon chain
single bonds
name occurrence
HCOOH C1 : meth -an- -COOH
(oic acid)
methanoic acid
(formic acid)
CH3COOH C2 : eth -an- -COOH
(oic acid)
ethanoic acid
(acetic acid)
C2H5COOH C3 : prop -an- -COOH
(oic acid)
propanoic acid
(propionic acid)
C3H7COOH C4 : but -an- -COOH
(oic acid)
butanoic acid
(butyric acid)
rancid butter
C4H9COOH C5 : pent -an- -COOH
(oic acid)
pentanoic acid
(valeric acid)
valerian root

Physical Properties of Carboxylic (Alkanoic) Acids

Carboxylic (alkanoic) acids are weak acids, and as such are expected to have a sour taste, conduct electricity in solution, and turn blue litmus red

Carboxylic (alkanoic) acids are polar molecules.

2 hydrogen bonds (shown in dotted red lines) can form between them.

For this reason, carboxylic (alkanoic) acids have higher melting and boiling points than compounds with the same number of carbon atoms:

compound formula melting point (oC) boiling point (oC)
ethanoic acid CH3COOH 17 118
ethanol CH3CH2OH -114 78
ethanal CH3CHO -123 20
ethane CH3CH3 -183 -89

Short chain carboxylic (alkanaoic) acids are soluble in water as a result of hydrogen bonding between the OH group of the carboxyl group and water molecules.

Chemical Properties of Carboxylic (Alkanoic) Acids

Carboxylic (alkanoic) acids are weak acids, the acid dissociation constant, Ka, is small.

Soluble carboxylic (alkanoic) acids dissociate slightly in water.

The acidity of carboxylic (alkanaoic) acids increases with the substitution of highly electronegative atoms, such as chlorine, in the molecule:

name formula Acid Dissociation Constant (Ka)
ethanoic acid CH3COOH 10-4.8
chloroethanoic acid ClCH2COOH 10-2.9
dicholorethanoic acid Cl2CHCOOH 10-1.3
trichloroethanoic acid Cl3CCOOH 10-0.7

Neutralisation Reactions

neutralisation: acid + base → salt + water

carboxylic (alkanoic) acid + base → salt (metal alkanoate) + water


eg,       CH3COOH + NaOH → CH3COO-Na+ + H2O

ethanoic acid + sodium hydroxide → sodium ethanoate + water

Soluble salts of long-chain (fatty) acids are soaps

eg,       C17H35COOH + NaOH → C17H35COO-Na+ + H2O

stearic acid + sodium hydroxide → sodium stearate + water

Reaction with Carbonates

acid + carbonate → salt + carbon dioxide gas + water

carboxylic (alkanoic) acid + metal carbonate → metal alkanoate + carbon dioxide + water

eg,       2CH3COOH + Na2CO3 → 2CH3COO-Na+ + CO2 + H2O

ethanoic acid + sodium carbonate → sodium ethanoate + carbon dioxide + water

eg,       CH3COOH + NaHCO3 → CH3COO-Na+ + CO2 + H2O

ethanoic acid + sodium bicarbonate → sodium ethanoate + carbon dioxide + water

Reaction with Active Metals

acid + metal → salt + hydrogen gas

carboxylic (alkanoic) acid + metal → metal alkanoate + hydrogen

eg,       2CH3COOH + 2Na(s) → 2CH3COO-Na+ + H2(g)

ethanoic acid + sodium → sodium ethanoate + hydrogen

Esterification Reactions

Esters are produced in a condensation reaction between a carboxylic (alkanoic) acid and an alkanol (alcohol).

This is known as an esterification reaction.

carboxylic (alkanoic) acid + alkanol (alcohol) ester + water

eg,       2CH3COOH + CH3OH CH3COOCH3 + H2O

ethanoic acid + methanol methyl ethanoate + water

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