Key Concepts
- Number the longest carbon chain starting with the carbon atom of the COOH functional group.
- Name the carbon chain.
- Add the suffix -oic acid
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- Properties of carboxylic (alkanoic) acids:
- Reactions of carboxylic (alkanoic) acids:
- react with bases in neutralisation reactions
- react with carbonates to produce salt, carbon dioxide gas and water
- react with active metals to produce hydrogen gas
- react with alkanols (alcohols) in esterification reactions
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Examples of Carboxylic (Alkanoic) Acids
| formula |
longest
carbon chain |
C-C
single bonds |
functional
group |
name |
occurrence |
|---|
| HCOOH |
C1 : meth |
-an- |
-COOH
(oic acid) |
methanoic acid
(formic acid) |
ants |
| CH3COOH |
C2 : eth |
-an- |
-COOH
(oic acid) |
ethanoic acid
(acetic acid) |
vinegar |
| C2H5COOH |
C3 : prop |
-an- |
-COOH
(oic acid) |
propanoic acid
(propionic acid) |
dairy
products |
| 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
RCOOH + MOH → RCOO-M+ + H2O
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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