6) Carboxylic acids. Nomenclature. Physical and chemical properties of carboxylic acids.

Carboxylic acids are weak organic acids which contain the carboxyl group (RCO2H):

Nomenclature of Carboxylic Acids:

• Select the longest carbon chain containing the carboxyl group. The -e ending of the parent alkane name is replaced by the suffix -oic acid.

• The carboxyl carbon is always numbered “1” but the number is not included in the name.

• Name the substituents attached to the chain in the usual way.

• Aromatic carboxylic acids (i.e., with a CO2H directly connected to a benzene ring) are named

after the parent compound, benzoic acid.

Physical Properties of Carboxylic Acids:

• Since carboxylic acids can form more than one set of hydrogen bonds, their boiling points are usually higher than those of other molecules of the same molecular weight(MW).

• Low-MW carboxylic acids are generally liquids at room temp. (often, they are somewhat oily); higher-MW carboxylic acids are generally waxy solids.

• Carboxylic acids with 12 to 20 carbon atoms are often referred to as fatty acids, since they are found in the triglycerides in fats and oils (more later).

• Short-chain carboxylic acids are also generally more soluble in water than compounds of similar MW, since they can hydrogen bond to more than one water molecule.

Chemical Properties of Carboxylic Acids:

Carboxylic acids are weak acids, the acid dissociation constant, K_a, is small. Soluble carboxylic acids dissociate slightly in water. The acidity of carboxylic 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⁺ + H_2(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