2.6 Carboxylic Acids

General formula of carboxylic acids
■ Carboxylic acids

	►	General formula: CnH2n+1COOH with n = 0, 1, 2, 3, ....
	►	Every member of carboxylic acid contain a functional group that is known as the carboxyl group, -COOH.
	►	Carboxylic acids are a group of organic compounds, which are also composed of the elements carbon, hydrogen and oxygen.

■ Nomenclature of carboxylic acid members

	►	The longest carbon chain containing the functional group is determined.
	►	The name of the carboxylic acid is named by replacing the suffix-a for the corresponding alkane with the suffix-oic.
	►	Note that carboxyl group, COOH has always been on the other end of the carbon chain.
	►	The term 'acid' placed after the name of the carboxylic acid. Number of carbon, n Molecular formula CnH2n+1COOH Name Structural formula 0 HCOOH Methanoic acid 1 CH3COOH Ethanoic acid 2 C2H5COOH Propanoic acid 3 C3H7COOH Butanoic acid

Preparation of carboxylic acids
■ Preparation of Ethanoic Acid

	►	Ethanoic acid can be prepared in the laboratory by heating a mixture of ethanol and acidified potassium dichromate(VI) solution under reflux. $C_2{H}_5\mathit{OH}(l)+2\mathrm{[}O\mathrm{]}\begin{array}{c}\to \\ \text{acidified potassium dichromate(VI)}\end{array}C{H}_3\mathit{COOH}(l)+H_{2}O(l)$
	►	Both ethanoic acid and ethanol are volatile liquids, thus heating under reflux is used to avoid loss of volatile substances.

Physical properties of carboxylic acids
■ Several physical properties of carboxyl acids.

►

Name Molecular formula Boiling point (°C) Physical state Methanoic acid HCOOH 101 Liquid Ethanoic acid CH3COOH 118 Liquid Propanoic acid C2H5COOH 141 Liquid Butanoic acid C3H7COOH 164 Liquid

■ Smell, color and physical state at room conditions

	►	Colorless liquid with a sharp and pungent smell.
	►	Boiling point ○ The boiling points of carboxylic acids increases with increasing number of carbon per molecule. ○ Boiling point of carboxylic acids is higher than the boiling point of alkanes, alkenes or alcohol equivalent. ○ The presence of a carboxyl group -COOH in carboxylic acids resulted in additional forces (hydrogen bonding) between carboxylic acid molecules. So the more heat energy is required to break the bond between the acid molecule.
	►	Soluble in water. Solubility of carboxylic acids in water decreases with increasing number of carbon atoms per molecules.

Chemical properties of ethanoic acids
■ This video contains information on the chemical properties of ethanoic acid.

■ Ethanoic acid

	►	Reacts as a weak acid because the ethanoic acid molecule ionises partially in water to produce hydrogen ion, H+. ○ CH3COOH(aq) ⇌ CH3COO-(aq) + H+(aq)
	►	Changes moist blue litmus paper to red.
	►	Can react with base, metal carbonate, metal and alcohol.

■ Reaction with a base

►

An acid and a base react to form salt and water. Similarly, ethanoic acid reacts with sodium hydroxide to form a salt, sodium ethanoate, and water. Sodium ethanoate is commonly known as sodium acetate. ○ CH3COOH(aq) + NaOH ➝ CH3COONa(aq) + H2O(aq)

■ Reaction with carbonates and hydrogen carbonate:

►

Carbonates and bicarbonates are also basic in nature and react with ethanoic acid to form salt and water. Carbon dioxide is also formed in the reaction. The chemical equations involved are given as: ○ 2CH3COOH(aq) + Na2CO3 ➝ 2CH3COONa(aq) + H2O(aq) + CO2(g) ○ CH3COOH(aq) + NaHCO3 ➝ CH3COONa(aq) + H2O(aq) + CO2(g)

■ Reaction with metals

►

React with a reactive metal to release hydrogen gas, H2. ○ 2CH3COOH(aq) + Mg(s) ➝ (CH3COO)2Mg(aq) + H2(g)

■ Reaction with alcohol or esterification reaction:

►

The reaction of a carboxylic acid with an alcohol to form an ester is known as esterification reaction. When ethanoic acid reacts with ethanol in the presence of an acid, ethyl ethanoate is formed. Esters react in the presence of an acid or a base to give back alcohol and carboxylic acid as follows: ○ $\begin{array}{c}{C}_2{H}_5\mathit{OH}\\ \text{ethanol}\end{array}+\begin{array}{c}{\mathit{CH}}_3\mathit{COOH}\\ \text{ethanoic acid}\end{array}\mathrm{\iff }\begin{array}{c}{\mathit{CH}}_3{\mathit{COOC}}_2{H}_5\\ \text{ethyl ethanoate}\end{array}+H_{2}O$

■ Prediction on the chemical properties of other carboxylic acids

	►	All the members of the carboxylic acid homologous series have the same functional group, that is, carboxyl , -COOH, group.
	►	Thus, the other carboxylic acid members should be able to exhibit similar chemical properties as ethanoic acid, CH3COOH.
	►	Based on the reaction of ethanoic acid, the chemical properties of other carboxylic acids can be predicted. ○ Reaction with bases: carboxylic acid + metallic carbonate ➝ carboxylate salt + water ○ Reaction with carbonates and hydrogen carbonate: carboxylic acid + metallic carbonate ➝ carboxylate salt + water + carbon dioxide ○ Reaction with metals: carboxylic acid + reactive ➝ carboxylate salt + hydrogen ○ Esterification reaction: alcohol + carboxylic acid ➝ ester + water

The uses of carboxylic acid in daily life
■ Uses of carboxylic acid

►

Manufacture medicine Food preservative Coagulant in the manufacture of rubber Manufacture soap Manufacture dye, perfumes and rayon

⇲ For exercise(objective and subjective), download for free on Android OS.