Classwork Series and Exercises {Chemistry- SS3}: Alkanoic Acids

Introduction

Alkanoic acids are also known as carboxylic acids. A carboxylic acid can he identified from the carboxyl functional group and the ‘-oic’ name ending.

Structural Examples of carboxylic acids

Nomenclature (Naming Alkanoic Acids)

Alkanoic acids are named as follows:

• The number of carbon atoms in the longest carbon atom chain is noted
• The corresponding alkane is then named
• Finally, the name of this corresponding alkane is modified by removing the ane and replacing it with anoic acid

For example:

Preparation of Ethanoic Acid

The Oxidation of Ethanol to Ethanoic Acid

 The reaction of sodium dichromate(VI) solution with ethanol gives a carboxylic acid, ethanoic acid, a dilute solution of which is sold as vinegar.

Physical Properties

1. Ethanoic acid is usually a colourless liquid with a very pungent smell
2. It is soluble in water and a dilute solution has the usual sour taste of acid
3. It has a boiling point of 118⁰C
4. Pure anhydrous ethanoic acid freezes into ice-like crystals at temperature below 17⁰C
5. It turns blue litmus paper red

Chemical Properties of Carboxylic (Alkanoic) Acids

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

Soluble carboxylic (alkanoic) acids dissociate slightly in water.

• Neutralization Reactions

Neutralization: acid + base → salt + water

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

RCOOH + MOH → RCOO^–M⁺ + H₂O

e.g. CH₃COOH + NaOH → CH₃COO^–Na⁺ + H₂O

Ethanoic acid + sodium hydroxide → sodium ethanoate + water

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

e.g. 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

e.g. 2CH₃COOH + Na₂CO₃ → 2CH₃COO^–Na⁺ + CO₂ + H₂O

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

e.g. 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

e.g. 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

e.g. 2CH₃COOH + CH₃OH CH₃COOCH₃ + H₂O

Ethanoic acid + methanol methyl ethanoate + water

Uses

1. It can be used as a solvent
2. It is used in food industries as vinegar for preserving and flavouring food
3. It is used In the manufacture of cellulose ethanoate which is used for making synthetic fibres, such as rayon

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