Alkanoic (Carboxylic) acids

Alkanoic Acids

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

General formula of carboxylic (alkanoic acids) : C_nH_2n+1COOH or R-COOH

Examples of Carboxylic (Alkanoic) Acids

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 alkanoic acid

For example:

methylpentanoic

 2-ethylpentanoic acid  

benzoic acid   

Preparation of ethanoic acid

The Complete Oxidation of Ethanol to Ethanoic Acid by acidified sodium heptaoxochromate(VI) solution. Ethanol undergoes oxidation first to ethanal and then 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.

Ethanoic acid can also be prepared by

•  distilling anhydrous sodium ethanoate, CH₃COONa with concentrated H₂SO₄
• or boiling methyl cyanide CH₃CN with an acid

CH₃COONa_(aq)+ H₂SO_4(aq)  → CH₃COOH_(g) + NaHSO_{4 (aq)}

CH₃CN_(aq) + HCl_(aq) → CH₃COOH_(g) + NH₄Cl_(aq)

Physical Properties

1. Ethanoic acid is usually a colourless liquid with a characteristics sharp and 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
6. Dilute solution has a sour taste

Chemical Properties of Carboxylic (Alkanoic) Acids

1. Carboxylic (alkanoic) acids are weak acids, the acid dissociation constant, K_a, is small.
2. 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
4.  It is used in making compounds like ethyl ethanoate, ethanoic anhydride (used in aspirin), cellulose ethanoate (used for packing), propanone etc

 Assessment

1. Ethanoic acid is usually a colourless liquid with a characteristics sharp and ……. smell
   a. pungent
   b. annoying
   c. foul
   d. dirty
2. Esters are produced in a condensation reaction between a carboxylic (alkanoic) acid and an alkanol (alcohol) known as …….
   a. Polymerization
   b. Esterification
   c. Dehydration
   d. Hydrolysis
3. Ethanoic acid reacts with active metal to give off ……….. gas
   a. Oxygen
   b. Hydrogen
   c. Carbon
   d. OH
4. Boiling point of Ethanoic acid is …….
   a. 117⁰C
   b. 119⁰C
   c. 118⁰C
   d. 120⁰C

It is used in food industries as vinegar for preserving and flavouring food. True/False