Acetic Acid or Ethanoic Acid: Introduction, method of preparation, properties
Acetic acid is known since ancient times in the form of vinegar. Vinegar contains about 10% acetic acid. It was first made in impure form by Stahl in 1720. It was first obtained in pure form by Lavoisier in 1786 by the oxidation of ethyl alcohol. In 1845, Kolbe synthesized it from its elements. Its molecular formula is C2H4O2 and structural formula is as follows-
H O
| ||
H—C—C—O—H or CH3COOH
|
H
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H—C—C—O—H or CH3COOH
|
H
Methods of manufacture
Following are the main methods of making acetic acid-
1. From ethyl alcohol - Acetic acid is obtained by oxidation of ethyl alcohol with acidic potassium dichromate solution.
(K2Cr2O7 + H2SO4) (K2Cr2O7+ H2SO4)
C2H5OH → CH3CHO → CH3COOH
C2H5OH → CH3CHO → CH3COOH
O O
2. From methyl cyanide - Acetic acid is obtained by hydrolysis of methyl cyanide with dilute acid or dilute alkali.
CH3CN + 2H2O + HCl → CH3COOH + NH4Cl
3. From ethyl acetate - Acetic acid is obtained by hydrolysis of ethyl acetate with dilute acid or dilute alkali.
H+
CH3COOC2H5 + H2O → CH3COOH + C2H5OH
CH3COOC2H5 + H2O → CH3COOH + C2H5OH
4. From acetamide - Acetic acid is obtained by the action of nitrous acid on acetamide.
CH3CONH2 + HNO2 → CH3COOH + N2 + H2O
Hydrolysis of acetamide also gives acetic acid.
5. Industrial methods - There are three main methods of industrial manufacture of acetic acid—
(i) from acetylene
(ii) by the destructive distillation of wood
(iii) Fermentation of sugarcane juice or ethyl alcohol.
The fermentation of ethyl alcohol takes place due to the presence of mycoderma aceti bacteria present in the air. The reaction of fermentation of ethyl alcohol is as follows-
C2H5OH + O2 → CH3COOH + H2O
Mycoderma aceti
Physical properties
- It is a colourless, strong smelling and corrosive liquid. The pungent odor of vinegar is due to the presence of acetic acid in it. Its boiling point is 118°C. It freezes like ice at 16.5°C. It is soluble in water, alcohol and ether. It is heavier than water and its relative density at 16°C is 1.045.
Chemical properties
One molecule of acetic acid (CH2COOH) contains one methyl (—CH3) group and one carboxyl (—COOH) group. Hence, it exhibits reactions of methyl and carboxyl groups. Due to the presence of methyl group, it shows only halogenation reaction. Carboxyl group participates in all its remaining reactions. These reactions of this show the general reactions of carboxyl group.
Its main chemical reactions are as follows-
1. Reaction with bases : Acetic acid is a weak monobasic acid and ionizes in aqueous solution to form acetate ion (CH3COO-) and hydrogen cation. Due to its acidic nature, it reacts with bases to form salts. For example -
CH3COOH + NaHCO3 → CH3COONa + H2O + CO2⬆
CH3COOH + NH3 → CH3COONH4
On heating ammonium acetate (CH3COONH4) acetamide (CH3CONH2) is obtained.
on heating
CH3COONH4 → CH3CONH2 + H2O
2. Esterification - Just as salt and water are formed by the reaction of acid and base, in the same way ester and water are formed by the reaction of acid and alcohol. This reaction is called esterification. Esterification of carboxylic acids is usually carried out in the presence of concentrated sulfuric acid. It acts as both a dehydrating agent and a catalyst. For example
CH3COOH + C2H5OH → CH3COOC2H5 + H2O
concentrated H2SO4
3. Halogenation - When chlorine or bromine is passed through acetic acid in the presence of red phosphorus, hydrogen atoms of methyl radical get displaced one by one from chlorine or bromine atoms.
CH3COOH + Cl2 → CH2ClCOOH + HCl
CH2ClCOOH + Cl2 → CHCl2COOH + HCl
CHCl2COOH + Cl2 → CCl3COOH + HCl
CH2ClCOOH + Cl2 → CHCl2COOH + HCl
CHCl2COOH + Cl2 → CCl3COOH + HCl
Other carboxylic acids react in the same way with chlorine and bromine in the presence of red phosphorus. In this reaction, the hydrogen atom present on the adjacent carbon atom of the carboxylic acid group i.e. ca-hydrogen atom is replaced by halogen atoms. This reaction is called Hell-Volhard-Zelinsky reaction or HVZ reaction.
4. PCI5 reaction - It reacts with PCI5 to form acetyl chloride (CH3COCI). In this reaction the —OH group of the —COOH group is displaced from the chlorine atom.
CH3COOH + PCI5 → CH3COCI + POCI3 + HCI
It reacts in the same way with PCI3 and SOCI2.
5. Dehydration - On heating acetic acid alone or in the presence of dehydrating agents like phosphorus penta-oxide, one molecule of water is separated from its two molecules and acetic anhydride is obtained.
CH3COOH P2O5 CH3CO
+ → 〉O + H2O
CH3COOH Δ CH3CO
+ → 〉O + H2O
CH3COOH Δ CH3CO
6. Reduction - It is reduced by lithium aluminum hydride to form ethanol.
LiAlH4
CH3COOH → CH3CH2OH
-H2O
CH3COOH → CH3CH2OH
-H2O
The above reduction can also be carried out by H2/Ni, Na/alcohol or NaBH4.
7. Reactions of salts - (i) Methane is obtained when sodium acetate is heated with soda lime.
CaO
CH3COONa + NaOH → CH4 + Na2CO3
CH3COONa + NaOH → CH4 + Na2CO3
(ii) Ethane is obtained on electrolysis of sodium or potassium acetate.
2CH3COONa + 2H2O → C2H6 + 2CO2 + 2NaOH + H2
(at anode) (at cathode)
(iii) Acetone is obtained on heating calcium acetate alone.
on heating
(CH3COO)2Ca → CH3COCH3 + CaCO3
(iv) Acetaldehyde is obtained when calcium acetate is mixed with calcium formate and heated.
on heating
(CH3COO)2Ca + (HCOO)2Ca → 2CH3CHO + 2CaCO3
(v) Silver salt of acetic acid reacts with bromine in dry CCI4 solution to give methyl bromide (CH3Br). This reaction is called Hunsdecker reaction.
CCI4
CH3COOAg + Br2 → CH3Br + CO2 + AgBr
8. Schmidt reaction - Carboxylic acids react with hydrazoic acid (N3H) in the presence of concentrated sulfuric acid to form primary amines. This reaction is called Schmidt reaction. Acetic acid reacts with hydrazoic acid in the presence of concentrated sulfuric acid to form methylamine (CH3NH2).
concentrated H2SO4
CH3COOH + N3H → CH3NH2 + CO + N2
Uses
Following are the uses of acetic acid-
- As a reagent in the laboratory.
- In the manufacture of acetone and esters, which are used in the manufacture of perfumes, dyes, plastics and medicines.
- In the manufacture of artificial fibers called cellulose acetate and poly-vinyl acetate.
- White means in making artificial vinegar.
- In the manufacture of lead tetra-acetate which is an oxidising agent.
- In rubber coagulation.
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