Question

A carbon compound 'A' is widely used as a preservative in pickles and has a molecular formula C\(_2\)H\(_4\)O\(_2\). This compound reacts with ethanol to form a sweet smelling compound 'B'.
(i) Identify the compound 'A' and write its structure.
(ii) Write the chemical equation for the reaction of 'A' with ethanol to form compound 'B'. State the role of presence of an acid in the reaction.
(iii) How can we get compound 'A' back from 'B' ?
(iv) How can 'A' be obtained from ethanol ?
(v) Name the gas produced when compound 'A' reacts with washing soda.

Answer 1

Reactions and Properties of Compound 'A' (Ethanoic Acid)

(i) Identify the compound 'A' and write its structure.

Compound 'A' is used as a preservative in pickles and has the molecular formula C₂H₄O₂, which corresponds to Ethanoic acid (Acetic acid).

Structure: CH₃COOH

   

(ii) Reaction of 'A' with ethanol to form compound 'B'

Ethanoic acid reacts with ethanol (C₂H₅OH) to form an ester called Ethyl ethanoate (sweet-smelling). This is an esterification reaction.

Equation:

CH₃COOH + CH₃CH₂OH --(conc. H₂SO₄, heat)--> CH₃COOCH₂CH₃ + H₂O  

Role of acid: Conc. H₂SO₄ acts as a catalyst and a dehydrating agent to shift the equilibrium toward ester formation.

(iii) Recovery of 'A' from 'B' (Hydrolysis of Ester)

Method 1: Alkaline hydrolysis (Saponification):

CH₃COOCH₂CH₃ + NaOH → CH₃COONa + CH₃CH₂OH   CH₃COONa + HCl → CH₃COOH + NaCl  

Method 2: Acid hydrolysis (reverse of esterification):

CH₃COOCH₂CH₃ + H₂O --(H⁺, heat)--> CH₃COOH + CH₃CH₂OH  

(iv) Preparation of 'A' from Ethanol

Ethanoic acid can be prepared by oxidation of ethanol.

Chemical Method:

CH₃CH₂OH --(alk. KMnO₄ or acid. K₂Cr₂O₇, heat)--> CH₃COOH  

Biological (Fermentation) Method:

CH₃CH₂OH + O₂ --(Acetobacter)--> CH₃COOH + H₂O  

(v) Gas produced when 'A' reacts with washing soda

Washing soda is sodium carbonate (Na₂CO₃). Reaction with ethanoic acid produces carbon dioxide (CO₂).

2CH₃COOH + Na₂CO₃ → 2CH₃COONa + CO₂ + H₂O  

Gas released: Carbon dioxide (CO₂)