Question

Mohan heated ethanol with a compound 'X' in the presence of a few drops of conc. \( H_2SO_4 \) and observed a sweet smelling compound 'Y' is formed. When 'Y' is treated with sodium hydroxide it gives back ethanol and a compound 'Z'.
(i) Identify 'X', 'Y' and 'Z'.
(ii) Write the role of conc. \( H_2SO_4 \) in the reaction.
(iii) Write the chemical equations involved and name the reactions.

Hint:

Esterification: Alcohol + Carboxylic acid \(\xrightarrow{\text{conc. H}_2\text{SO}_4}\) Ester + Water (sweet smell) Saponification: Ester + NaOH \(\Rightarrow\) Alcohol + Sodium salt of carboxylic acid Conc. H₂SO₄ acts as catalyst and dehydrating agent.

Answer 1

Part (i): Identification of X, Y and Z

Step 1: Identify the type of reaction.
Ethanol reacts with compound X in presence of concentrated H₂SO₄ to form a sweet-smelling compound Y.
Sweet-smelling substances in organic chemistry are generally esters.
Therefore, Y must be an ester.

Step 2: Identify compound X.
Esters are formed by the reaction of an alcohol with a carboxylic acid in presence of concentrated H₂SO₄.
Since ethanol is the alcohol given, compound X must be a carboxylic acid.
The most common reaction involves ethanoic acid (CH₃COOH).
Thus:
X = Ethanoic acid (CH₃COOH)
Y = Ethyl ethanoate (CH₃COOC₂H₅)

Step 3: Identify compound Z.
When ester Y reacts with NaOH, it gives back ethanol and a sodium salt.
This reaction is called alkaline hydrolysis or saponification.
Therefore, Z is the sodium salt of ethanoic acid.
Z = Sodium ethanoate (CH₃COONa)

Final Identification:
X = Ethanoic acid (CH₃COOH)
Y = Ethyl ethanoate (CH₃COOC₂H₅)
Z = Sodium ethanoate (CH₃COONa)

Part (ii): Role of concentrated H₂SO₄

Step 1: Nature of esterification reaction.
The reaction between ethanol and ethanoic acid is slow and reversible.

Step 2: Functions of concentrated H₂SO₄.
It acts as a catalyst and increases the rate of reaction.
It acts as a dehydrating agent by removing water formed during the reaction.
By removing water, it shifts the equilibrium toward ester formation according to Le Chatelier’s principle.

Final Answer:
Concentrated H₂SO₄ acts as both a catalyst and a dehydrating agent in the esterification reaction.

Part (iii): Chemical equations and names of reactions

Step 1: Esterification reaction.
CH₃COOH + C₂H₅OH → CH₃COOC₂H₅ + H₂O (in presence of conc. H₂SO₄)
Name of reaction: Esterification (Fischer esterification).

Step 2: Saponification reaction.
CH₃COOC₂H₅ + NaOH → CH₃COONa + C₂H₅OH (on heating)
Name of reaction: Saponification or alkaline hydrolysis of ester.