Question

A carbon compound 'A' on heating with excess conc. H\(_2\)SO\(_4\) forms a compound 'B', which on addition of one mole of hydrogen gas in the presence of nickel catalyst forms a compound 'C'. 'C'. on combustion in air forms 2 moles of carbon dioxide and 3 moles of water. Identify 'A', 'B' and 'C' and write their structures. Give chemical equations of the reactions involved. Also state the role of concentrated sulphuric acid in the formation of 'B' from 'A'.

Answer 1

Analysis and Identification of Compounds A, B, and C

1. Analysis of Compound C (Combustion)

Compound C gives 2 moles of CO₂ and 3 moles of H₂O on combustion. This indicates:

• 2 carbon atoms ⇒ C₂
• 3 × 2 = 6 hydrogen atoms ⇒ H₆

Molecular formula of C: C₂H₆ (Ethane)

Structure of C: CH₃–CH₃

2. Formation of C from B (Hydrogenation)

Compound B adds H₂ in presence of Ni catalyst to form Ethane:

B = C₂H₄ (Ethene)

Reaction:

CH₂=CH₂ (B) + H₂ → CH₃–CH₃ (C) [Ni catalyst, heat]  

This is a hydrogenation reaction.

3. Formation of B from A (Dehydration)

Compound A undergoes dehydration with conc. H₂SO₄ to form Ethene (B).

A = C₂H₅OH (Ethanol)

Reaction:

CH₃–CH₂–OH (A) --(conc. H₂SO₄, heat ~170°C)--> CH₂=CH₂ (B) + H₂O  

4. Combustion of Compound C

Full reaction for 1 mole:

C₂H₆ + (7/2)O₂ → 2CO₂ + 3H₂O + heat + light  

5. Role of Concentrated H₂SO₄

In the conversion of Ethanol (A) to Ethene (B), conc. H₂SO₄:

• Acts as a dehydrating agent
• Protonates the -OH group
• Promotes elimination to form a double bond (C=C)
• Forms water as a leaving group

Summary of Identified Compounds

• A: Ethanol (CH₃–CH₂–OH)
• B: Ethene (CH₂=CH₂)
• C: Ethane (CH₃–CH₃)

Summary of Chemical Equations

1. Dehydration of A to B:CH₃CH₂OH --(conc. H₂SO₄, heat)--> CH₂=CH₂ + H₂O      
2. Hydrogenation of B to C:CH₂=CH₂ + H₂ --(Ni catalyst)--> CH₃–CH₃      
3. Combustion of C:C₂H₆ + (7/2)O₂ → 2CO₂ + 3H₂O