Question

In which of the following reactions, hydrogen is evolved?
I. $\text{Reaction of sodium borohydride with iodine}$
II. $\text{Oxidation of diborane}$
III. $\text{Reaction of boron trifluoride with sodium hydride}$
IV. $\text{Hydrolysis of diborane}$

• A. I, IV only
• B. I, II only
• C. III, IV only
• D. I, II, IV only

Hint:

$\text{B}_2\text{H}_6$ (Diborane) reactions are key. It is a highly reactive, electron-deficient compound. It evolves hydrogen upon hydrolysis (reaction with protic solvents) and is prepared via reduction reactions (like reaction with $\text{NaH}$ or $\text{NaBH}_4$).

Answer 1

The Correct Option is A

Step 1: Analyze Reaction I: $\text{Reaction of sodium borohydride with iodine$.}
$\text{NaBH}_4$ is a source of hydride ions ($\text{H}^-$). The reaction is a synthesis of diborane, which typically evolves hydrogen gas, especially when reacting with an oxidizing agent like iodine ($\text{I}_2$). \[ 2\text{NaBH}_4 + \text{I}_2 \xrightarrow{\text{ether}} \text{B}_2\text{H}_6 + 2\text{NaI} + \text{H}_2 \uparrow \] Hydrogen gas is evolved. So, I is correct.

Step 2: Analyze Reaction II: $\text{Oxidation of diborane$.}
Diborane ($\text{B}_2\text{H}_6$) is oxidized (burned) in the presence of oxygen ($\text{O}_2$). The reaction is highly exothermic. \[ \text{B}_2\text{H}_6 + 3\text{O}_2 \to \text{B}_2\text{O}_3 + 3\text{H}_2\text{O}. \] No hydrogen gas is evolved; water vapor is produced. So, II is incorrect.

Step 3: Analyze Reaction III: $\text{Reaction of boron trifluoride with sodium hydride$.}
This reaction is a laboratory method for the preparation of diborane ($\text{B}_2\text{H}_6$). It is a reduction reaction. \[ 2\text{BF}_3 + 6\text{NaH} \xrightarrow{450 \text{ K}} \text{B}_2\text{H}_6 + 6\text{NaF}. \] No hydrogen gas is evolved. So, III is incorrect.

Step 4: Analyze Reaction IV: $\text{Hydrolysis of diborane$.}
Diborane reacts with water (hydrolysis) to form boric acid and hydrogen gas. \[ \text{B}_2\text{H}_6 + 6\text{H}_2\text{O} \to 2\text{H}_3\text{BO}_3 + 6\text{H}_2 \uparrow \] Hydrogen gas is evolved. So, IV is correct.

Step 5: Conclude the final correct option.
Hydrogen gas is evolved in reactions I and IV.