Question

Consider the following (unbalanced) reactions:

\[ \text{BF}_3 + \text{NaH} \xrightarrow{450\,\mathrm{K}} X + \text{NaF} \]

\[ X + \text{H}_2\text{O} \rightarrow Y + \text{H}_2 \uparrow \]

The correct statements about \(X\) and \(Y\) are:

1. \(X\) is an electron-deficient molecule
2. In \(X\), a B--B bond is present
3. \(Y\) is a weak tribasic acid
4. \(Y\) acts as a Lewis acid

• A. I & IV
• B. II & III
• C. II & IV
• D. I & III

Hint:

Diborane (\(\text{B}_2\text{H}_6\)) is electron-deficient because of its 3-center-2-electron bonds. - Boric acid (\(\text{H}_3\text{BO}_3\)) is a \emph{Lewis acid}, not a true tribasic acid in water.

Answer 1

The Correct Option is A

Step 1: Identifying \(X\) and \(Y\)

• When \(\text{BF}_3\) reacts with \(\text{NaH}\) at \(\pu{450 K}\), the product \(X\) is \(\text{B}_2\text{H}_6\) (diborane).
• Diborane then reacts with water to form boric acid \(\text{H}_3\text{BO}_3\) (denoted here as \(Y\)) and hydrogen gas.\[ \text{B}_2\text{H}_6 + 6\,\text{H}_2\text{O} \longrightarrow 2\,\text{H}_3\text{BO}_3 + 6\,\text{H}_2 \]

Step 2: Verifying the Statements

1. \(X\) is an electron-deficient molecule: \(\text{B}_2\text{H}_6\) has 3-center-2-electron (banana) bonds and is electron-deficient. This is true.
2. In \(X\), a B--B bond is present: There is no direct B--B single bond in diborane; instead, there are bridging hydrogens. This is false.
3. \(Y\) is a weak tribasic acid: Boric acid (\(\text{H}_3\text{BO}_3\)) is monobasic in water because it primarily acts via Lewis acidity (accepting \(\text{OH}^-\)), not by losing 3 protons. This is false.