Compound $(X)$ on reduction with $\mathrm{LiAlH}_{4}$ gives a hydride $(Y)$ containing $21.72 \%$ hy

Question

Compound $(X)$ on reduction with $\mathrm{LiAlH}_{4}$ gives a hydride $(Y)$ containing $21.72 \%$ hydrogen along with other products. The compound $(Y)$ reacts with air explosively resulting in boron trioxide. Identify $(X)$ and ( $Y$ ).

Professor PC Thomas Classes · Answer

$Y+\operatorname{air} \longrightarrow \mathrm{B}_{2} \mathrm{O}_{3}$ From this we can guess that $Y$ must be $\mathrm{B}_{2} \mathrm{H}_{6}$.\ $\mathrm{B}_{2} \mathrm{H}_{6}+3 \mathrm{O}_{2} \longrightarrow \mathrm{~B}_{2} \mathrm{O}_{3}+3 \mathrm{H}_{2} \mathrm{O}+$ heat\ $\%$ of hydrogen in $\mathrm{B}_{2} \mathrm{H}_{6}$\ $=\frac{6}{27.62} 	imes 100=21.72 \%$\ [Mol. wt. of $\mathrm{B}_{2} \mathrm{H}_{6}=21.62+6=27.62$ ]\ Thus percentage of hydrogen is 21.72 in $\mathrm{B}_{2} \mathrm{H}_{6}$.\ $X \xrightarrow[	ext { (reduction) }]{\mathrm{LiAlH}_{4}} \mathrm{~B}_{2} \mathrm{H}_{6}$\ From this we can guess that $X$ is boron trihalide.\ $\underset{X}{4 \mathrm{BX}_3}+\mathrm{LiAlH}_4 \longrightarrow \underset{Y}{2 \mathrm{~B}_2 \mathrm{H}_6}$
                                    $+3 \mathrm{Li} X+3 \mathrm{Al} X_3$[ $X=\mathrm{Cl}$ or Br ]\ Structure of $Y$, i.e., $\mathrm{B}_{2} \mathrm{H}_{6}$ \In it, two bonds are involved in the formation of three centre 2 electron bond (Banana bond). These bonds are shown by dotted lines in the following diagram in which one of these bonds lies above and the other lies below the main plane.\ \