Question

The relative strengths of trichlorides of boron group to accept a pair of electron is given by

• A. GaCl$_3$ < AlCl$_3$ < BCl$_3$
• B. AlCl$_3$ < BCl$_3$ < GaCl$_3$
• C. AlCl$_3$ < GaCl$_3$ < BCl$_3$
• D. BCl$_3$ < AlCl$_3$ < GaCl$_3$
• E. GaCl$_3$ < BCl$_3$ < AlCl$_3$

Hint:

BCl$_3$ is strongest Lewis acid due to highest electron deficiency despite back bonding.

Answer 1

The Correct Option is A

Concept: Electron pair accepting ability corresponds to Lewis acidity. In group 13 trichlorides, Lewis acidity depends on:
• Size of central atom
• Back bonding (p$\pi$–p$\pi$ overlap)
• Availability of vacant orbital

Step 1: Understand BCl$_3$.
Boron is very small → effective p$\pi$–p$\pi$ back bonding between B and Cl reduces electron deficiency. However, due to very small size, B still remains highly electron deficient.

Step 2: Understand AlCl$_3$ and GaCl$_3$.
Down the group:
• Atomic size increases
• Back bonding decreases
• But availability of orbitals and shielding increases

Step 3: Key comparison logic.
Even though back bonding reduces in AlCl$_3$, its larger size reduces its tendency to accept electrons compared to BCl$_3$. For GaCl$_3$:
• Even larger size
• Poor overlap → weaker Lewis acidity

Step 4: Final order. \[ GaCl_3 < AlCl_3 < BCl_3 \]

Step 5: Answer. \[ \boxed{(A)} \]