Question

Which of the following statements is correct?

• A. $BCl_{3}$ and $AlCl_{3}$ are both Lewis acids and $BCl_{3}$ is stronger than $AlCl_{3}$
• B. Both $BCl_{3}$ and $AlCl_{3}$ are Lewis acids and $AlCl_{3}$ is stronger than $BCl_{3}$
• C. Both $BCl_{3}$ and $AlCl_{3}$ are equally strong Lewis acids
• D. Both $BCl_{3}$ and $AlCl_{3}$ are not Lewis acids

Answer 1

Answer: (A)

In $BCl_{3},p\pi -p\pi $ back-donation occurs between the vacant 2p-orbital of 'B' and fully-filled 3p-orbital of Cl of a $B-Cl$ bond. This $p\pi -p\pi $ overlapping is not much effective, because of dissimilar sizes of 2p and 3p-orbitals. But in $AlCl_{3}$ , $p\pi -p\pi $ overlapping is more effective, because p-orbitals of Al and Cl belongs to the same energy levels and hence, they are of similar sizes. As a result of back-donation, the electron-deficiency of Al atom gets more compensated as compared with 'B' in $BCl_{3}$ and therefore, Lewis acid character of $AlCl_{3}$ is lesser than that of $BCl_{3}$ .