Question

$AlCl_{3}$ forms dimer in vapour phase but $BCl_{3}$ does not because

• A. In $Al$ there are vacant d orbitals in which it accommodates lone pair from chlorine atoms
• B. In $B C l_{3}$ there is back bonding
• C. There is hydrogen bonding in between two $AlCl_{3}$ molecules in vapour phase
• D. None of the above

Answer 1

Answer: (A)

The formation of stronger backbond ( $3p \rightarrow 2p$ overlap in $BCl_{3}$ and weaker $3p \rightarrow 3p$ overlap in $AlCl_{3}$ ) reduces the electron deficiency of the central atom and thus dimerisation tendency is reduced. So $AlCl_{3}$ can form a dimer but $BF_{3}$ and $BCl_{3}$ cannot.