Question

which of the following statement is correct.
A. BCL3 and ALCL3 are both lewis acids and ALCL3 is stronger lewis acid
B. BCL3 and ALCL3 are both lewis acids and BCL3 is stronger lewis acid
C. BCL3 is a lewis but ALCL3 is not.
D. Both BCL3 and ALCL3 are not lewis acid.

Answer 1

Dear Student,

BCl₃ is stronger lewis acid than AlCl₃ because of the size difference of B and Al. For B it use its 2P orbital for bonding and for Al it use its 3P orbital for bonding. Al is an element of third period and it has a vacant 3d which is not filled and for boron, it has no vacant 3d orbital. Side by side, in case of Al 3p orbital is the valance orbital and for boron 2p orbital is the valance orbital. So, electron acceptance property in case of boron is high due to less energy of the 2p orbital than the 3p orbital of Al.
So for BCl₃ B - Cl bond distance is less compared to the AlCl₃ Al - Cl and we know that increasing bond length decrease lewis acid character. When bond length increase then electon acceptence ability decrease i.e. BCl₃ is stronger lewis acid than AlCl₃.


Regards