question_answer

Which of the following shows correct Lewis acidic strength order of trihalides of boron?

A)  \[BC{{l}_{3}}>BB{{r}_{3}}>B{{F}_{3}}>B{{l}_{3}}\]

B)  \[B{{F}_{3}}<BB{{r}_{3}}<BC{{l}_{3}}<B{{l}_{3}}\]

C)  \[B{{F}_{3}}<BC{{l}_{3}}<BB{{r}_{3}}<B{{l}_{3}}\]

D)  \[B{{F}_{3}}>BC{{l}_{3}}>BB{{r}_{3}}>B{{l}_{3}}\]

Correct Answer: C

Solution :

This cannot be explained on the basis of electronegativity of substituent attached actually. This can be explained on the basis of \[{{p}_{\pi }}-{{p}_{\pi }}\] back bonding, greater the \[{{p}_{\pi }}-{{p}_{\pi }}\] back bonding lower is its lew is acidic strength \[\to \] Back bonding tendency decreases \[\to \] Lewis acidity increases.