The pBlock Elements - Result Question 6
####7. The tendency of $BF_3, BCl_3$ and $BBr_3$ to behave as Lewis acid decreases in the sequence: [2010]
(a) $BCl_3>BF_3>BBr_3$
(b) $BBr_3>BCl_3>BF_3$
(c) $BBr_3>BF_3>BCl_3$
(d) $BF_3>BCl_3>BBr_3$
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Solution:
- (b) In $BF_3, p-p$ overlap between $B$ and $F$ is maximum due to identical size and energy of $p$-orbitals, so electron deficiency in boron of $BF_3$ is neutralized partially to the maximum extent by back donation. Also, the tendency to back donate decreases from $F$ to $I$. So the order will be:
$ BF_3<BCl_3<BBr_3 . $
