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Question

The correct order of strengths of back-bonding and lewis acidic strengths in Boron trihalides respectively is:

A
BF3<BCl3<BBr3 and BF3<BCl3<BBr3
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B
BF3>BCl3>BBr3 and BF3>BCl3>BBr3
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C
BF3>BCl3>BBr3 and BF3<BCl3<BBr3
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D
BF3<BCl3<BBr3 and BF3>BCl3>BBr3
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Solution

The correct option is C BF3>BCl3>BBr3 and BF3<BCl3<BBr3

Lewis acids are electron pair acceptors.
In BF3, though the partial positive charge on B atom is more due to the high electronegativity of flourine, the tendency to accept a lone pair is reduced because of the ability of the fluorine atoms to donate their own lone pair to the empty boron orbitals forming p(π)p(π) back bonding. F forms the strongest back bond due to the effective overlap between 2p (F) and 2p (B) orbitals. So, the electron deficiency of boron gets compensated and the lewis acidic character decreases.
But as the size of the orbital increases, the extent of back-bonding decreases. This is due to the ineffective overlap as in case of 3p and 4p obitals of Cl and Br respectively with that of 2p orbital of B. So, the order of back bonding is BF3>BCl3>BBr3 and hence that of lewis acidity is BF3<BCl3<BBr3


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