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Monday, December 21, 2020

What are the order of extent back bonding, Lewis acid character and nucleophilicity of (BF3, BCl3, BBr3, BI3)boron trihalides?

(1) Back bonding extent in boron tri halides decreases from BF3 to BI3 because on increasing of size of p-orbital of halogens atom the strength of back bond decreases thus extent of back bonding:

(2) Lewis acid character of Boron Halides is inversely proportional to extent of back bonding because on decreasing back bonding tendency to accept lone pair from base is increases thus the order of Lewis acid character is:

Hence it is clear that BF3 is weakest Lewis acid due to stronger 2pπ-2pπ back bonding (stronger partial double bond character) in BF3 (lone pair orbital of fluorine into vacant orbital of boron) and consequently behaves as less electron deficient. The back bonding gradually decreases (From BF3 to BI3) and becomes weakest in BI3. So that BI3 become strong Lewis acid

(3) The nucleophilicity(affinity towards nucleophile/water) order is inversely proportional to the Lewis acid character thus the nucleophilicity order is:

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