Manipal Entrance Test 2016 Chemistry Paper

Generally, as the electronegativity increases, acidic character increases, but in case of boron halides. BI3 is the most acidic, i.e. strongest Lewis acid. This is because there is back donation of electrons from F-atom to boron in caseof BF This makes BF3 less electron deficient. The tundency of back donation decreases as the size of halogen atom increases due to large energy difference. Hence, BI3 is the most electron deficient and thus, the strongest Lewisacid.