sri last edited by
Isn't the acidic strength given here wrong in point 12 as BF3 should be most acidic
Sarthak Basak last edited by
Amongst the boron trihalides, the acidity order is BI3>BBr3>BCl3>BF3.
Despite of having high electronegativity of F atoms in BF3, there is another reason that overweigh this effect & follows the above order. This is back bonding.
Back-bonding: In boron trihalides the B atom is a sp2 hybrid carbon atom & has its vaccant 2p orbital perpendicular to the plane of the molecule.
F atoms have filled 2p orbitals perpendicular to the plane of the molecule. These filled orbitals sends the electron density back to the B atom through the back bonding. This reduces lewis acidity on the B atom.
But in case of heavier congeners the ligand orbital is 3p, 4p etc. Thus, the orbital overlap is not so good for them. So the extent of back bonding decreases for the heavier congeners. Thus, the lewis acidity increases.