Why BF3 is weaker acid than BCl3?
Answers
Answer:
In contrast, toward weak bases such as CO, BF3 is a stronger Lewis acid than BCl3. ... In contrast, in the formation of a complex with a weak base such as CO, the BX3 is barely distorted from planarity and so the acidity of BF3 is greater than that of BCl3 because the charge on boron is greater in BF3 than BCl3.
Boron : (1s)2 (2s)2 (2px)1 (2py)0 (2pz)0 ,valence orbitals are 2s,2px,2py,2pz
Fluorine: (1s)2 (2s)2 (2px)2 (2py)2 (2pz)1 ,valence orbitals are 2s,2px,2py,2pz
Boron in excited state: (1s)2 (2s)1 (2px)1 (2py)1 (2pz)0
As fluorine molecules approach boron, the 2s, & two ‘p’ orbitals hybridize to form three (sp)2 hybridized orbitals.
Boron in (sp)2 hybridized state: (1s)2 (sp2)1 ( sp2)1 (sp2)1 (p)0
3 Fluorine atoms have an unpaired electrons in their (2pz)1 orbitals, which pair with the unpaired electrons of (sp2) 1 orbitals of Boron. Thus finally resulting in BF3 molecule with empty ‘p’ orbital on Boron. The empty ‘p’ orbital attributes to lewis acidity of BF3.
As the orbitals of Fluorine containing lone pair of electrons and vacant ‘p’ orbital of Boron are having same energy, overlap of those orbitals take place, resulting in donation of lone pair electrons of ‘F’to ‘B’ termed as backbonding.
Where as in case of BI3 , the empty ‘p’ orbital of ‘B’ and orbitals containing lone pair of electrons of ‘I’ are not of same energy levels, due to which the ‘p’ orbital remains empty in BI3.
BF3 is weaker lewis acid than BI3.