Tip: To know the acidic character of haloform, we have to look for the stability of conjugate base
What is acidic character?
Acidic character is the ability to or a tendency to lose a proton and form a stable conjugate base.
Factor acidic character depends
Acidic character of the acid depends upon the stability of conjugate base formed.
Acid → Conjugate base + H+
Explanation of acidic character of haloform
To identify the relative strength of the acidic character, we have to check the relative stability of their respective conjugate base formed after losing H+ ion.
Conjugate base of CHF3, CHCl3, CHBr3, CHI3 are CF3–,CCl3–, CBr3–, CI3– respectively.
Stability order of the conjugate base
Fluorine is most electronegative atom, so it will stabilize CF3– more. As we know electronegativity decrease from F to I, so the stability of conjugate base decrease from CF3– to CI3–.
So, the expected acidic character is:
CHF3 > CHCl3 > CHBr3 > CHI3
But it is not so. Instead, the actual acidic order is as:
CHCl3 > CHF3 > CHBr3 > CHI3
This order is because of the concept of effective back bonding in case of CCl3– conjugate base.
Here, the negative charge gets stabilised by back bonding to the vacant 3d orbital of chlorine (Cl).
This is because, CHCl3 is stronger acid than CHF3.
Now, the acidic character of the other three haloform can be compared based on the inductive effect. F is very electronegative and hence stabilizes the negative charge on the carbon atom. So, CHF3 is more acidic than CHBr3 and CHBr3 is more acidic than CHI3.
Hence, the correct order of acidic character of haloform is:
CHCl3 > CHF3 > CHBr3 > CHI3
Read More: Arrange the following in acidic character