Acidic character of Haloform

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

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