What is correct acidic strengths order of the haloforms acids ? Give correct explanation.

We know that acidic strength of the acid also depends upon stability of conjugate bases, so for relative strength of acid, we need to check the relative stabilities of their conjugate bases.

                                                     CF₃^-, CCl₃^-, CBr₃^- CI₃^-

We are expecting the acidic strength haloform acids asCHF₃, CHCl₃, CHBr₃, CHI₃ in decreasing order. Because Fluorine is most electronegative atom so it would be stabilize CF₃^- more, as electronegativity decreases from F to I the stability of conjugate -ve ion would be but that is not correct the actual order isCHCl₃ > CHF₃ > CHBr₃ > CHI₃.

This is because there is effective back bonding in CCl₃-and hence the negative charge partially gets stabilised by back donation to the vacant 3d orbitals of Cl. Thus, CHCl₃ is a stronger acid than CHF₃ and also among them due to 2pπ-3dπ back bonding.

The acidic strengths of the other three haloforms can be compared the inductive effects of their anions. F is very electronegative and hence stabilises the negative charge on the C atom. So, CHF₃ is a better acid than CHBr₃, and the least acidic is CHI₃.

 The overall acidic strength order is:

     CHCl₃ > CHF₃ > CHBr₃ > CHI₃.