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.

Why are bridge head carbocations unstable?

According to Bredt’s rule a bridgehead carbon atom of bicyclo compound cannot be sp2 hybridised or in other word a bridgehead carbon atom cannot be form double bond. unless the ring that contains at least eight atoms.

For more details on Bredt's rule click here

Related Questions:

Why is cyclopropyl methyl carbocation exceptionally stable?

Which carbocation is more stable : Benzyl or Tertiary?

Trifluoroethyl carbocation is less stable than trifluromethyl carbocation why ?

Which is more stable ,vinyl carbocation or allyl carbocation?

Why is cyclopropyl methyl carbocation exceptionally stable?

The exceptional stability of cyclopropane methyl cation can be explained by the concept of dancing resonance concept. The stability of additional cyclopropyl group , is result of more conjugation between the bent orbital of cyclopropyl ring and cationic carbon.

The most stable carbocation known till date in organic chemistry is explain by Dancing resonance.

Related Questions:

What is dancing resonance or Sigma Resonance or p- orbital resonance ?

What is Bredt's rule ?

Azabicyclo[2,2,1]heptane is more basic thantriethylamine why?.

1-Azabicyclo[2,2,1]heptane is more basic than triethylamine . Because in case of triethylamine the lone pair of electrons is less available in the latter due to rapid nitrogen inversion. Nitrogen inversion is not possible in the bicyclic amine.

Which carbocation is more stable : Benzyl or Tertiary?

Actually answers of this question is always confusing, most of the authors believe benzyl carbocation is more stable than tertiary because benzyl carbocation involves in resonance.

But some of the authors believe that Tertiary carbocation is more stable as it involves maximum +I effect and maximum hyperconjuation +H (9-alpha hydrogens). Maximum +I and +H is more dominant than +M effect. Thus tertiary carbocation is more stable than benzyl carbocation.

Important note:

Stability of Benzyl , allylic and tertiary alkyl carbocation is practically almost same .so that stabilities infact cannot be compared.

Similar Questions:

Which carbocation is more stable : Benzyl or Tertiary?

Which is more stable ,vinyl carbocation or allyl carbocation?

Why benzene is reluctant to show addition reaction?

Trifluoroethyl carbocation is less stable than trifluromethyl carbocation why ?