Compare the acidity of the following compounds: $\ce{CF3OH}$ and $\ce{CCl3OH}$.

My attempt: according to me $\ce{CCl3}$ group will show a stronger Negative hyperconjugation effect than $\ce{CF3}$, and thus would stabilize the anion more effectively.

But the answer is $\ce{CF3OH}$ is more acidic. What is wrong in my approach?

  • $\begingroup$ What is negative hyperconjugation? $\endgroup$ – electronpusher Aug 10 '19 at 19:06
  • $\begingroup$ Fluorine is the boss of all electronegative things. $\endgroup$ – Oscar Lanzi Aug 10 '19 at 19:33
  • $\begingroup$ @electronpusher typically, lone pair to $\sigma^*$ donation $\endgroup$ – orthocresol Aug 10 '19 at 20:13

$\ce{CF3OH}$ is more acidic because of the inductive effect of the $\ce{-CF3}$ group:
Fluorine is very electronegative and therefore it pulls the electron-density toward itself weakening the $\ce{O-H}$ bond in $\ce{CF3OH}$.
Chlorine is less electronegative than fluorine, thus $\ce{CCl3OH}$ is less acidic than $\ce{CF3OH}$.
Because $\ce{H}$ is even less electronegative, then $\ce{CH3OH}$ is less acidic than all its halogenated derivatives.

A related question can be found here: Why is CF3COOH exceptionally acidic?

The negative hyperconjugation is a type of resonance effect where there is a donation of electron density from a filled π- or p-orbital to a neighboring σ*-orbital. It does not occur in your molecules.

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