Negative hyperconjugation

Question

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?

Answer 1

$\ce{CF3OH}$ 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.

Answer 2

According to me, while comparing acidic strength, we should remove $\ce{H+}$ from both the compounds, hence we get $\ce{CF3-O-}$ and $\ce{CCl3-O-}$.

Now, we will compare stability using priority order:

Aromaticity > Resonance > Hyper-conjugation > Inductive effect

Now, we can see that, in $\ce{CF3O-}$ only inductive effect is applicable whereas in $\ce{CCl3O-}$ both inductive effect and d-orbital resonance can be seen.

So, as we know acidic strength is directly proportional to the stability of the conjugate base. In other words, it is only dependant on the stability of the negative charge on the compound, and as we can see the negative charge is more stable on (b) than in (a).

Therefore, the acidic strength order would be $\ce{CCl3O-}$>$\ce{CF3O-}$.