Why is CHF3 less acidic than CHCl3?

Why is CHF3 less acidic than CHCl3?



The idea behind determining which compound is more acidic is linked to the stability of the conjugate bases. The more stable the conjugate base, the easier it is for the compound to lose a proton, the more acidic the compound.

Comparing the two conjugate bases, CF3- and CCl3-. The negative charge on CF3- is stabilised by the induction effect of the electronegative fluorine while the negative charge on CCl3- is stabilised by both induction effects of chlorine and by delocalisation. This delocation can happen in CCl3- because the filled 2p orbitals of C can overlap with the empty 3d orbitals of Cl which lowers the energy. Some people call this backbonding. However F does not have empty d orbitals and this overlap cannot occur.
Therefore, although the induction effect of F is stronger, the combination of induction and back bonding effect better stabilises the conjugate base in CHCl3, making it a stronger acid.
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