Question

Why do C-C bond break earlier than C-F in the pyrolysis of c2f6?

According to me C-F bond is highly polar and so a WEAKER bond from C-C hence C-F must break early why it is not so?

Answer 1

C-F bond has higher bond energy than C-C bond hence is harder to break.

Explanation:

Let's compare the bond energies of these bonds at `25^@ "C"`:

C-F bond 488 kJ/mol
C-Cl bond 330 kJ/mol
C-Br bond 288 kJ/mol
C-C bond 348 kJ/mol

It is true that C-F bond is highly polar as fluorine is more electronegative than carbon. That would suggest a large amount of ionic character, which weakens the bond.

However, fluorine is very small, so the `"C"-"F"` bond is exceptionally short, and much stronger than the `"C"-"C"` bond (this is also why the electron affinity of fluorine is almost smaller than that of bromine).

This makes C-F bond extremely stable and hard to break, and explains why there is a big jump from C-Cl to C-F.

In fact in Organic Chemistry we usually say that the C-F bond is not broken in reactions.

Hope this helps!