According to me $\ce{C-F}$ bond is highly polar and thus is a weaker bond as compared to $\ce{C-C}$. Hence, $\ce{C-F}$ should break first in the pyrolysis of perfluroethane. Instead, in J. D. Lee, it is stated that $\ce{C-C}$ bond will break first. Why?

  • $\begingroup$ I think that the main reason is that the CF3 radical would be delocalised (hyperconjugation?). Another reason could be stearic repulsion. $\endgroup$ – Anurag B. Jun 8 '18 at 6:35
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    $\begingroup$ What makes you think the C-F bond is weaker? In the tables of bond enthalpies I've seen, the C-F bond is considerably stronger than the C-C bond. $\endgroup$ – Christopher Grayce Jun 8 '18 at 9:09
  • $\begingroup$ @Christopher Grayce i had misconception that high polarity always make a bond weak but now i got cleared that it can even make a bond strong by making bond ionic...... $\endgroup$ – Rafael Nadal Jun 8 '18 at 13:08
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    $\begingroup$ I would be cautious about the relationship between bond polarity and bond strength. A bond can be strong and nonpolar (N2), strong and polar (C-F), weak and nonpolar (I2), or weak and polar (HI). There's definitely correlation, but it isn't perfect, and should be used with caution. After all, the two phenomena arise from related but not identical causes. $\endgroup$ – Christopher Grayce Jun 8 '18 at 17:36

I got this explanation from one of my seniors.

It is true that $\ce{C-F}$ bond is highly polar as fluorine is more electronegative than carbon. In fact it is this very difference in electronegativity that makes C-F bond very strong. This is because, in addition to the covalent bond strength between carbon and fluorine, there is an additional electrostatic attraction between positive carbon and negative fluorine.

We can think of it has the $\ce{C-F}$ bond has both covalent and ionic character. This makes C-F bond extremely stable and hard to break.

In fact in organic chemistry we usually say that the $\ce{C-F}$ bond is not broken in reactions.

We can also verify this from their bond energies:

$\ce{C-F}$: $\pu{544 kJ mol-1}$
$\ce{C-C}$: $\pu{346 kJ mol-1}$

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    $\begingroup$ Another way to think about the difference in bond energies is to think about the stability of radical species formed after bond homolysis, which is likely the process occurring during pyrolysis. The general trend is that free radical stability decreases with increasing electronegativity. See: masterorganicchemistry.com/2013/08/14/…. $\endgroup$ – Dennis Cao Jun 8 '18 at 14:33
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    $\begingroup$ Lemal mentions that the charge on C is +0.76 while that on F is -0.19. This shows how large the separation of charge actually is and how strong the ionic contribution to the bond is. Lemal, D. M. (2004). Perspective on Fluorocarbon Chemistry [Abstract]. The Journal of Organic Chemistry,69(1), 1-11. doi:10.1021/jo0302556 $\endgroup$ – Tan Yong Boon Jun 8 '18 at 23:25
  • $\begingroup$ One more reason is that-----Even CF being so polar bond but due to very small bond length and very small F size IT IS MORE STRONG THAN CC BOND $\endgroup$ – Rafael Nadal Jun 10 '18 at 15:12

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