From this wiredchemist.com, I obtained values of the bond dissociation energies of the $\ce{C-O}$ and $\ce{C-F}$ bonds:

\begin{align} D_0(\ce{C-F}) &= \pu{485 KJ mol-1}\\ D_0(\ce{C-O}) &= \pu{358 KJ mol-1} \end{align}

I have read an explanation saying that is because of fluorine being more electronegative than oxygen, consequently the dipole moment across the bond is greater. As a result the delta negative on $\ce{F}$ and delta positive on $\ce{C}$ attracts each other more compared to the situation in $\ce{C-O}$.

There is another explanation of better overlap resulting in stronger bond. However in this case the orbital overlap seems quite similar and if not $\ce{C-O}$ has better overlap than $\ce{C-F}$, so this explanation doesn't apply.

However, if this is the case, then why in many organic reaction mechanisms, the polar bonds are more prone to break when the atom with delta positive on is attacked by a nucleophile? Would you give examples for nucleophilic addition-elimination and for nucleophilic substitution?

  • $\begingroup$ Bond Dissociation Energy (BDE) is a homolytic process. More difficult to break a polar bond homolytically than a less polar bond. $\endgroup$
    – user55119
    Commented Nov 28, 2020 at 17:12
  • $\begingroup$ You may want to read this. chemistry.stackexchange.com/questions/124896/… $\endgroup$
    – user55119
    Commented Nov 28, 2020 at 17:19
  • $\begingroup$ Thank you for this, @user55119. If so, then e.g. why reaction of propan-1-ol with HI has higher rate than with HCl? Textbooks use bond enthalpy to explain this but in this case heterolytic fission occurs. $\endgroup$
    – Yushi Li
    Commented Nov 28, 2020 at 18:20
  • $\begingroup$ Rate is a kinetic issue where iodide is better nucleophile than chloride in an SN2 displacement of protonated propan-1-ol. $\endgroup$
    – user55119
    Commented Nov 28, 2020 at 19:43
  • $\begingroup$ Sorry, I meant a given nucleophile like hydroxide attacking for example 1-iodopropane compared to 1-chloropropane. Textbooks give explanations of C-Cl has higher BDE than C-I but as you pointed out BDE is a measure of homolysis but C-X would break by heterolytic fission. $\endgroup$
    – Yushi Li
    Commented Nov 28, 2020 at 20:21


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