2
$\begingroup$

According to "Organic Chemistry as a secondary language", during elimination-addition, formation of benzyne involves 2 steps (as shown in the bottom photo). I wonder during 2nd step, it is just electron transfer and 'kick out' the Cl as leaving group.

Can the 2 step process be simplified as 1 step? (as shown in the upper photo) The only difference is that I used the electrons from C-H bond to form the 3rd C-C bond, where the book suggest a deprotonated anion is first formed then C-C is subsequently formed. I wonder what causes the difference? Thank you.

Improve this question
$\endgroup$
6
  • 1
    $\begingroup$ The loss of HX from chloro- and bromobenzene is concerted with amide ion while fluorobenzene loss of HF is stepwise. J. D. Roberts, et al., J. Am. Chem. Soc., 78, 611 (1956). $\endgroup$
    – user55119
    Jun 16, 2020 at 18:10
  • $\begingroup$ But I would really like to know why it must be a 2- instead of 1-step process, is the (central) anion more stable? What caused the electron to stay at 1 point but not immediately transferred to form 3rd C-C bond? Thanks $\endgroup$
    – 234ff
    Jun 17, 2020 at 1:14
  • 1
    $\begingroup$ look carefully at user55119's comment and you will realize that it is satisfying the criteria you have yourself stated. The anion formed in the fluorobenzene case will be quite a bit more stable than that in chlorobenzene and bromobenzene, hence theformer goes through a stepwise process, while the other halobenenes adopt a concerted path $\endgroup$
    – Yusuf Hasan
    Jun 17, 2020 at 3:52
  • $\begingroup$ OK thanks. So in this case, forming benzyne from chlorobenzene is actually a concerted process rather than 2-step process as the book says? (since the anion is much less stable) $\endgroup$
    – 234ff
    Jun 17, 2020 at 3:58
  • $\begingroup$ Yes, benzyne to chlorobenzene is concerted $\endgroup$
    – Yusuf Hasan
    Jun 17, 2020 at 4:32

0

Reset to default

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Browse other questions tagged or ask your own question.