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I am having trouble understanding this concept. Vinyl cation was a test question in my organic chem class. The professor mentioned that a vinyl cation cannot make the leaving group leave and that it must be on the double bond for it to be vinyl. The questioned asked as following:

Which is the best to make the vinyl cation?

a) $$\ce{C-C#C - H+ ->}$$

b) C-C=C --KOH-->

    |
    Cl

c) C-C=C --H+-->

    |
    OH

d) a+c

I know the answer but I don't understand why? Can someone please help in easy terms?

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  • $\begingroup$ Don't confuse a high energy vinyl cation (charge on the sp^2 center of the olefin, C=C+) with a low energy delocalized allyl cation (C=C-C+). $\endgroup$ – Uncle Al Apr 3 '14 at 17:58
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Addition of $\ce{HX}$ to $\ce{R1-C#C-R2}$ is assumed to proceed via cyclic intermediate in which the proton bridges the sp centres.

Vinyl cations can be formed from alkenes, supposed they are substituted with a super leaving group, such as $\ce{-OSO2CF3}$ (OTf, trifluoromethanesulfonate, triflate) or $\ce{-OSO2(CF2)3CF3}$ ((ONf, nonafluorobutanesulfonate, nonaflate).

If you are interested, look for some articles by Michael Hanack from the 1970s on the topic.

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