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A double bond/alkene reacts as a nucleophile. I have never seen a reaction where it takes part in an $S_N1$ or $S_N2$ reaction with a halogenoalkane for example. This would presumably be a very useful reaction yet I have never seen it. I assume this is because it doesn't work. Is this the case? If so why?

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  • $\begingroup$ A double bond/alkene reacts as a nucleophile. I have never seen a reaction where it takes part in an $S_N1$ or $S_N2$ reaction : If you've never seen a reaction where a double bond participates in a nucleophilic reaction, how can you say that it reacts as a nucleophile? $\endgroup$ – Binary Geek Apr 15 '15 at 15:20
  • $\begingroup$ ...with a halogenoalkane (as it says in the question) $\endgroup$ – RobChem Apr 15 '15 at 15:54
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The pi electrons in an alkene or alkyne can act as a nucleophile. This class of reactions is often taught a bit later than the standard $\ce{S_{N}1}$ and $\ce{S_{N}2}$ reactions and comes up when neighboring group participation or anchimeric assistance is covered.

The pi electrons in a double or triple bond are not strongly nucleophilic but when you arrange things in their favor, pi electrons can act as a nucleophile. The reactions involving double bonds that I am familiar with involve an $\ce{S_{N}1}$ mechanism. Here are a few examples.

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(image source)

In example (1) notice how the compound with the double bond solvolyzes $10^{11}$ times faster than the saturated model compound! Note too that only the product with acetate group anti to the double bond is formed.

Triple bonds and benzene rings can participate as well. Often in cases involving double or triple bonds new carbon-carbon bonds are formed between the unsaturated linkage and the carbon with the leaving group [examples (2) and (3) above]. In cases involving aromatic rings aromaticity is restored so no new carbon-carbon bonds are formed between the aromatic ring and the carbon with the leaving group.

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  • $\begingroup$ Just in terms of a non intramolecular reaction. Like, for example, the reaction of propene with methyl iodide, is this possible? I suspect not but I can see no clear reason why. $\endgroup$ – RobChem Apr 15 '15 at 18:39
  • $\begingroup$ Pi electrons are a weak nucleophile. They need that entropic help that comes with intramolecular reactions. Still, like you say, I don't see why it can't happen intermolecularly, but I can't think of an example. Perhaps someone else reading this can come up with such an example. I'll think about it some more. $\endgroup$ – ron Apr 15 '15 at 18:47

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