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I have to put a methyl group on a cyclopentene. My teacher made this solution (the picture). But my first guess was just to use $\ce{CH3Cl}$ and $\ce{AlCl3}$. Why can't you do that?

Teachers solution

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    $\begingroup$ It doesn't appear to be a known reaction, as far as I can tell from SciFinder (possibly just because theres no point in doing it rather than it being impossible). Your suggestion of a Friedel-Crafts type reaction is, however, unlikely to work as cyclopentene isn't aromatic. Your teachers route seems plausible, if not a little steppy $\endgroup$
    – NotEvans.
    Aug 4, 2018 at 9:54

1 Answer 1

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In Friedel–Crafts alkylation you form a strong electrophile to break the aromaticity of a benzene ring, resulting in a species that is also electrophilic, but not on the carbon atom. It is much more energetically favorable to reform the aromatic ring by losing a proton to $\ce{AlCl4-}$, evolving $\ce{HCl}$ gas.

The reaction of the electrophile with an alkene forms another carbon centered electrophile, however, which can rearrange and will go on to react with other alkenes or with $\ce{AlCl4-}$. There is also nothing to stop the reformed $\ce{AlCl3}$ from continuing to react with the newly formed chloroalkanes, so overall it would probably be a big mess. I found a resource here that discusses the reaction of $\ce{AlCl3}$ with alkenes:

reaction

For your reaction, though, I would propose something a little simpler and probably higher yielding like this:

reaction

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