Two reasons to avoid a Friedel–Crafts alkylation
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص492-493
2025-06-09
609
When electron-donating substituents are added, multiple substitution is always a threat. The principal reaction where multiple substitution is a genuine problem is the Friedel–Crafts alkyl ation reaction. Here’s an example: preparation of diphenylmethane from benzene and benzyl chloride is a useful reaction but the product has two benzene rings, each more reactive than benzene itself. A 50% yield is the best we can do and that requires a large excess of benzene to ensure that it competes successfully for the reagent with the reactive, electron-rich product.
Multiple substitution is just one of the potential pitfalls of Friedel–Crafts alkylations. The other is important to be aware of too: Friedel–Crafts alkylations work well only with stable cations. This is what happens when we try a Friedel–Crafts reaction with n-propyl chloride.
Recall from Chapter 15 that primary halides don’t form cations easily, so the Friedel–Crafts reaction with n-propyl chloride has to go via an SN2 mechanism.
So where does the major product of the reaction come from? The three carbons are arranged not as an n-propyl group but as an iso-propyl group: a rearrangement has occurred. This is the mechanism:
The green hydrogen migrates to allow a secondary rather than a primary alkyl cation to be formed, and iso-propylbenzene results. This leaves us with a problem.
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