Ch_322b_15.06

Ch_322b_15.06 - Chapter 15.6 lecture note

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Some Limitations of the Friedel-Crafts Reactions (1) Rearrangements during Alkylations Whenever carbocation intermediates are formed, they are subject to rearrangement reactions producing more stable species. Example: The Friedel-Crafts reaction of benzene and butyl bromide CH 3 CH 2 CH 2 CH 2 Br AlCl 3 CH 3 CH 2 CH 2 CH 2 -Br-AlCl 3 - + complex
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A 1,2-hydride shift, possibly concurrent with dissociation, produces a more stable secondary-butyl carbocation. A mixture of products results. CH 3 CH 2 CHCH 2 -Br-AlCl 3 - + ~H: 1,2-hydride shift H CH 3 CH 2 CHCH 3 + AlBrCl 3 - + CH 2 CH 2 CH 2 CH 3 butylbenzene (~32%) sec-butylbenzene (~68%) CHCH 2 CH 3 CH 3 Overall + CH 3 CH 2 CH 2 CH 2 Br AlCl 3 CH 2 CH 2 CH 2 CH 3 butylbenzene (~32%) + sec-butylbenzene (~68%) CHCH 2 CH 3 CH 3 The rearrangement product dominates.
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(2) Friedel-Crafts reactions do not proceed well with electron-deficient benzenes, or with aromatics that have amino groups. The following
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Ch_322b_15.06 - Chapter 15.6 lecture note

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