Ch_322a_8.013

Ch_322a_8.013 - (-X-) X-nucleophilic addition R-C=C-H H X...

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Addition Reactions of Alkynes Alkynes undergo many of the same addition reactions as alkenes. Since the initial products are alkenes, they may react further. Addition of Bromine or Chlorine R-C C-R' Br 2 CCl 4 C=C R R' Br Br Br 2 CCl 4 C C R R' Br Br Br Br With care, the dihaloalkene products can be isolated: CH 3 CH 2 CH 2 CH 2 C C-H 1-hexyne Br 2 1 mol C=C Br Br CH 3 CH 2 CH 2 CH 2 H (72%) C=C Br Br CH 3 CH 2 CH 2 CH 2 H + (28%) trans- and cis -1,2-dibromo-1-hexene A mixture of the diastereomeric dihaloalkenes is often produced. Example
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Addition of Hydrogen Halides to Alkynes The addition of hydrogen chloride and hydrogen bromide to alkynes is regioselective (Markovnikov's rule) and may involve one or two equivalents of HX. R-C C-H HX RC=CH X H HX RC CH X X H H haloalkene geminal dihalide
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A Mechanism for the Hydrohalogenation of Alkynes R-C C-H + H-X R-C=C-H H + electrophilic addition The more stable vinyl cation is produced.
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Unformatted text preview: (-X-) X-nucleophilic addition R-C=C-H H X The regiochemistry of the addition of the second HX reflects the stabilizing influence of the halogen on the carbocation intermediate: R-C=C-H H X H-X + (-X-) R-C C-H H H :X: : + R-C C-H H H :X: + = resonance stabilization by nonbonding electrons X-R-C C-H H H X X Some Examples CH 3 CH 2 CH 2 C C-H 1-pentyne HBr CH 3 CH 2 CH 2 C=CH 2 Br 2-bromo-1-pentene HBr CH 3 CH 2 CH 2 CCH 3 Br Br 2,2-dibromopentane CH 3 CH 2 C CCH 3 2-pentyne HBr CH 3 CH 2 CH=CCH 3 Br + CH 3 CH 2 C=CHCH 3 Br 2-bromo-2-pentene 3-bromo-2-pentene formed as (E) and (Z) isomers With 2-pentyne, there is no regioselectivity because of the similar stability of the two vinyl cation intermediates....
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Ch_322a_8.013 - (-X-) X-nucleophilic addition R-C=C-H H X...

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