Ch_322a_7.014

Ch_322a_7.014 - Hydrogenation of Alkynes Catalytic...

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Hydrogenation of Alkynes If the catalyst is "poisoned" with a compound that selectively interferes with the hydrogenation of the alkene, the alkene product may be isolated. "Catalyst poisons" are salts of Pb 2+ and some nitrogen compounds such as pyridine and other amines. Example: Hydrogenation of Diphenylethyne C 6 H 5 C CC 6 H 5 diphenylethyne + 2H 2 Pd/C (no poison) Pd/C is palladium metal finely dispersed on powdered charcoal. C 6 H 5 CH 2 CH 2 C 6 H 5 1,2-diphenylethane (95%) C=C H H C 6 H 5 C 6 H 5 cis-1,2-diphenylethene "cis-stilbene" (87%) C 6 H 5 C CC 6 H 5 diphenylethyne + H 2 Pd/C (poisoned with pyridine) N Catalytic hydrogenation of the triple bond occurs step-wise and may be controlled to yield alkenes: CH 3 C CCH 3 2-butyne H 2 Pt CH 3 CH=CHCH 3 H 2 Pt CH 3 CH 2 CH 2 CH 3 2-butene butane
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Syn-Hydrogenation of Alkynes: cis- Alkenes The selective catalytic hydrogenation of alkynes with a "poisoned" catalyst is a syn-addition that yields cis-alkenes . A second way to carry out this transformation is with a
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Ch_322a_7.014 - Hydrogenation of Alkynes Catalytic...

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