l18 - CH 203 O R G A N I C C H E M I S T R Y I Alkynes:...

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Alkynes • Reaction syllabus © Bruno I. Rubio 1 CH 203 O R G A N I C C H E M I S T R Y I Alkynes: Reaction syllabus The alkyne reactions discussed in this lecture are divided into two groups: addition reactions and acid–base reactions. Addition to the carbon–carbon triple bond of an alkyne closely parallels addition to the carbon–carbon dou- ble bond of an alkene in that, in both cases, the multiple bond is degraded and new atoms add to the carbon atoms that comprised the original multiple bond. Hydrogenation In the presence of metal catalysts such as platinum (Pt) or palladium (Pd), the atoms of the hydrogen (H 2 ) molecule add across the carbon–carbon triple bond of an alkyne. The catalyst surface is first dosed with H 2 , which disso- ciates into surface-adsorbed H atoms: H catalyst surface H H H H––H ! H•(ads) + H•(ads) where the H·(ads) denotes a surface-adsorbed hydrogen radical. One of the carbon–carbon " bonds of the alkyne undergoes a similar dissociative adsorp- tion to form a surface-adsorbed alkene diradical: C C C C C C C C (ads) an alkene diradical
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Alkynes • Reaction syllabus © Bruno I. Rubio 2 The surface-adsorbed hydrogen radicals encounter the surface-adsorbed alkene diradical, the unpaired electrons team up, and the alkene product pulls away from the surface: H H H C H C C C C C H H (ads) + 2 H•(ads) C C If an excess of H 2 is available, the alkene itself undergoes hydrogenation by a mechanism that is described in a previous lecture. The final product is an alkane: C C H H C C H 2 cat H 2 cat C C H H H H Because the reaction takes place on the catalyst surface, both H atoms are delivered to the same side of the original carbon–carbon triple bond, that is, hydrogenation is a syn or cis addition. Hydrogenation of an alkyne to a cis -alkene is normally conducted in the pres- ence of a special catalyst called the Lindlar catalyst, which is Pd catalyst that has been deactivated (poisoned) by a mixture of a tertiary amine and Pb 2+ . The Lindlar catalyst is active enough to convert an alkyne to a cis - alkene, but it is not active enough to convert a cis -alkene to an alkane even if excess H 2 is present. Thus, hydrogenation of an alkyne over the Lindlar catalyst stops at the cis -alkene stage. Examples of hydrogenation of alkynes H 3 C CH 3 excess H 2 Ni H 3 C CH 3 CH 3 (77%)
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Alkynes • Reaction syllabus © Bruno I. Rubio 3 OH H 2 Lindlar OH (90–95%) H 2 Lindlar (87%) H 3 C CH 3 H H CH 3 H 3 C Dissolving-metal reduction Hydrogenation of an alkyne over the Lindlar catalyst produces a cis -alkene. A trans -alkene is the product of the reaction of an alkyne with Group IA metals (e.g., Li, Na) in liquid ammonia (NH 3 (l)), for example: H 3 C CH 3 H 3 C + 2 Li + 2 NH 3 CH 3 + 2 LiNH 2 The first steps of the reaction mechanism feature the transfer of an electron
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l18 - CH 203 O R G A N I C C H E M I S T R Y I Alkynes:...

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