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When my textbook talks about hydrogenation using Pt, Pd or Ni heterogeneous catalysts, it never mentions if it is anti or syn addition. It simply jumps on to say that for alkynes, NiB2 (P-2) catalysts and Lindlar's catalysts are syn hydrogenation. Can someone tell me if using Pt gives syn or anti for alkynes?

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Using any transition metal catalyst (Pt, Pd, Ni) for hydrogenation of alkynes results in syn hydrogenation to not alkenes but alkanes. Using Lindlar's catalyst results in syn hydrogenation of an alkyne to an alkene.

In the case of sodium metal (not a transition metal) in ammonia, hydrogenation of alkynes results in a trans alkene.

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Pt gives you syn addition of hydrogens to alkynes, but Pt without anything else (Lindlar's catalyst) will hydrogenate your alkyne to an alkane. –  Dissenter Aug 18 at 1:33
    
Really!? So, if I write a reaction of CH3CCCH3 --> CH3CH=CHCH3 ( H2,Pt) I am actually wrong because that will directly undergo another hydrogenation? My whole life has been a lie :O –  yolo123 Aug 18 at 1:36
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Ok perfect. Now I get the whole point of using Lindlar's or the NiB2 (P-2). GREAT HELP AS USUAL. –  yolo123 Aug 18 at 1:42
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One final comment, some metals (Na\NH3 - it's not a transition metal, but still...) produce pure trans hydrogenation of an alkyne. –  ron Aug 18 at 2:09
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You should edit in any information that is in addition to your original answer and then remove the comments. It is now really hard to follow. I even do not see any real relationship to the original question. It seems like the answer is only hidden in the comments. In general it should be avoided to discuss anything in the comments. –  Martin Aug 18 at 2:34

All catalytic hydrogenations involving solid metal catalysts transfer hydrogen in a syn fashion to the alkyne to produce the corresponding cis-olefin. The following figure provides a mechanism that illustrates why the

enter image description here

addition must be syn.

But, with alkynes there are problems. First, reduction of the resultant alkene is faster than reduction of the starting alkyne, so product mixtures of alkenes and alkanes will result unless poisoned catalysts (Lindlar catalyst) are used or the reaction is stopped early. A second problem often observed is that the resultant alkene usually remains adsorbed on the catalyst surface. Consequently cis-trans isomerization of the alkene can take place leading to further contamination of the initial cis-product.

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"First, reduction of the resultant alkene is faster than reduction of the starting alkyne". Why? –  yolo123 Aug 18 at 1:46
    
@yolo123 the first pi bond to be reduced in an alkyne is stronger than the second pi (alkene) bond, so once the strong (alkyne) bond is reduced, the second pi bond goes quick. –  ron Aug 18 at 1:56
    
By reduced, you mean hydrogenated? By the way, oxidizing something is equivalent to dehydrogenating? And oxylation= adding oxygen atom? Sorry, I'm just orgo newbie! –  yolo123 Aug 18 at 1:59
    
1) "By reduced, you mean hydrogenated?" Yes; 2) Yes, dehydrogenation is an example of oxidation. –  ron Aug 18 at 2:01
    
I would expect that the coordination of the alkyne is the rate determining step. After that the alkyne would most likely already be coordinated and therefore be further reduced. –  Martin Aug 18 at 2:38

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