Syn Addition H2/Pt: Kaplan Wrong?

[gabe85]

According to Kaplan H2/Pt syn addition to an alkene produces a racemic mixture of both products. They say that the hydrogens can attack from either side. However, according to McMurry H2/Pt produces only one product because both hydrogens add to the double bond from the catalyst's surface. Also, the double bond approaches the catalyst from the more accessible (less steric) side, producing one product.

Which will occur, racemic mixture or one product. McMurry seems to be more of a plausible answer because the H2 is absorbed on the catalyst surface, and the alkene adjusts accordingly to its substituents.

Any ideas?

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[yuppers]

I agree with McMurry. Since there is a double bond there is no rotation so the cataylst will procede on the less hindered side.
However, the hydrogens can attack on the more hindered side creating the other product.

 

[Shjfr]

According to my KAPLAN Blue Book for this reaction (H2, Pd and alkene):

"The reaction takes place on the surface of the metal. One face of the double bond is coordinated to the metal surface, and thus the two hydrogen atoms are added to the same face of the double bond. This type of addition is syn addition."

Maybe you are using a different Kaplan book?

 

[krajapak]

From what i researched Catalytic hydrogenation is sterospecific. Both of the Hydrogens are added to the same side (syn). Building on this:
1. in the absence of steric hinderance you can add to either side of the pi bond, giving an equal ratio of enantiomers= resulting in a racemic mixture.
2. with steric hinderance esp ( if you have two bulky things in cis orientation) you will get one product.