[Organic Chemistry 1] If a reaction involves alkene reactants and the reaction generates a molecule with 2 chiral centers, why do certain reactions favour syn vs. anti vs. a mixture of both?

[u/throw-away3105]

I'm reading David Klein's textbook and I'm making more detailed notes about the reaction mechanisms of alkene reactants. But it doesn't really go over the details of WHY some reactions favour the syn vs. a mixture. I know how to draw reaction mechanisms and all that.

I know the anti additions occur because an SN2 backside attack happens (ex: halogenation, halohydrin formation).

Examples:
1.) Catalytic hydrogenation favours syn addition like in this reaction: 1-ethyl-2-methylcyclohex-1-ene + H₂ yields a molecule with two chiral centers.
Theoretically, there should be 4 stereoisomer products but in reality, there are only two.

2.) Hydrohalogenation favours both syn and anti additions. If a reaction produces a molecule with two chiral centers, then there will be 4 stereoisomer products (if it's not a meso compound): (R, R), (S, S), (R, S), and (S, R).

So why the difference between, say, catalytic hydrogenation and hydrohalogenation? So far, I just memorized which reactions are syn vs. anti vs. mixture but I never really understood why they are the way they are.

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[u/veryjewygranola]

Catalytic hydrogenation is syn because you actually have alkene and hydrogens lying flat on the catalyst surface: the only available bonding sites are on the same side of the original double bond. This diagram is helpful here.

Hydrohalogenation goes through a carbocation intermediate which is sp2, so you have trigonal planar geometry and the nucelophile can attack from either side, leading to equal parts of syn and anti addition

[u/throw-away3105 → u/veryjewygranola]

Thank you so much for the detailed reply! I appreciate the diagram, too!