[BeepoGirl]

Reactions of Alkenes:

Bromination is aparently trans selective.
Hydrogenation is cis selective.
Oxidation using osmium tetroxide with water is also cis.

What exactly does this mean?
Does this mean that bromination can only occur when the alkene in question is trans, and the others vice versa?

Does it mean the resulting product is cis or trans? - if this is the case then surely because a single bond is produced the bond can rotate so there is no geometrical differentiation between the 2 isomers

Or does it only apply for ring structures so the single bond can't rotate?

Thanks for your help.

[sjb]

Reactions of Alkenes:

Bromination is aparently trans selective.
Hydrogenation is cis selective.
Oxidation using osmium tetroxide with water is also cis.

What exactly does this mean?
Does this mean that bromination can only occur when the alkene in question is trans, and the others vice versa?

Does it mean the resulting product is cis or trans? - if this is the case then surely because a single bond is produced the bond can rotate so there is no geometrical differentiation between the 2 isomers

Or does it only apply for ring structures so the single bond can't rotate?

Thanks for your help.

It's probably best to start with ring structures, but the same does apply for non-rings as well - it's just harder to perhaps visualise to begin with.

Bromination of cyclohexene, for instance will give (1R,2R)-1,2-dibromocyclohexane (and its enantiomer). If you brominate (Z)-hex-3-ene (so in effect removing a carbon-carbon bond from the other side of the ring) you'll get (3R,4R)-3,4-dibromohexane in a similar way.

Similarly, hydrogenation of cyclohexane (with deuterium, just so you can see what's going on) gives (1R,2S)-1,2-dideuteriocyclohexane, or dihydroxylation will give (1R,2S)-1,2-dihydroxycyclohexane

It does not mean that the reaction will only occur with cis or trans olefins.

[Arctic-Nation]

Sjb did a fine job explaining what happens, but some rationalization as to why one reagent is cis- and the other trans-selective might be useful.

In the case of bromination, the first step is the formation of a cyclic bromonium ion (google it if you don't understand what it is) by nucleophilic attack of the double bond on the bromine molecule. As bromine is quite a large atom, the bromide anion is forced to attack the bromonium ion on its C-C side, not one of the C-Br sides. This addition can be considered to happen trans, and the resulting product, if looked at from the side, will have one bromine atom pointing upwards and one downwards.

Hydrogenation is a bit easier, in that the reactant will interact with the catalyst (palladium-on-carbon or something related), bonding with it, and presenting only one side to it. This means that the hydrogen atoms can only 'hop onto' the substrate on one side of the double bond. Once released from the catalyst, both hydrogen atoms on the (now) product will point in the same direction, in a cis manner.

Osmium tetroxide is even more easy: the oxidizer binds to the alkene forming a five membered ring intermediate, and that means that transfer of oxygen to carbon can only happen in a cis manner. Both oxygen atoms are already bonded to the same side of the alkene in the first step of the reaction, and aren't able to move to opposite sides after that.

If you'd like some drawings to go with that, just ask.

[sjb]

True, I just focussed on one part of the query here. Thanks for picking up the slack!

[BeepoGirl]

Thanks a lot, that was really helpful!