Todd:Chem3x11 ToddL10
Chem3x11 Lecture 10
Being constructed Sat May 26
This lecture is about additions to cyclohexenes again, specifically how to add water the wrong way round, and how to convert a double bond to a diol.
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Key concepts
- Addition of water to a double bond can be made to be anti-Markovnikov through the use of mercury or boron reagents
- A double bond can be converted to a diol using an osmium reagent.
Oxymercuration
Mercury diacetate will add to double bonds in a way not dissimilar to bromine addition. The mercurinium ion intermediate is like the bromonium ion we saw earlier, and can be opened by water. If we are using a familiar cycohexene ring, we will see the trans diaxial product being favoured. If the alkene is not symmetrically substituted, the more substituted position is attacked by water (as we say with bromonium ions and acid-catalysed epoxide opening) since that position is best able to stabilise a developing partial positive charge. The C-Hg bond in the intermediate product can be converted to a C-H bond with a reducing agent. If we employ NaBD4 as the reducing agent, we can observe that the reduction goes with retention of stereochemistry. You don't need to know why. But you should ask yourself why.
So the initial addition will be trans, given the nature of the intermediate. The overall addition follows Markovnikov because the OH ends up on the more hindered position. You should therefore ask yourself an obvious question.
The stereochemistry of the addition is something that should by now be quite simple, given that we've seen such similar cases with bromination and epoxide opening. Here is, in any case, an example that you can check for corectness.
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