Yesterday we looked at how alkenes can react as nucleophiles. The reasoning behind this activity is due to the fact that the pi electrons are not trapped between the nuclei, as are electrons in a sigma bond, but are out in a more open space (remember--side to side overlap of the fat parts of p-orbitals). As a result, the pi electrons can act somewhat like a lone pair of electrons.
The reaction that we looked at was the reaction of alkenes with HBr. It is a two-step process, leading to an intermediate cation which is then attacked by the generated bromide ion. The entire process is just the reverse of an E1 reaction.
The regiochemistry of the reaction is governed by Markovnikov's Rule -- the cation formed is the more stable of the two possibilities. Note that the intermediate cation has an unhybridized p-orbital so the bromide can attack from either side; this can have implications for stereochemistry.
Problem Set 7 and its key have been added to the list of useful links. At this point, we have only covered how to do reaction series (a).
The picture is of Vladimir Markovnikov.
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