Help needed with Markovnikov's "Law"

[george huber]

Eleusis (ele...@netcom.com) wrote:

: Alright... Markovnikov said, take an electrophilic compound and react it
: with a bunch of chained carbons, the anion of the compound will attach to
: the carbon with the *least* number of hydrogens, correct?

: So, theoretically speaking, the following would be true (ignoring
: regiochemistry effects if they are more predominant):

: /\ // /\ /
: / \ // + CrO3/H2SO4 ---> / \ /
: / \// [Jone's Reagent] / \/
: ||
: ||
: O

Actually, Markovnikov original law was used to explain the orientation of
addition of HBr to an alkene (in the absence of peroxides). A good paraphrase
of his law is that the carbon with the most hydrogens gets the incoming
hydrogen from HBr. Now, we currently understand Markovnikov's rule to be
based on carbocation stability. When an electophile (Br- in the case of HBr,
however it could be anything - for example BH3 is the electrophilic species in
the hydroboration/oxidation reaction) attacks the double bond of an
unsymetrical alkene two possiple carbocations are formed. The carbocation
that is the stablest will be the one that predominates. The resulting
carbocation is then attacked by any nucleophile that is in solution to give
the final product.

Now in the above reaction, there is no electrophilic species so addition
to the alkene as shown will not occure. However, there is a synthetic method
where an alkene can be oxidized by strong oxidents (eg MgO4) to yield a
vinicial diol. Furthremore if the conditions are strong enough it is possible
to oxidativley cleave a vinicial diol the either acids or aldehydes. This is
a classic proceedure in sugar chemistry where a 1,2-diol is treated with
something like lead acetate or periodate to cleave the carbon-carbon bond.

George Huber
Boston University Chemistry Department
geoh...@chem.bu.edu

[Dr. Pwatt will see you now....]

In article <43q4es$9kk...@netcom.com> ele...@netcom.com (Eleusis) writes:
>
>Alright... Markovnikov said, take an electrophilic compound and react it
>with a bunch of chained carbons, the anion of the compound will attach to
>the carbon with the *least* number of hydrogens, correct?
>

As I learned it, Markovnikov's Law stated that the addition would
proceed such that the most stable carbocationic intermediate
could be formed, rather than simply adding to a carbon with so
many hydrogens.

8)