Monday, February 12, 2007

Markovnikov's rule

I was reading an essay about named things and saw this picture. (The Road to Chemical Names and Eponyms: Discovery, Priority, and Credit, Angew. Chem. int. ed., 2004, 43, 5888)
Figure 3. V.V. Markovnikov (1838-1904). He
would hardly recognize the rule now bearing
his name.

In the text there is no reference to this Figure 3, but I thought it was probably a nice example of how things become named to people who had just 'a role in it'.

We all know Markovnikov's rule:

In elctrophillic additions of H-X to alkenes the hydrogen ends up attached to the carbon of the double bond that had more hydrogens to start with. (Clayden, Greeves, Warren and Wothers' textbook).

Nowadays we learn the reason behind this observation, which is far more important. Markovnikov's rule is not a law.

Now I wanted to know how Markovnikov formulated this rule himself so I searched for his paper and found it quite easily. The rule can be found in an addendum of a paper on isomeric buturic acid. (Ueber die Abhängigkeit der verschiedenen Vertretbarkeit des Radicalwasserstoffs in den isomeren Buttersäuren, Annalen der Chemie und Pharmacie, 1870, 153, 2, 228)

He starts to say : 'I cannot here enter into a detailed examination of the various facts that alows us to postulate this rule rule.'

Then he says (in italics): 'When an unsymmetrical alkene combines with a hydrohalic acid, the halogen adds on to the carbon atom containing the fewer hydrogen atoms, that is the carbon that is more under the influence of other carbons.'

Markovnikov said exactly the same thing as the Clayden book does. Why would Markovnikov hardly recognize the rule now bearing his name?? The only reason I can think why he would not recognize it now is because he is dead!!!

Markovnikov 's paper is quite sloppy. It contains several errors and it is not obvious which chemistry was done by himself. It is obvious he relied on the work of people like Butlerov.It took about 60 years before his rule became well known because of the work done by Maas (1924) and later Kharasch (1933). In Markovnikov's time stereochemistry was about to be born, nobody saw the important addendum in his paper, but he formulated the rule himself, Prof. Pedro can deny it, but it is true.

2 comments:

Russ said...

Perhaps the statement was referring more to the reasons behind the rule?

I usually teach Markovinkov's rule as the product arising from the most stable carbocationic intermediate (rather than oversimplifying it to counting C's and H's).

Even still, I doubt that he 'would hardly recognize' the rule, given a very brief primer on carbocations (which I don't think would have existed back in his day...)

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