Imagine the International Baccalaureate started in the nineteenth century and that it is now 1872. You are an IB student looking for a research question for your internal assessment. You have just read a paper by a Russian chemist called Vladimir Markovnikov published two years earlier which states that when hydrogen halides add to an asymmetric alkene the hydrogen atom bonds to the carbon atom that already contains the most hydrogen atoms. For example, when hydrogen bromide adds to propene the major product is 2-bromopropane with 1-bromopropane being the minor product. You decide to investigate how the solvent used alters the ratio of the two products. You perform your experiments and like a good IB student record all the uncertainties associated with your measurements and work out the total uncertainty for the ratio you obtain. You then repeat your experiments and get a very different result each time and in fact in some cases the ratio reverses so that 1-bromopropane is the major product. The results clearly lie way outside the possible limits associated with your calculated total uncertainty.

This scenario mimics exactly what did happen after Markovnikov published his paper. Chemists that tried to replicate his results found that sometimes his prediction worked and other times the minor product became the major product. This became known as anti-Markovnikov addition. It was thought that the solvent influenced the outcome but no one could actually achieve consistent results to explain this.

Professor Morris Kharash (Image from University of Chicago)

In the 1930s an American chemist, Morris Kharash, published a paper on research he had done with adding hydrogen bromide to allyl bromide (3-bromopropene), CH₂BrCH=CH₂. He showed that in the complete absence of oxygen or air the reaction took an average of ten days and yielded 88% of the expected Markovnikov addition product 2,3-dibromopropane. However if air or oxygen was present, even in only small amounts, the reaction proceeded much faster (often only hours) and on average produced 87% of the anti-Markovnikov product, 1,3-dibromopropane.

An IB student should be able to deduce at least two conclusions from this. Firstly, the emphasis that the IB places on uncertainties of measurements in the physical data may well be trivial compared to the conditions under which the experiment is performed. Students often do not comment upon the conditions used in their IA evaluation and yet these can have a profound effect on the results.  Secondly, different mechanisms may be happening which could explain the difference in the nature of the products formed.

Markovnikov addition assumes an electrophilic addition mechanism and is explained by the relative stabilities of the possible intermediate carbocations formed. When even small amounts of air or oxygen are present peroxides may be formed. The weak O−O bond can then break homolytically to form free radicals and the mechanism will proceeds via free radical addition.

Free radical intermediates are very reactive and cannot easily be isolated. However strong evidence for this mechanism is provided by the fact that adding a peroxide catalyst makes the reaction even faster whereas if free-radical inhibitors are added the anti-Markovnikov product is not formed as the reaction then has to proceed by an electrophilic addition mechanism.

The current IB Chemistry guide actually states “Markovnikov’s rule can be applied to predict the major product in electrophilic addition reactions of unsymmetrical alkenes with hydrogen halides and interhalogens.” This is a correct statement but questions in examination usually ask what the product will be without clarifying what the conditions are, i.e. which mechanism is involved. If the reaction is done in a school laboratory (where it is very difficult to remove air and or oxygen altogether) it will be the anti-Markovnikov product that will be the major product.  This could be an interesting TOK/NOS point to bring in for the next programme when single electron sharing reactions and electron-pair reactions are both on the syllabus under R3 What are the mechanisms of chemical change.