Morris S. Kharasch

Morris Selig Kharasch (August 24, 1895 – October 9, 1957) was a pioneering organic chemist best known for his work with free radical additions and polymerizations.

When World War II began, the US government recognized the need for a synthetic rubber and employed the best chemists around to aid in this effort.

In 1869, a Russian chemist named Vladimir Markovnikov demonstrated that the addition of HBr to alkenes usually but not always resulted in a specific orientation.

Markovnikov's rule, which stems from these observations, states that in the addition of HBr or another hydrogen halide to an alkene, the acidic proton will add to the less substituted carbon of the double bond.

Elsewhere in the literature, other examples of anti-Markovnikov additions were observed by Whitmore and Homeyer as well as Sherril, Mayer and Walter, all of whom rejected Kharasch's conclusions.

When allyl bromide reacted with HBr in vacuo (in the absence of air or other oxygen source), the average reaction time took about 10 days with an approximate yield of 88%, the majority of which was the expected (according to Markovnikov's rule) 1,2-dibromopropane (65-85%).

In contrast, when the reaction was run in the presence of air or oxygen, it lasted a markedly shorter time (with great variation), in one case only taking one hour to reach completion.

[1] Because other experimenters had reported anti-Markovnikov products and had attributed them to other factors, Kharasch addressed several variables to see if they also had an effect on the orientation of HBr addition to allyl bromide.

From this continued research, industrial polymerization reactions of unsaturated hydrocarbons were discovered and mass production of synthetic rubber and plastics was possible.

While standard conditions generally support one orientation of addition, in some cases it may be advantageous to have the halide on the less highly substituted carbon, in the anti-Markovnikov position.

In this case a free radical addition step may be the key to obtaining the desired ultimate product, and is possible because of the work of Morris Kharasch.