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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. He defined the peroxide effect, explaining how an anti-Markovnikov orientation could be achieved via free radical addition. Kharasch was born in the Russian Empire in 1895 and immigrated to the United States at the age of 13. In 1919, he completed his Ph.D. in chemistry at the University of Chicago and spent most of his professional career there. Most of his research in the 1920s focused on organo-mercuric derivatives. He synthesized an important anti-microbial alkyl mercuric sulfur compound, thimerosal,〔 "Alkyl Mercuric Sulphur Compound"〕 commercially known as Merthiolate, which he patented in 1928 and assigned to the pharmaceutical company Eli Lilly and Company. Merthiolate was introduced as a vaccine preservative in 1931, and by the late 1980s thimerosal was used in all whole-cell DPT vaccines. Nobel laureate Herbert C. Brown was one of his students during the 1930s. When World War II began, the US government recognized the need for a synthetic rubber and employed the best chemists around the nation to aid in this effort. In 1942, Kharasch joined the American Synthetic Rubber Research Program and applied his knowledge of radical reactions to aid in the polymerization of synthetic styrene. In his later years, Kharasch devoted his attention to studying the Grignard reaction and in 1954 co-authored a book with O. Reinmuth entitled ''Grignard Reactions of Nonmetallic Substances''. == Proposal for anti-Markovnikov addition: The peroxide effect == 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.〔Wade, L.G. Organic Chemistry. Ed 5. Prentice Hall: 2003. 314-20.〕 This directed addition of a proton results in the more thermodynamically stable carbocation intermediate, as determined by degrees of substitution; more highly substituted carbocations are stabilized by the electron-pushing inductive effect of the surrounding carbon molecules. Kharasch, in his seminal 1933 paper entitled "The Addition of Hydrogen Bromide to Allyl Bromide", proposed that the anti-Markovnikov addition of HBr to allyl bromide to yield 1,3-dibromopropane was due to the presence of peroxides. He termed this the "peroxide effect", which he proposed proceeds through a free radical chain reaction addition. 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. They instead argued that the direction in which the reaction proceeds is determined not by the presence or absence of peroxides, but by the nature of the solvent in which the reaction is taking place. In this paper, Kharasch analyzed one at a time the effects of temperature, solvent, and light on the direction in which the reaction proceeded. He concluded that the presence of peroxides was the driving force for anti-Markovnikov addition and that any changes in temperature, solvent, or light affected the orientation of addition only through the chemistry of the peroxides. Once Kharasch began determining the dibromopropane compositions of the products under various conditions, he made a startling discovery. 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. More importantly, however, is that the major product of these additions was the 1,3-dibromopropane, constituting approximately 87% of the product. Since the only apparent variable that had changed was the presence of oxygen (other gases found in air were tested individually and did not show the same effect), Kharasch hypothesized that the rapid anti-Markovnikov addition of HBr to allyl bromide was the result of trace amounts of peroxide in the reaction mixture that could have resulted from the interaction of molecular oxygen in its diradical triplet state and allyl bromide to form allyl bromide peroxide . From there, the weak peroxide O-O bond (~51 kcal/mol)(3) could be cleaved by incident light, causing homolytic cleavage and creating the peroxide radical. Even trace amounts of this allyl bromide peroxide radical would then be sufficient to begin a chain reaction whereby a hydrogen atom would be abstracted from the HBr, leaving a Br radical. This Br radical would then combine with an electron from the double bond of allyl bromide at the less-highly substituted carbon, giving the more stable 2o radical. Reaction of this radical with another HBr molecule would cause the abstraction of another H molecule and would complete the anti-Markovnikov addition. Since the Br radical is regenerated, the reaction would continue to proceed at a fairly quick pace until the reactants were exhausted and/or the radical species were terminated.〔 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Morris S. Kharasch」の詳細全文を読む スポンサード リンク
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