Markownikoff's Rule: What Did He Say and When Did He Say It? John Tierney Pennsylvania State University, Delaware County Campus, Media, PA 19063 One of the simplest and most fundamental rules in organic chemistry, Markownikoffs rule, appears in a number of different versions in just about every organic text (1-11) that has been published over the past three decades. Further intrigue is added by the varied spelling of Markownikoffs name and from the date that he initially proposed his rule which ranges from 1868 (1) to 1905 (11). Some authors (1214)..olavsafe hveivine aversion of the rule and no . vear.. while others, invariibiy mire advanced texts (15), mention the rule without statine it. One monoaranh (16) in organic chemistry chose to ignore the rule complktely and a more recent text (17) states the rule in terms of carbocation stabilities. Markownikoff (18)actually wrote in Annalen d e r Chemie, in 1870, the following:
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. wenn ein unsymmetriseh eonstituirter Kohlenwasserstoff sich mit einer Haloidwasserstoffsaiire uerbindet, so addirt sich Kohlenstoffatom. mit dns Haloid an das wenieer " hvdroeenisirte , d. h. 2 u d m Kohlensroff,u,drher rich mchr unlor d m Emflusse nnderer K,.hlenm.ffebefinder. Translated, this statement obviously covers both forms of the rule that follow: (A) When a hydrogen halide adds to an unsymmetric alkene the addition occurs such that the halogen attaches itself to the carbon atom o f the alkene hearing the least numher of hydrogen atoms. (B) When a hydrogen halide adds to an unsymmetric alkene the addition occurs such that the halogen attaches itself to the carbon atom of the alkene hearing the greater number o f carbon atoms.
However, the third form of the rule, shown below, that regularly appears in texts (I, 3.4, 6, 7, 9, 12), though not incorrect, is only obtained by inference from the original statement written in German (shown above). (C) When a hydrogen halide adds to an unsymmetric alkene the addition occurs such that the hydrogen of the hydrogen halide attaches itself to the carhon atom of the alkene bearing the most number of hydrogens.
The snelline of Markownikoffs name has alreadv been set in this akiclesnd is the way i t appeared in the original paper in Annalen. The consistencv in translatine his name from the Cyrillic alphabet into ~ e i m a or~rencLdoes n not appear to create the same diversity that exists in a translation into English (11). This was clearly shown when, five years after his initial statement of what is now known as Markownikoffs rule, he wrote in French (19): Lorsqu'd une molecule non soturee C.HrnXs'ajoute un autre s y s t h e moleculoire YZd une temperature basse, I'G-ment ou le groupe le plus negatif Y se combine avec l'atome de carbone le moins hydrogen&,ou auec celui qui etait dejds en liason directe avee quelque element nhgatif; mais d des temperatures cornparativement plus haute, c'est l'hlement Z, qui se fire sur le eorbone le moins hydrogbn6,c'est-d-direque,pour les memessubstances, In reoctionprend une marehe tout d fait opposee d lapremi2re.
In this modified version of Markownikoff s rule he was addressing the more general problem of the addition of mixed
halogens and hydrogen halides to vinyl alkenes. He had noted that a temperature effect created the possibilities of both regular "Markownikoff' addition at low temperatures and "anti-Markownikoff" addition at high temperatures. The different mechanistic pathways that are available for the addition of hydrogen halides to alkenes were obviously unknown to him a t the time. Saytzeff (20) extended Markownikoffs rule in the same year to include additions of hydrogen halides to alkenes containine two eauallv hvdroaenated carbon atoms where one c o n t a h a mkthyi g r k p . v ~ a y t z e f f smodification said that the haloaen of a haloaen acid would attach itself to the carbon atombf the alkene hearing the methyl group. This has since been interpreted as the halogen adding to the carbon atom of the alkene with the shorter chain. Exceptions to Markownikoffs rule were discovered almost as soon as the rule had been formulated (19. . . 21-23):.. however, Markownikoff considered these miscellaneous alkene derivatives to be totallv irrelevant to his rule (24). This is an often-used ploy, even today, when chemists have problems explaining troublesome results. Ipatieff (25) tried to achieve a broader statement to include the nature of the groups .. . linked to the carbon-carbon double bond, the solvent and the renvtion temperature employed,allofwhich play a part inrheregioselecrivity: however, he did not succeed.^ In conclusion, even though there appear to he many chemists todav who have never seen the orieinal form of Markownikoffs rule; i t is interesting that every organic chemistry student is exnosed to one form or another of the rule and that i t has withstood the test of time. The rule is wonderful in its simplicity and predictability, working irrespective of the mechanism of addition, he it ionic (26) or molecular (27). Fortunately, the subiect has crown to the point where the exceptions~canbe readily explained in the-knowledge that the addition may follow different pathways to give the saturated product. Literature Clied
8. Hine, J. Physical Organic Chemirfry, 2nd ed.: MeGrsw-Hill: New York. 1962:p 225. 9. Hendriehosan, J.B.:Crsrn,D. J.; Harnrnond G. S. Organic Chemisfry.3rded.; MeGrswHill: NPWYwk, 1970; p 622. la. Lin~trornberg,W. W.:Bsurngar*n. H. E. orgonir Chemialry-ABlielCouraa;Heath: Lexington, 1987:p. 92. 11. McMurry, J. Orgonic Chemistly: Brooks Cole: Mantomy, CA, 198< p 144. 12. Brown, W. H. Introduction ra Organic Chemialry, 3rd ed.: Grant: Boaton. 1981:p 56. 13. Gould. E. S. Mechanism ond Structure in Oreonic Chemisfrw Holt. Rinehsrf snd
MA. 1965. 17. Kernp. D. 8.; Vellacchio. F. Organic Chemistry: Worth: New York, 1980; p. 261.
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18. 19. 20. 21. 22.
Msrkounikaff.V.V.Ann. 1870,153,256. Markounikaff, V. V. Compl. Rend. 1875.81,670. SsytLeff, A. Ann. 1875,159,324. Wagner, G.; Linnernsn,A.Ann. 1872,163.95. Papn8techer.A. J. Ann. 1879,195,108.
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23. Wisclicemus, J.: Talbaf, H.P.; Herre, W. Ann. 19W. 313,231 24. Khsrasch. M.S. J Chem. Edue. 1931.8.1703.
