Markovnikov's Rule

Abstract

Letters Markovnikov's RuleThe recent contribution by Ilich, Rickertsen, and Becker (1) provides abundant reasons for abandoning the description of addition reactions and their products as "Markovniknov" or (even worse) "anti-Markovnikov". In a recent review (2) I have surveyed the history of this usage and concluded that its use in teaching has long outlived its utility. The deeper understanding of these reactions as involving appropriate reactive intermediates (e.g., carbocations), whose relative stabilities can be understood on a general basis, should have superseded the use of an empirical rule that is often inapplicable, often restated in ways unrecognizable to its originator, and never an adequate substitute for understanding. If we wish students to understand organic reactivity as a whole, we should not provide them with this cognitive crutch. If our use of the description of one product as a "Markovnikov product" is meant to honor a great 19th century chemist, what is the effect of describing another as an "anti-Markovnikov product"? Unfortunately, the authors of the article draw back from the simple conclusion that the rule should be abandoned, but rather attempt once again to restate or reformulate it, while clinging to the name. In agreement with Gooch (3), I urge teachers and textbook authors to move past Markovnikov's rule once and for all. The author replies: Literature CitedThe principal goal of our work published in this Journal (1) has been to show that electrophilic addition reactions to alkenes can be adequately described by the standard parameters of chemical reactivity: charge distribution of the substrate and energy of the reaction-determining intermediate. Through the analysis of six representative addition reactions we also show that the term "anti-Markovnikov" is incorrect and redundant. Dr. Kerber's plea-and the stronger message of his article published in Foundations of Chemistry (2)-is to avoid artificial and non-pedagogical extensions of the Markovnikov rule and phase it out from organic chemistry textbooks. We completely agree with and support Dr. Kerber's call for making teaching and learning of second-year organic chemistry more accurate, more effective, and less difficult.However, beyond undergraduate chemistry courses and textbooks there is a vast and very important world of practicing organic chemists. They keep using the "Markovnikov terminology" as useful shorthand for classification of products in an ever-increasing number of what seem to be electrophilic addition reactions. We posit that the use of this terminology is often incorrect. Our extensive analysis (submitted for publication) of the energy-charge profile of a number of reactions reported as "anti-Markovnikov" suggest that they follow mechanisms different from those of the reactions originally described some 140 years ago by Vladimir Markovnikov. However, we also feel that more insight into the mechanisms of addition reactions to alkenes is needed before requesting radical changes in the organic chemistry terminology...

Towards the 150th Anniversary of the Markovnikov Rule

“…However, chemists of the 21st Century have a much better understanding of what is going on in addition reactions, so keeping Markovnikov's rule in the role of an actual rule—that is of a statement which can be directly applied to predict the outcome of a given reaction—cannot be justified. Is it now just a sentimental tribute to famous figures of the past? Moreover, even in terms of electrophilic additions to alkenes, the options are wider than just following the stability of carbocations, and a more competent picture can only be drawn through the inclusion of quantum chemical modeling of reaction pathways …”

Organic Chemistry and Synthesis Rely More and More upon Catalysts

“…Similarly, tertiary systems are more stable than their secondary isomers [51]. The formulation of Markovnikov's rule as a kinetic control is not always valid [52][53][54][55]. Additions to alkenes are exothermic but have negative entropies (condensations) that can cause the reactions to be reversible (with standard Gibbs energy variation ∆ r G T = ±1 kcal/mol).…”

Untitled

“…This feud lasted until Markovnikov's death. The possible origins of this feud have been discussed, 39 but the question of why the feud arose is still open to interpretation. 40…”