Chemistry
Markovnikov Rule
The Markovnikov Rule is a principle in organic chemistry that states that when a hydrogen halide (HX) is added to an unsymmetrical alkene, the hydrogen atom of HX attaches to the carbon atom that has the most hydrogen atoms already attached to it. This rule helps predict the outcome of reactions involving unsymmetrical alkenes and hydrogen halides.
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3 Key excerpts on "Markovnikov Rule"
eBook - ePubAddition, Elimination and Substitution: Markovnikov, Hofmann, Zaitsev and Walden
Discovery and Development
- David E. Lewis(Author)
- 2022(Publication Date)
- Elsevier(Publisher)
89 and other major textbooks of this era). Since its rewriting in terms of the protonation-first mechanism, the origins of the regiochemistry of electrophilic additions to alkenes have continued to intrigue chemists. In May 2020, there were over 4000 hits in a Google Scholar search for “Markovnikov's Rule,” and 158 explicit and over 30,000 concept references returned by SciFinder Scholar.In 1949, Batty and B. C. L. Weedon (Fig. 3.5 ) were first to use the term “anti-Markownikoff” to replace the term “abnormal,” when describing addition regiochemistry.90In many ways, this has been an unfortunate development because the term implies that the addition does not obey Markovnikov's Rule. The term may have been an oddity initially, but it moved squarely into the mainstream when H. C. Brown (Fig. 3.5 ) used it to describe the regiochemistry of alkene hydration by the hydroboration–oxidation sequence.91With so many chemists using the method because of its regio- and stereospecificity, it was to be expected that Brown's usage would prevail and it has.From a pedagogical standpoint, the term “anti-Markovnikov” has long been a source of confusion for beginning students in organic chemistry. This stems—in part, at least—from students now being taught the rule from a mechanistic perspective. Early in the course, however, students are often not ready to appreciate the changes in the course of the reaction when the mechanism changes. Additions that do not fall neatly under the electrophilic addition mechanistic purview (e.g., the addition of HBr under free-radical conditions or the two-stage hydroboration-oxidation) lead to confusion.
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2017(Publication Date)
- Wiley(Publisher)
344 CHAPTER 8 ALKENES AND ALKYNES II: Addition Reactions 8.2B General Statement of Markovnikov’s Rule With this understanding of the mechanism for the ionic addition of hydrogen halides to alkenes, we can now generalize about how electrophiles add to alkenes. • General statement of Markovnikov’s rule: In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate. • Addition of the electrophile determines the overall orientation of the addition, because it occurs first (before the addition of the nucleophilic portion of the adding reagent). Notice that this formulation of Markovnikov’s rule allows us to predict the outcome of the addition of a reagent such as ICl. Because of the greater electronegativity of chlorine, the positive portion of this molecule is iodine. The addition of ICl to 2-methylpropene takes place in the following way and produces 2-chloro-1-iodo-2-methylpropane: H 3 C I I I Cl Cl Cl − C C + CH 2 CH 2 + CH 3 CH 2 CH 3 C 2-Methylpropene 2-Chloro-1-iodo- 2-methylpropane H 3 C H 3 C H 3 C δ+ δ− PRACTICE PROBLEM 8.1 Give the structure and name of the product that would be obtained from the ionic addi- tion of IBr to propene. PRACTICE PROBLEM 8.2 Write mechanisms for the following addition reactions: (a) HBr (b) ICl (c) HI PRACTICE PROBLEM 8.3 Write a mechanism that accounts for the formation of ethyl isopropyl ether as one of the products in the following reaction. HCl EtOH Cl OEt + 8.2C Regioselective Reactions Chemists describe reactions like the Markovnikov additions of hydrogen halides to alkenes as being regioselective. Regio comes from the Latin word regionem meaning direction. • A regioselective reaction is one that can potentially yield two or more constitutional isomers but that actually produces only one (or a predominance of one).
eBook - PDF- T. W. Graham Solomons, Craig B. Fryhle, Scott A. Snyder(Authors)
- 2016(Publication Date)
- Wiley(Publisher)
344 CHAPTER 8 ALKENES AND ALKYNES II: Addition Reactions 8.2B General Statement of Markovnikov’s Rule With this understanding of the mechanism for the ionic addition of hydrogen halides to alkenes, we can now generalize about how electrophiles add to alkenes. • General statement of Markovnikov’s rule: In the ionic addition of an unsymmetrical reagent to a double bond, the positive portion of the adding reagent attaches itself to a carbon atom of the double bond so as to yield the more stable carbocation as an intermediate. • Addition of the electrophile determines the overall orientation of the addition, because it occurs first (before the addition of the nucleophilic portion of the adding reagent). Notice that this formulation of Markovnikov’s rule allows us to predict the outcome of the addition of a reagent such as ICl. Because of the greater electronegativity of chlorine, the positive portion of this molecule is iodine. The addition of ICl to 2-methylpropene takes place in the following way and produces 2-chloro-1-iodo-2-methylpropane: H 3 C I I I Cl Cl Cl - C C + CH 2 CH 2 + CH 3 CH 2 CH 3 C 2-Methylpropene 2-Chloro-1-iodo- 2-methylpropane H 3 C H 3 C H 3 C δ+ δ- PRACTICE PROBLEM 8.1 Give the structure and name of the product that would be obtained from the ionic addi- tion of IBr to propene. PRACTICE PROBLEM 8.2 Write mechanisms for the following addition reactions: (a) HBr (b) ICl (c) HI PRACTICE PROBLEM 8.3 Provide mechanistic explanations for the following observations: (a) HCl Cl Cl + (b) HCl + Cl Cl 8.2C Regioselective Reactions Chemists describe reactions like the Markovnikov additions of hydrogen halides to alkenes as being regioselective. Regio comes from the Latin word regionem meaning direction. • A regioselective reaction is one that can potentially yield two or more constitutional isomers but that actually produces only one (or a predominance of one).
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