Markovnikovs Rule Explained

To explain Markovnikov s Rule, let us consider a specific example, the addition of H—Cl to propene. The first step is addition of a proton to the double bond. This can occur in two ways, to give either an isopropyl cation or a propyl cation. 

At this stage of the reaction, the structure of the product is already determined-, when combining with chloride ion, the isopropyl cation can give only 2-chloropropane, and the propyl cation can give only 1-chloropropane. The only observed product is 2-chloropropane, so we must conclude that the proton adds to C-1 to form only the isopropyl cation. Why  

Carbocations can be classified as tertiary, secondary, or primary, depending on whether the positive carbon atom has attached to it three organic groups, two groups. 

Alkyl groups stabilize carbocations by donating electron density from C—H and C—C sigma bonds that can line up with the empty p orbital on the positively charged carbon atom. 

Markovnikov s Rule can now be restated in modern and more generally useful terms The electrophilic addition of an unsymmetric reagent to an unsymmetric double bond proceeds in such a way as to involve the most stable carbocation. 

---

Exceptions to the Markovnikov rule when hydrogen bromide reacts with unsym-metric alkenes have long been known.117,118 The reaction for this anti-Markovnikov addition was explained as being a chain reaction with the involvement of bromine atoms influenced by the presence of peroxides.119-121 Both added peroxides and peroxides formed by the action of oxygen (air) on the alkene are effective. 

The Chemistry of Vision Addition and Substitution Reactions Compared Polar Addition Reactions Addition of Unsymmetric Reagents to Unsymmetric Alkenes Markovnikov s Rule Mechanism of Electrophilic Addition to Alkenes Markovnikov s Rule Explained Reaction Equilibrium What Makes a Reaction Go ... 

This regioselectivity was noted by Vladimir Markovnikov who made the generalization known as Markovnikov s rule in the addition of H—X to an alkene, hydrogen adds to the double-bonded carbon that has the greater number of hydrogens already bonded to it. Although MarkovnikoVs rule provides a way to predict the products of many alkene addition reactions, it does not explain why one product predominates over other possible products. 

We can see from these examples that, if the carbon atoms participating in the double bond are not equally substituted, the proton from the hydrogen halide attaches itself to the less substituted carbon. As a consequence, the halogen ends np at the more snbstituted carbon. This phenomenon, referred to as the Markovnikov rule, can be explained by what we know about the mechanism of electrophile additions of protons to alkenes. The key is the relative stability of the resulting carbocation intermediates. 

Would you expect addition of HCl to the double bond of 3-buten-2-one (shown below) to follow Markovnikov s rule Explain your answer by a mechanistic argument. 

Sir Robert Robinson played an early central role in the development of the electronic theoiy of organic chemistry (5S). For example, he was the first to use the now commonplace curly arrow to imply the reorganization of electron density during the course of a chemical reaction (59). The constract of stereoelectronic control (which, in its most literal sense, explains all chemical reactions) underpins many concepts now in the lexicon of mechanistic and synthetic organic chemistry. These include the Hammond postulate, the Curtin-Hammett Principle, the Markovnikov rule (for additions to alkenes), the Thorpe-lngold effect (on rates of cyclization), the Btirgi-Dunitz approach trajectory, Cram/Comforth/Felkin-Ahn controlled additions (to chiral ketones... 

Markovnikov s rule can be restated by saying that, in the addition of HX to an aikene, the more stable carbocation intermediate is formed. This result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an aikene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formation. 

Consequently, 2-bromopropane, rather than 1-bromopropane, will be the major product of the reaction. This helps to explain why Markovnikov s rule applies to the addition of a hydrogen halide to an unsymmetrlcal alkene. 

Prototem 6.23 Unsymmetrical reagents like HX add to unsymmetrical alkenes such as propene according to Markovnikov s rule the positive portion, e.g., H of HX, adds to the C that has more H s ( the rich get richer ). Explain by stability of the intermediate cation. ... 

Explain Markovnikov s rule, that is, in an electrophilic addition of HX to an olefin, the hydrogen goes to the carbon atom which has the most hydrogens. 

Explain Markovnikov s rule and complete the reactions below. 

A modernized version of Markovnikov s rule often explains the anti-Markovnikov behavior. 

The regioselective addition of HX to alkenes produces the more substituted alkyl halide, which is known as the Markovnikov (Markovnikoff) product. Markovnikov s rule states that on addition of HX to an alkene, H attaches to the carbon with fewest alkyl groups and X attaches to the carbon with most alkyl groups . This can be explained by the formation of the most stable intermediate carbocation. 

This mechanism explains the formation of the more highly substituted alcohol from unsymmetrical alkenes (Markovnikov s rule). A number of other points must be considered in order to provide a more complete picture of the mechanism. Is the protonation step reversible Is there a discrete carbocation intermediate, or does the nucleophile become involved before proton transfer is complete Can other reactions of the carbocation, such as rearrangement, compete with capture by water ... 

Complexity follows in its own time, but we have deliberately omitted detailed discussion of obscure reactions of little value, or of variants of reactions which lie a simple step of mechanistic logic from our main story some of these are explored in the problems associated with each chapter, which are available online. We have similarly aimed to avoid exhuming principles and rules (from those of Le ChateUer through Markovnikov, Saytseff, least motion, and the like) to explain things which are better understood in terms of unifying fundamental thermodynamic or mechanistic concepts. 

Some reactions do not follow Markovnikov s Rule, and onf/-Markovnikov products are isolated. The outcome of the regioselectivity may be explained by tte relative stability of the radical intermediates. 

Markovnikov s rule can be explained by realizing that two different carbocations can be formed from the reaction between an unsymmetrical alkene and an electrophile. Formation of the more stable carbocation will predominate and wiU lead to the product predicted by Markovnikov s rule. 

Markovnikov s rule is explained by the mechanism of the reaction. Consider the reaction ofpropene MECHANISTIC BAS IS HBr. The n electrons in propene act as a Lewis base, and react with a proton, which is an elec-... 

The order of carbocation stability explains Markovnikov s rule. Addition of the electrophile to the less substituted double bonded carbon atom gives the more stable carbocation. The formation of the more stable carbocation controls the product formed. 

---