Electrophilic Addition of a Hydrogen Halide to an Alkene

Electrophilic Addition of a Hydrogen Halide to an Alkene THE OVERALL REACTION  

Step 1 Protonation of the carbon-carbon double bond by the hydrogen halide  

The characteristic chemical property of a C=C structural unit is susceptibility to attack by electrophiles. Electrons flow from the tt component of the double bond toward the electrophile and ultimately become a shared-electron pair in a covalent bond. WeTl see numerous other examples of electrophilic addition to alkenes in this chapter. First, however, we need to extend our discussion of hydrogen halide addition to alkenes of various types. 

5 Regioselectivity of Hydrogen Halide Addition Markovnikov s Rule 

In principle, a hydrogen halide can add to an unsymmetrical alkene (an alkene in which the two carbons of the double bond are not equivalently substituted) in either of two directions. In practice, addition is so highly regioselective as to be considered regiospecific. 

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Alkyl halides having a proton attached to a neighbouring (1-carbon atom can undergo an elimination reaction to produce an alkene and a hydrogen halide (Following fig.)- This reaction is the reverse of the electrophilic addition of a hydrogen halide to an alkene. There are two mechanisms by which this elimination can occur. These are E2 mechanism and the El mechanism. 

Electrophilic addition of a hydrogen halide to an alkene is the addition of a halogen (Cl, Br, or I) and H across the carbon-carbon double bond. The reaction occurs with Markovnikov regioselectivity with the H adding to the carbon with the greater number of hydrogens. 

The addition of a hydrogen halide, e.g. HCl, to an alkene is a simple example of an electrophilic addition. 

This reaction is typified by the addition of a hydrogen halide, HX, or water, to an alkene. The process involves two steps, with a carbocation as the intermediate. In the first step (Figure 11.1), an electrophile, for example, a proton, is added to the carbon-carbon bond to form a carbocation. Notice how the curly arrow is drawn—the proton is being added to the upper carbon of the double bond and the electrons are taken away from the lower carbon, leaving it positively charged. In the second step, the counterion, for example, bromide, attacks the carbocation to give a saturated product. Note that we only show one of the new bonds we have made in this example (in red) a carbon-hydrogen bond has also been made. 

The addition of hydrogen halides to simple alkenes, in the absence of peroxides, takes place by an electrophilic mechanism, and the orientation is in accord with Markovnikov s rule. " When peroxides are added, the addition of HBr occurs by a free-radical mechanism and the orientation is anti-Markovnikov (p. 985). It must be emphasized that this is true only for HBr. Free-radical addition of HF and HI has never been observed, even in the presence of peroxides, and of HCl only rarely. In the rare cases where free-radieal addition of HCl was noted, the orientation was still Markovnikov, presumably beeause the more stable product was formed. Free-radical addition of HF, HI, and HCl is energetically unfavorable (see the discussions on pp. 900, 910). It has often been found that anti-Markovnikov addition of HBr takes place even when peroxides have not been added. This happens because the substrate alkenes absorb oxygen from the air, forming small amounts of peroxides... 

The first two steps of the mechanism for the acid-catalyzed addition of water to an alkene are essentially the same as the two steps of the mechanism for the addition of a hy- 4 drogen halide to an alkene The electrophile (H ) adds to the sp carbon that is bonded to the greater number of hydrogens, and the nucleophile (H2O) adds to the other sp carbon. 

Besides the addition of halides and hydrogen-halide adds to alkenes or alkynes, other industrially relevant electrophilic addition reactions involve hydratization reactions (addition of water to alkenes and alkynes, forming alcohols), cationic polymerization (addition of carbocation to an alkene), hydrogenation (addition of hydrogen to alkenes to form alkanes), and Diels-Alder reactions (addition of an alkene to a conjugated diene to form complex, unsaturated hydrocarbon structures). 

In the addition of hydrogen halides to alkenes, it is generally found that the halogen atom becomes attached to the more substituted carbon atom. The statement of this general observation is called Markownikoff s rule. The basis for this regioselectivity lies in the relative abilities of the carbon atoms to accept positive charge. The addition of hydrogen hahde is initiated by an electrophilic protonation of the alkene. The new C—H bond is formed from the it electrons of the carbon-carbon double bond. It is easy to see that if a carbocation is formed as an intermediate, the halide would be added to the more substituted carbon, because addition of the proton at the less substituted carbon atom provides the more stable carbocation intermediate. 

Hydrogenolysis (Section 28.7) A reaction that cleaves a o bond using H2 in the presence of a metal catalyst, a Hydrogens (Section 23.1) The hydrogen atoms on the carbon bonded to the carbonyl carbon atom (the a carbon). Hydrohalogenation (Section 10.9) An electrophilic addition of hydrogen halide (HX) to an alkene or alkyne. 

Vladimir Markovnikov was the first to recognize that the major product obtained when a hydrogen halide adds to an alkene is a result of the addition of the to the sp carbon bonded to the most hydrogens. Consequently, this is often referred to as Markovnikov s rule. However, Markovnikov s rule is valid only for addition reactions in which the electrophile is H. A better rule and the one we will use in this book, is one that applies to all electrophilic addition reactions of alkenes ... 

The reactions of halogens and hydrogen halides with alkenes are electrophilic addition reactions. This means that the initial attack on the organic molecule is by an electron-deficient species that accepts a lone pair of electrons to form a covalent bond. This species is called an electrophile. In the case of the reaction with hydrogen bromide, the mechanism for the reaction is as shown. 

With the exception of hydrogenation, the addition reactions of alkenes presented in this text occur by an electrophilic addition mechanism. The electrophile (H+ or X+) attacks the electron-rich pi-bond of the double bond. The pi electrons are used to form a single bond between the carbon and attacking species the other carbon becomes a carbocation. The carbocation is then neutralized by halide ion or water the addition is complete. In bromination reactions, the bromine adds in a trans fashion. 

Alkynes undergo many of the same electrophilic additions as alkenes. A characteristic reaction is a 7t bond acting as a nucleophile to make a new bond with an electrophile. In this section, we study the addition of bromine and chlorine as well as the addition of hydrogen halides. 

The product of the electrophilic addition reaction of an alkyne with HCI is an alkene. Therefore, a second addition reaction can occur if excess hydrogen halide is present. The second addition—like other addition reactions to alkenes— is regioselective the adds... 

The order of reactivity of hydrogen halides with aUtenes is HI > HBr > HCl, which corresponds to the order of decreasing acidity. The reaction is an electrophilic addition, in which the proton is the electrophile. The alkene, a Lewis base, accepts a proton from the acid and forms a carbocation. Then, the nucleophilic halide ion reacts with the carbocation to form the addition product. 

Introduction to Addition Reactions Reactions of Alkenes—The characteristic reaction of alkenes is the addition of one substituent to each of the carbon atoms in the double bond (reaction 27.7). A variety of addition reactions are possible hydrogenation addition of hydrogen halides (reaction 27.8) hydration (reaction 27.9), producing a vicinal dihalide or a geminal dihalide and halogenation (reaction 27.10). In these reactions, a transfer of electron density occurs from the tt bond of the alkene to an electrophilic atom. In all these reactions— except halogenation— the electrophilic atom tends to add to... 

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