Hydrogen halide addition Markovnikov’s rule

REGIOSELECTIVITY OF HYDROGEN HALIDE ADDITION MARKOVNIKOV S RULE... 

Regioselectivity of Hydrogen Halide Addition Markovnikov s Rule... 

As shown in this equation, addition of both the first and second moles of HBr is regiose-lective. Addition of hydrogen halides follows Markovnikov s rule (Section 6.3A) hydrogen adds to the carbon that has the greater number of hydrogens. We can account for this regioselectivily of addition of HX by a two-step mechanism for each addition. 

Let s compare the carbocation intermediates for addition of a hydrogen halide (HX) to an unsymmetrical alkene of the type RCH=CH2 (a) according to Markovnikov s rule and (b) opposite to Markovnikov s rule (a) Addition according to Markovnikov s rule... 

A proton and a halogen add to the double bond of an alkene to yield an alkyl halide Addition proceeds in ac cordance with Markovnikov s rule hy drogen adds to the carbon that has the greater number of hydrogens halide to the carbon that has the fewer hydro gens... 

Hydrogen halides add to alkynes in accordance with Markovnikov s rule to give alkenyl halides In the presence of 2 moles of hydrogen halide a second addition occurs to give a geminal dihalide... 

Ion 21 can either lose a proton or combine with chloride ion. If it loses a proton, the product is an unsaturated ketone the mechanism is similar to the tetrahedral mechanism of Chapter 10, but with the charges reversed. If it combines with chloride, the product is a 3-halo ketone, which can be isolated, so that the result is addition to the double bond (see 15-45). On the other hand, the p-halo ketone may, under the conditions of the reaction, lose HCl to give the unsaturated ketone, this time by an addition-elimination mechanism. In the case of unsymmetrical alkenes, the attacking ion prefers the position at which there are more hydrogens, following Markovnikov s rule (p. 984). Anhydrides and carboxylic acids (the latter with a proton acid such as anhydrous HF, H2SO4, or polyphosphoric acid as a catalyst) are sometimes used instead of acyl halides. With some substrates and catalysts double-bond migrations are occasionally encountered so that, for example, when 1 -methylcyclohexene was acylated with acetic anhydride and zinc chloride, the major product was 6-acetyl-1-methylcyclohexene. ... 

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... 

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. 

Alkenes are converted to alkyl halides by the addition of HX (HCl, HBr or HI). Addition of HX to unsymmetrical alkenes follows Markovnikov s rule. The reaction is regioselective, and occurs via the most stable carbocation intermediate. For example, addition of hydrogen bromide (HBr) to propene yields 2-bromopropane as the major product. 

Electrophilic addition to terminal alkynes (unsymmetrical) is regioselective and follows Markovnikov s rule. Hydrogen halides can be added to alkynes just like alkenes, to form first the vinyl halide, and then the geminal alkyl dihalide. The addition of HX to an alkyne can be stopped after the first... 

Addition of chlorine or bromine in the presence of water can yield compounds containing halide and hydroxyl on adjacent carbon atoms (haloalcohols or halohydrins). The same products can be obtained in the presence of methanol (13) or acetic acid (14). As expected from the halonium ion intermediate, the addition is anti. As expected from Markovnikov s rule, the positive halogen goes to the same carbon that the hydrogen of a protic reagent would. 

Among the hydrogen halides, only hydrogen bromide reacts with alkenes by both electrophilic and free-radical addition mechanisms. Hydrogen iodide and hydrogen chloride always add to alkenes by electrophilic addition and follow Markovnikov s rule. Hydrogen bromide normally reacts by electrophilic addition, but if peroxides are present or if the reaction is initiated photochemically, the free-radical mechanism is followed. 

Markovnikov s rule is obeyed because the mechanism of sulfuric acid addition to alkenes, illustrated for the case of propene in Figure 6.8, is analogous to that described earlier for the electrophilic addition of hydrogen halides. 

Section 6.8 Hydrogen bromide is unique among the hydrogen halides in that it can add to alkenes either by electrophilic or free-radical addition. Under photochemical conditions or in the presence of peroxides, free-radical addition is observed, and HBr adds to the double bond with a regio-selectivity opposite to that of Markovnikov s rule. 

Halogen halides add across carbon-carbon double bonds. These additions follow Markovnikov s rule, which states that the positive part of a reagent (a hydrogen atom, for example) adds to the carbon of the double bond that already has more hydrogen atoms attached to it. The negative part adds to the other carbon of the double bond. Such an arrangement leads to the formation of the more stable carbocation over other less-stable intermediates. 

When a terminal alkyne is treated with an excess of hydrogen halide the halogens both end up on the more substituted carbon (Fig. F). This is in accordance with the Markovnikov s rule which states that the additional hydrogens end up on the carbon which already has the most hydrogens. The same rule applies for the reaction with acid and mercuric sulphate which means that a ketone is formed after keto-enol tautomerism instead of an aldehyde (Fig. G). 

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