Addition of hydrogen halides or halogens to alkenes

Direct addition of a hydrogen halide to an alkene gives rise to an alkyl halide, the order of reactivity being HI HBr HCl. In the case of an unsymmetrical alkene, the regioselectivity of the reaction may be predicted from the mechanism of the reaction. Thus, the carbocation which is the most stabilised by charge dispersal will be the one which is formed preferentially. Classically the mode of addition is described as proceeding in the Markownikoff manner. 

Addition of hydrogen halide (1 mol) to a diene (1 mol) is a method of greater preparative value. This reaction is illustrated by the addition of hydrogen bromide to isoprene (Expt 5.64) the overall 1,4-addition process (thermodynamically controlled), as opposed to the 1,2-addition (kinetically controlled), predominates under the conditions specified. 

The addition of hydrogen bromide (but not the iodide or chloride) in the presence of an added peroxide catalyst proceeds by a radical mechanism, giving rise to a regioselectivity which is opposite to that of the ionic mechanism (anti-Markownikoff). An example is provided in the preparation of 11-bromo-undecanoic acid (Expt 5.65). 

Such anti-MarkownikofT regioselectivity may be achieved in the formation of iodides by proceeding via the corresponding organoborane, which is then decomposed with iodine in the presence of sodium methoxide93 (Expt 5.66). More recently iodine monochloride has been found to be an economic alternative iodinating agent.94 

The addition of bromine usually proceeds the most smoothly, and is conveniently carried out in a solvent such as carbon tetrachloride. The examples of the addition of bromine to allyl bromide and the addition of bromine to undec-10-enoic acid are illustrative (Expts 5.67 and 5.23). 

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The addition of hydrogen halides or halogens to alkenes (Expts 5.64 to 5.67). 

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... 

Reactions of alkenes involve the carbon-carbon double bond. The key reaction of the double bond is the addition reaction. This involves the addition of two atoms or groups of atoms to a double bond. The major alkene addition reactions include addition of hydrogen (Hj), halogens (Clj or Brj), water (HOH), or hydrogen halides (HBr or HCl). A generalized addition reaction is ... 

Ester- and amide-substituted radicals bearing an adjacent stereocenter abstract hydrogen with high diastereoselectivity1. The radicals are generated via intra- or intermolecular radical addition to alkenes, halogen abstraction from alkyl halides or reductive cleavage of alkylmercury compounds. Some examples are shown in Table 1. 

The addition of a hydrogen halide to an alkyne can be stopped after the addition of one equivalent of HBr or HCl because an alkyne is more reactive than the halo-substituted alkene that is the product of the first addition reaction. The halogen substituent withdraws electrons inductively (through the cr bond), thereby decreasing the nucleophilic character of the double bond. 

Because radical chlorination of an alkane can yield several different monosubstitution products as well as products that contain more than one chlorine atom, it is not the best method for synthesizing an alkyl halide. Addition of a hydrogen halide to an alkene (Section 4.1) or conversion of an alcohol to an alkyl halide (a reaction we will study in Chapter 12) is a much better way to make an alkyl halide. Radical halogenation of an alkane is nevertheless still a useful reaction because it is the only way to convert an inert alkane into a reactive compound. In Chapter 10, we will see that once the halogen is introduced into the alkane, it can be replaced by a variety of other substituents. 

In organic chemistry, a reaction in which two substances join to produce one compound is called an addition reaction. Addition at the carbon-carbon double bond is the most common reaction of alkenes. Hydrogen, halogens (CI2 or Br2), hydrogen halides, and water are some of the reagents that can be added to unsaturated hydrocarbons. Ethylene, for example, reacts in this fashion ... 

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. 

An attractive pathway with a lot of potential uses the transition metal mediated reaction of organic halides with carbon monoxide. Suitable substrates are organic halides capable of oxidative addition to low-valent transition metal compounds. Insertion of carbon monoxide and reductive elimination of an acid halide will complete the catalytic cycle. In tins way it was shown tiiat allyl chloride yields butenoic acid chloride in >80% yield accor g to equation 22)P As well as palladium, rhodium and iridium also act catalytically. It is of no surprise that allylic halides, benzylic halides and aryl halides in particular are readily converted to acid halides. Simple aliphatic halid undergo the oxidative addition step more slowly and, if they cany hydrogen atoms on an sf hybridized C atom in the -position to the halogen atom, may give alkenes via 3-hydrogen elimination. Alkenes can also be converted to acid halides widi carbon monoxide in the presence of transition metal catalysts in solvents such as methylene chloride or tetrachloromethane. ... 

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