Alkenes addition reactions with hydrogen halides

Alkenes undergo electrophilic addition reactions with hydrogen halides, to form alkyl halides. 

If excess reagent is available, alkynes undergo a second addition reaction with hydrogen halides and halogens because the product of the first reaction is an alkene. 

Alkenes undergo addition reactions with hydrogen to form alkanes, with halogens to form dihaloalkanes, with hydrogen halides to form monohaloalkanes and with water to form alcohols. For example ... 

Alkenes. The reaction of hydrogen halides with alkenes is generally an electrophilic addition. It occurs by a variety of mechanisms and involves a carbocation... 

When formulating a mechanism for the reaction of alkynes with hydrogen halides we could propose a process analogous to that of electrophilic addition to alkenes m which the first step is formation of a carbocation and is rate determining The second step according to such a mechanism would be nucleophilic capture of the carbocation by a halide ion... 

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. 

The addition of a hydrogen halide, such as HBr, is an important addition reaction for alkenes often seen in Organic Chemistry 1. However, conjugated dienes may behave differently. An example is the reaction of HBr with 1,3-butadiene as illustrated in Figure 4-11. 

Alkynes. Because of their less nucleophilic character, alkynes react less readily with hydrogen halides than do alkenes and often require the use a metal halide catalyst. Vinyl halides are formed in the reaction with one equivalent of HHlg. They may react further in an excess of the reagent to yield geminal dihalides. High yields of these compounds can be achieved. The addition of HC1 to acetylene was studied in detail because of the practical importance of the product vinyl chloride (see Section 6.2.4). 

In the absence of a catalyst, alkynes react very slowly with bromine. Scheme 3.28 particularly when compared to alkenes. When a choice exists, bromine reacts preferentially with an alkene rather than an alkyne. It is possible that radical reactions play a more important role in the addition to alkynes. When the reaction of acetylene with chlorine is catalysed by iron(lll) chloride, the reaction is fast and 1,1,2,2-tetrachloroethane is formed. The uncatalysed addition of a hydrogen halide gives a tram alkenyl halide. Further addition is restricted but can give rise to dihalides. 

In view of the very low reactivity of simple alkenes towards hydra-zoic acid, 2ind since these electron-withdrawing groups render the homopolar double bond less basic, it is unlikely that the reactions are Adfi in type. By comparison with hydrogen halide additions to such... 

Addition reactions with halogens, hydrogen halides hydrogenation to yield alkenes and alkanes... 

Addition reactions involve unsaturated compounds containing double or triple bonds, particularly C=C and C=C. Hydrogenation and reactions of hydrogen halides and halogens with alkenes and alkynes are examples of addition reactions. 

Energy diagrams comparing addition of a hydrogen halide HX with an alkene H2C=CHR according to Markovnikov s rule (solid red) and opposite to Markovnikov s rule (dashed blue). The energy of activation is less and the reaction is faster for the reaction that proceeds through the more stable secondary carbocation. 

Some specific reactions of this type that we shall study in this chapter include addition of hydrogen halides, sulfuric acid, water (in the presence of an acid catalyst), and halogens. Later we shall also study some specialized reagents that undergo addition reactions with alkenes. 

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. 

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