Hydrohalogenation—Electrophilic Addition of HX

The addition reactions of aikenes are discussed in Sections 10.9-10.16 and in Chapter 12 (Oxidation and Reduction). 

What kinds of reagents add to the weak, electron-rich n bond of aikenes There are many of them, and that can make alkene chemistry challenging. To help you organize this information, keep in mind the following  

The stereochemistry of addition is often important in delineating a reaction s mechanism. Because the carbon atoms of a double bond are both trigonal planar, the elements of X and Y can be added to them from the same side or from opposite sides. 

Hydrohalogenation is the addition of hydrogen halides HX (X = Cl, Br, and 1) to aikenes to form alkyl halides. 

Hydrohalogenation of an alkene to form an alkyl halide is the reverse of the dehydrohalogenation of an alkyl halide to form an alkene, a reaction discussed in detail in Sections 8.4 and 8.5. 

Two bonds are broken in this reaction—the weak 7C bond of the alkene and the HX bond— and two new (T bonds are formed—one to H and one to X. Because X is more electronegative than H, the H-X bond is polarized, with a partial positive charge on H. Because the electrophihc (H) end of HX is attracted to the electron-rich double bond, these reactions are called electrophilic additions. 

The mechanism of electrophilic addition of HX consists of two steps addition of to form a carbocation, followed by nucleophilic attack of X . The mechanism is illustrated for the reaction of d -2-butene with HBr in Mechanism 10.1. 

The mechanism of electrophilic addition consists of two successive Lewis acid-base reactions. In Step [1], the alkene is the Lewis base that donates an electron pair to H-Br, the Lewis acid, while in Step [2], Br is the Lewis base that donates an electron pair to the carbocation, the Lewis acid. 

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Hydrohalogenation (Section 10.9) An electrophilic addition of hydrogen halide (HX) to an alkene or alkyne. 

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. 

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