Addition of a Hydrogen Halide to an Alkene

A mechanism for addition of a hydrogen halide to an alkene involves (he following two steps  

The It electrons of the alkene form a bond with a proton from HX to form a carbocation and a halide ion. 

The halide ion reacts with the carbocation by donating an electron pair the result is an alkyl halide. 

If the aUcene that undergoes addition of a hydrogen halide is an unsymmetrical aUcene such as propene, then step 1 could conceivably lead to two different carbocations  

These two carbocations are not of equal stability, however. The secondary carbocation is more stable, and it is the greater stability of the secondary carbocation that accounts for the correct prediction of the overall addition by Markovnikov s rule. In the addition of HBr to propene, for example, the reaction takes the following course  

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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. 

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. 

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. 

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

A MECHANISM FOR THE REACTION ] Addition of a Hydrogen Halide to an Alkene 341 [ A MECHANISM FOR THE REACTION ] Addition of HBr to 2-Methylpropene 343... 

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. 

When you look at the mechanism for the acid-catalyzed addition of water to an alkene, notice that the first two steps are the same (except for the nucleophile employed) as the two steps of the mechanism for the addition of a hydrogen halide to an alkene (Section 6.1). 

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