Electrophilic Addition of HX to Alkenes

Mechanism of the electrophilic addition of HBr to 2-methylpropene. The reaction occurs in two steps and involves a carbocation intermediate. 

The electrophile HBr is attacked by the IT electrons of the double bond, and a new C-H cr bond is formed. This leaves the other carbon atom with a + charge and a vacant p orbital. 

Br donates an electron pair to the positis ely charged carbon atom, forming a C-Br r bond and yielding the neutral addition product. 

Electrophilic addition of HX to alkenes is successful not only with HB but with HC2 and HI as well. Note that HI is usually generated in the reac tion mixture by treating potassium iodide with phosphoric acid. 

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Table 10.3 summarizes the characteristics of electrophilic addition of HX to alkenes. 

Electrophilic Addition of HX to Alkenes 206 Writing Organic Reactions 208... 

Electrophilic addition of HX to an alkene involves a two-step mechanism, the overall rate being given by the rate of the initial protonation step. Differences in protonation energies are usually explained by considering differences in carbocation stability, but the relief or buildup of strain can also be a factor. One of the following alkenes protonates much more easily than the other. 

Strategy All of these reactions are electrophilic additions of HX to an alkene. Use Markovnikov s rule to predict orientation. 

Like the electrophilic addition of HX to an alkene, the addition of HBr to a conjugated diene forms the more stable carbocation in Step [1], the rate-determining step. In this case, however, the carbocation is both 2° and allylic, and thus two Lewis structures can be drawn for it. In the second step, nucleophilic attack of Br can then occur at two different electrophilic sites, forming two different products. 

The standard mechanism written for electrophilic addition of HX to an alkene or alkyne is similar to acid-catalyzed hydration. As shown in Scheme 10.2, protonation is the first step, followed by nucleophilic attack of the halide on the resulting electrophilic carbenium ion. Just as with hydration, electrostatic effects guide our understanding of both steps of the reaction. As weTl show below, this standard electron pushing implies a mechanism that is much simpler than that often foimd experimentally. 

Markovnikov s rule (Section 6.8) A guide for determining the regiochemistry (orientation) of electrophilic addition reactions. In the addition of HX to an alkene, the hydrogen atom bonds to the alkene carbon thal has fewer alkyl substituents. 

When the addition is initiated by attack of the jr-electrons in an unsaturated bond on an electrophile to form a carbocation the reaction is an electrophilic addition, a very common class of reactions for alkenes. The reaction is governed by Markovnikov s rule, which states that in addition of HX to a substituted alkene, the H will form a bond to the carbon of the alkene carrying the greater number of hydrogen atoms. 

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... 

What can follow with an alkene is an ionic chain reaction with the following two propagating steps. First, the nucleophile attacks at carbon to form a carbon anion (carbanion) intermediate (Equation 10-8). Second, electrophilic transfer of a proton from HX to the carbanion forms the adduct and regenerates the nucleophile (Equation 10-9). The overall reaction is the addition of HX to the double bond ... 

Markovnikov s rule (Section 11.2) In addition reactions of HX to alkenes, the H bonds to the carbon with more hydrogens (fewer alkyl substituents) and the X bonds to the carbon with fewer hydrogens (more alkyl substituents). A more modern version of Markovnikov s rule, based on mechanistic reasoning, is that the electrophile adds so as to form the more stable carbocation. 

Electrophilic addition to alkenes, such as the addition of hydrogen bromide under ionic conditions, follows the Markownikoff rule which states that in the addition of HX to an alkene, the hydrogen atom (the electrophile) becomes attached to the less-substituted carbon atom. The addition of halogens such as bromine proceeds via a halonium ion, and takes place with an overall trans stereochemistry. 

The electrophilic addition of HX or X2 to alkenes generates alkyl halides with one or two halogen atoms, respectively (see Section 6.2.2 for a detailed discussion of the mechanisms). 

Scheme 2.31 Mechanism of electrophilic hydration and addition of HX to a simple alkene using HjO X in aqueous solution.

You may recall that alkenes (which are electron-rich due to an excess of electrons in the tt system) can react with an electrophilic reagent. The intermediate formed is electron-deficient. It reacts with the nucleophile to complete the reaction. The overall sequence is called electrophilic addition. Addition of HX to cyclohexene is an example. 

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