Addition of HX to an Alkene

Step 2 Attack by the halide ion gives the addition pjroduct. 

The ionic addition of HBr to propene shows pjrotonation of the less substituted carbon to give the more substituted carbocation. Reaction with bromide ion completes the addition. 

Positive charge on less substituted carbon. Less stable not formed. 

There are many examples of reactions where the proton adds to the less substituted carbon atom of the double bond in order to produce the more substituted carbocation. The addition rf HBr (and other hydrogen halides) is said to be regioselective because in each case, one rf the two possible orientations rf addition results ptreferentially over the other. 

Markovnikov s Rule A Russian chemist, Vladimir Markovnikov, first showed the orientation of addition of HBr to alkenes in 1869. Markovnikov stated  

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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. 

Markovnikov s rule In the addition of HX to an alkene, the H attaches to the carbon with fewer alkyl substituents and the X attaches to the carbon with more alkyl substituents. 

If an unsymmetrical alkene combines with a hydrogen halide, the halide ion adds to the carbon atom with fewer hydrogen atoms (The addition of HX to an alkene, the hydrogen atom adds to the carbon atom of the double that already has the greater number of hydrogen atoms). 

Figure 8.1 Free-energy diagram for the addition of HX to an alkene. The free energy of activation for step 1 is much larger than that for step 2.

When the carbocation initially formed in the addition of HX to an alkene can rearrange to a more stable one => rearrangements invariably occur. 

Mechanism 8-2 Ionic Addition of HX to an Alkene 332 Mechanism 8-3 Free-Radical Addition of HBr to Alkenes 334 8-4 Addition of Water Hydration of Alkenes 337... 

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

Markovnikov s rule can be restated by saying that, in the addition of HX to an alkene, the more stable carbocation intermediate is formed. I his result is explained by the Hammond postulate, which says that the transition state of an exergonic reaction step structurally resembles the reactant, whereas the transition state of an endergonic reaction step structurally resembles the product. Since an alkene protonation step is endergonic, the stability of the more highly substituted carbocation is reflected in the stability of the transition state leading to its formal ion. 

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. 

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 regioselective addition of HX to alkenes produces the more substituted alkyl halide, which is known as the Markovnikov (Markovnikoff) product. Markovnikov s rule states that on addition of HX to an alkene, H attaches to the carbon with fewest alkyl groups and X attaches to the carbon with most alkyl groups . This can be explained by the formation of the most stable intermediate carbocation. 

Just as addition of HX to an alkene produces haloalkanes, elimination of HX from a haloalkane by reaction with a strong base, such as potassium ethoxide, produces an alkene ... 

We met cftemoselectivity—which group reacts—in the last chapter. Chemoselectivity means that there are two separate functional groups and that a reagent must choose between them. By contrast, regioselectivity implies that there is one functional group that can react in two different places and a reagent must choose where to react. Simple examples include addition of HX to an alkene (Chapter 19) and nucleophilic attack on the epoxide derived from that alkene (Chapter 15). 

Vladimir Markovnikov observed this regioselectivity and made the generalization, known as Markovnikov s rule, that, in the addition of HX to an alkene, hydrogen adds to the doubly bonded carbon that has the greater number of hydrogens already bonded to it. Although Markovnikov s rule provides a way to predict the product of many alkene addition reactions, it does not explain why one product predominates over other possible products. 

Chemists account for the addition of HX to an alkene by a twonstep mechanism, which we illustrate by the reaction of 2-butene with hydrogen chloride to give 2step mechanism in general and then go back and study each step in detail. 

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