Stereochemistry of the Ionic Addition to an Alkene

With propene, for example, the addition takes place as follows  

In Section 10.9 we shall find that this addition occurs by a radical mechanism, and not by the ionic mechanism given at the beginning of Section 8.2. 

Consider the following addition of HX to 1-butene and notice that the reaction leads to the formation of a prodnct, 2-halobntane, which contains a chirality center  

The product, therefore, can exist as a pair of enantiomers. The question now arises as to how these enantiomers are formed. Is one enantiomer formed in greater amount than the other The answer is no the carbocation that is formed in the first step of the addition (see the following scheme) is trigonal planar and is achiral (a model will show that it has a plane of symmetry). When the halide ion reacts with this achiral carbocation in the second step, reaction is equally likely at either face. The reactions leading to the two enantiomers occur at the same rate, and the enantiomers, therefore, are produced in equal amounts as a racemic form. 

1-Butene donates a pair of electrons to the proton of HX to form an achiral carbocation. 

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Since the ionic addition of hydrogen bromide and the acid-catalyzed addition of water to an alkene proceed via planar carbocations, the formation of products derived from rearranged carbocations is relatively common. In contrast, the bromi-nation of alkenes, which is illustrated by the addition of bromine to cyclopentene (Eq. 10.18), occurs via a cyclic bromonium ion 38 related to 33, so skeletal rearrangements of such cations are not generally observed. Moreover, the intervention of this cyclic ion dictates that the stereochemistry of the addition is completely in the anti sense, as shown. The reddish color of bromine is discharged upon addition to an alkene, making this reaction a useful qualitative test for unsaturation (Sec. 25.8A). 

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. 

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