Addition Reactions of Alkenes Overview

The most common mode of addition of X-Y across the ir-bond of alkenes involves an ionic stepwise mechanism in which an electrophile, E, first attacks the ir-bond, functioning as a nucleophile, to produce a carbocation. In such cases, X-Y may be rewritten as E-Nu, wherein E is the less electronegative of the two atoms. The nucleophile, Nu , then attacks the carbocation to produce the final product. The overall process is termed electrophilic addition. 

In order to understand the mechanism of electrophilic addition, it is helpful to consider this type of addition in terms of sequential reactions of Lewis acids and Lewis bases. In the first step, the electrons of the polarizable ir-bond, a Lewis base, attack the electrophile E, an electron-deficient species and a Lewis acid, to give either an acyclic planar carbocation 32 or a cyclic cation 33 (Eq. 10.15). If carbocation 32 is formed as an intermediate in the electrophilic addition to an unsymmetrically 

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). 

The addition of reagents X-Y to carbon-carbon ir-bonds may also proceed via a concerted mechanism in which each new a-bond is formed simultaneously on the same face of the ir-bond. The stereochemistry of such reactions is necessarily syn. For example, the reaction of potassium permanganate, which is purple, with an alkene such as cyclohexene proceeds via si/H-addition of permanganate ion across the ir-bond to give 39, which is colorless. Subsequent decomposition of 39 gives a ds-l,2-diol and manganese dioxide, the brown precipitate that is observed as the other product of the reaction (Eq. 10.19). This decoloration of potassium permanganate by alkenes forms the basis of the Baeyer qualitative test for the presence of carbon-carbon ir-bonds (Sec. 25.8B). 

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