Halogen addition, alkene mechanism

MECHANISM OF HALOGEN ADDITION TO ALKENES HALONIUM IONS... 

Many of the features of the generally accepted mechanism for the addition of halogens to alkenes can be introduced by referring to the reaction of ethylene with bromine... 

Mechanism of Halogen Addition to Alkenes Halonium Ions... 

Both parts of the Lapworth mechanism enol formation and enol halogenation are new to us Let s examine them m reverse order We can understand enol halogenation by analogy to halogen addition to alkenes An enol is a very reactive kind of alkene Its carbon-carbon double bond bears an electron releasing hydroxyl group which makes it electron rich and activates it toward attack by electrophiles... 

Both ( )- and (Z)-l-halo-l-alkenes can be prepared by hydroboration of 1-alkynes or 1-halo-l-alkynes followed by halogenation of the intermediate boronic esters (244,245). Differences in the addition—elimination mechanisms operating in these reactions lead to the opposite configurations of iodides as compared to bromides and chlorides. 

The bromonium ion postulate, made more than 75 years ago to explain the stereochemistry of halogen addition to alkenes, is a remarkable example of deductive logic in chemistry. Arguing from experimental results, chemists were able to make a hypothesis about the intimate mechanistic details of alkene electrophilic reactions. Subsequently, strong evidence supporting the mechanism came from the work of George Olah, who prepared and studied stable... 

HC1, HBr, and HI add to alkenes by a two-step electrophilic addition mechanism. Initial reaction of the nucleophilic double bond with H+ gives a carbo-cation intermediate, which then reacts with halide ion. Bromine and chlorine add to alkenes via three-membered-ring bromonium ion or chloronium ion intermediates to give addition products having anti stereochemistry. If water is present during the halogen addition reaction, a halohydrin is formed. 

Although the reaction of ketones and other carbonyl compounds with electrophiles such as bromine leads to substitution rather than addition, the mechanism of the reaction is closely related to electrophilic additions to alkenes. An enol, enolate, or enolate equivalent derived from the carbonyl compound is the nucleophile, and the electrophilic attack by the halogen is analogous to that on alkenes. The reaction is completed by restoration of the carbonyl bond, rather than by addition of a nucleophile. The acid- and base-catalyzed halogenation of ketones, which is discussed briefly in Section 6.4 of Part A, provide the most-studied examples of the reaction from a mechanistic perspective. 

In addition to the polar mechanism already considered (p. 179), halogen addition to alkenes can proceed via radical intermediates. The former is favoured by polar solvents and by the presence of Lewis acid catalysts, the latter by non-polar solvents (or in the gas phase),... 

Michael Faraday reported in 1821 that chlorine addition to alkenes is Stimulated by sunlightand today this is taken to indicate the involvement of a free radical process (equation 26). Free radical chain mechanisms were proposed in 1927 by Berthoud and Beraneck for the isomerization of stilbene catalyzed by Br2 (equation 27), and by Wachholtz for bromine addition to ethyl maleate (equation 28).Later studies showed inhibition of halogen addition by reaction of the intermediate radicals with oxygen, and a free radical chain mechanism for solution and gas phase halogenations as in equation (26) was shown (equation 29). Kinetic and mechanistic... 

When mechanisms involving radicals can be discounted, the main and general reaction pathway of addition of halogens to alkenes is shown in Scheme 10, where X is mainly Cl and Br69-70. 

Mechanism 8-6 Hydroboration of an Alkene 345 8-8 Addition of Halogens to Alkenes 349... 

Mechanism 8-7 Addition of Halogens to Alkenes 350 8-9 Formation of Halohydrins 352... 

Vinylic halogen atoms in perhalogenated alkenes are easily substituted by an addition-elimination mechanism with secondary phosphanes. 

The stereoselective tram addition of sulfenyl halides and other sulfenyl derivatives to alkenes can be rationalized by assuming a bridged thiiranium ion, by analogy with the mechanism proposed for stereoselective halogenation of alkenes. 

The commonly accepted mechanism for addition of halogens to alkenes has two steps, and is quite analogous to the mechanism for addition of hydrogen-containing acids (protic acids). In step (1) halogen adds as a positive halogen ion... 

Then we shall examine the stereochemistry of several reactions we have already studied—free-radical halogenation of alkanes, and electrophilic addition of halogens to alkenes- and see how stereochemistry can be used to get information about reaction mechanisms. In doing this, we shall take up ... 

As our second example of the application of stereochemistry to the study of reaction mechanisms, let us take another familiar reaction addition of halogens to alkenes. In this section we shall look at the stereochemical facts and, in the next, see how these facts can be interpreted. 

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