Alkenes, halogenation mechanism

This is an example of electrophilic addition of Cl to an alkene. The mechanism of this reaction involves the following steps. In the first step, the ethylene reacts with chlorine to form the cyclic ethylene chloronium ion (intermediate) and chloride ion. Note that in this cyclic intermediate, the chlorine has a positive charge. This step is followed by the nucleophilic attack by chloride ion on the chloronium ion. The reaction is enhanced by electron-donating substituents such as alkyl groups on the carbon-carbon double bond, since such groups can further stabilize the formation of the transition state which results in the formation of the chloronium ion. Halogen addition is usually an anti addition process. 

All of the data cited so far are consistent with the mechanism suggested in Figure 9.2, but additional experimental results suggest a much more complex range of interactions among alkene, halogen, solvent, and added nucleophiles than is apparent from that figure. ... 

In methanol, pseudo-second-order kinetics are observed when a high concentration of Br is present. Under these conditions, the dominant contribution to the overall rate comes from the third term of the general expression. The occurrence of third-order terms suggests the possibility of a mechanism similar to the Ad S mechanism for addition of hydrogen halides to alkenes, namely, attack of halide ion on an alkene-halogen complex. 

Notice that addition occurs in a way that places the two halogen atoms on opposite sides of the Tt bond. This mode of addition is called an anti addition. For most simple alkenes, halogenation appears to proceed primarily via an anti addition. Any proposed mechanism must be consistent with this observation. 

If we take the addition of hydrogen bromide to an alkene as a model for the alkene halogenation reaction, we can build a mechanism quite quickly. Just as the alkene reacts with hydrogen bromide to produce a carbocation and bromide (Br ), so reaction with bromine-bromine might give a brominated cation and bromide (Fig. 10.9).The n bond of the alkene acts as a nucleophile, displacing Br from bromine. Addition of bromide to the carbocation would give the dibromide product. 

So, a modification of our earlier addition mechanism suffices to explain the reaction. In contrast to the addition to aikenes we saw in Chapter 9 (HX, carbocations, and boranes), in alkene halogenation (bromination and chlorination) a three-membered ring is produced as an intermediate. In some, very spedalized cases the bromonium ion can even be isolated, and it seems its intermediacy in most brominations is assured. 

The reaction is possible because the bond joining the two halogen atoms is relatively weak and therefore is easily broken (Table 5.1) and because Br2 and CI2 are polarizable. As the nucleophilic alkene approaches Br2 (or CI2), it induces a dipole, which causes one of the halogen atoms to have a partial positive charge. This is the electrophile that adds to the sp carbon of the alkene. The mechanism of the reaction is shown on page 257. 

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... 

N Bromosuccimmide provides a low concentration of molecular bromine which reacts with alkenes by a mechanism analogous to that of other free radical halogenations... 

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... 

It might be noted that most (not all) alkenes are polymerizable by the chain mechanism involving free-radical intermediates, whereas the carbonyl group is generally not polymerized by the free-radical mechanism. Carbonyl groups and some carbon-carbon double bonds are polymerized by ionic mechanisms. Monomers display far more specificity where the ionic mechanism is involved than with the free-radical mechanism. For example, acrylamide will polymerize through an anionic intermediate but not a cationic one, A -vinyl pyrrolidones by cationic but not anionic intermediates, and halogenated olefins by neither ionic species. In all of these cases free-radical polymerization is possible. 

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. 

An a-halosulfone 1 reacts with a base by deprotonation at the a -position to give a carbanionic species 3. An intramolecular nucleophilic substitution reaction, with the halogen substituent taking the part of the leaving group, then leads to formation of an intermediate episulfone 4 and the halide anion. This mechanism is supported by the fact that the episulfone 4 could be isolated. Subsequent extrusion of sulfur dioxide from 4 yields the alkene 2 ... 

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. 

The HX compounds are electrophilic reagents, and many polyhalo and polycyano alkenes, (e.g., Cl2C=CHCl) do not react with them at all in the absence of free-radical conditions. When such reactions do occur, however, they take place by a nucleophilic addition mechanism, (i.e., initial attack is by X ). This type of mechanism also occurs with Michael-type substrates C=C—Z, where the orientation is always such that the halogen goes to the carbon that does not bear the Z, so the product is of the form X—C—CH—Z, even in the presence of free-radical initiators. Hydrogen iodide adds 1,4 to conjugated dienes in the gas phase by a pericyclic mechanism ... 

This type of polyhalo alkane adds to halogenated alkenes in the presence of AICI3 by an electrophilic mechanism. This is called the Prins reaction (not to be confused with the other Prins reaction, 16-53). ... 

The reaction of an a-halo sulfone with a base to give an alkene is called the Ramberg-Bdcklund reaction. The reaction is quite general for a-halo sulfones with an (x hydrogen, despite the unreactive nature of a-halo sulfones in normal 8 2 reactions (p. 437). Halogen reactivity is in the order I>Br>Cl. Phase-transfer catalysis has been used. In general, mixtures of cis and trans isomers are obtained, but usually the less stable cis isomer predominates. The mechanism involves formation of an episulfone, and then elimination of SO9. There is much evidence for... 

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