Alkenes with Halogens

We will consider the reaction of ethylene with chlorine. Sample reaction 16-9 The overall reaction  

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. 

Halogen addition is usually an anti addition process. See the next reaction that exemplifies this aspect. 

---

Naphthalene dioxygenase from P. putida strain FI is able to oxidize a number of haloge-nated ethenes, propenes, and butenes, and d5 -hept-2-ene and cis-oct-2-ene (Lange and Wackett 1997). Alkenes with halogen and methyl substituents at double bonds form allyl alcohols, whereas those with only alkyl or chloromethyl groups form diols. 

Phenyl-substituted alkenes with halogen atoms at the double bond react with xenon difluoride in dichloromethane in the presence of hydrogen fluoride forming vicinal difluorides in 50% yield.35... 

Reaction of Conjugated Alkenes with Halogen Azides... 

The analogous reactions of alkenes with halogen azides are treated in Section 7.2.2 and those with iodine isocyanate and bromocyanamide are treated in Section 7.2.6. 

Reactions of alkenes with halogens or hydrogen halides... 

The greater stability of allylic radicals relative to their alkyl counterparts suggests that free-radical halogenation of alkenes should be both feasible and regioselective for the allylic position. Although, as we have already seen, the typical reaction of alkenes with halogens at room temperature and below is electrophilic addition to the double bond. 

The region of high electron density between the doubly bonded carbon atoms gives alkenes an additional reactivity and in addition to burning and reacting with halogens, alkenes will add on other molecules for example ... 

There are a wide variety of methods for introduction of substituents at C3. Since this is the preferred site for electrophilic substitution, direct alkylation and acylation procedures are often effective. Even mild electrophiles such as alkenes with EW substituents can react at the 3-position of the indole ring. Techniques for preparation of 3-lithioindoles, usually by halogen-metal exchange, have been developed and this provides access not only to the lithium reagents but also to other organometallic reagents derived from them. The 3-position is also reactive toward electrophilic mercuration. 

You have just seen that cyclic halonmm ion intermediates are formed when sources of electrophilic halogen attack a double bond Likewise three membered oxygen containing rings are formed by the reaction of alkenes with sources of electrophilic oxygen... 

Cyclopropanes can be generated in high yield by treatment of halogen-containing alkenes with Cp2ZrHCl eg,... 

Electrophilic addition of hydrogen bromide to alkenes follows Markovnikov s rule, leading to the product with halogen on the more-substituted position. However, trace amounts of hydroperoxides (among other impurities ) may initiate a reaction that gives rise to the anti-Markovnikov product, with bromine in the less-substituted position. 

In 1976, Sato reported the hydroalumination of terminal alkenes with LiAlH4 in the presence of ZrCh [13]. For example, 1-hexene was quantitatively converted into n-hexane at room temperature after hydrolytic workup, whereas no reaction occurred in the absence of a catalyst Halogenation of the reaction mixtures indicated that these reactions in fact proceed through organoaluminum intermediates. Later, TiCh was found to be an even more active catalyst [14, 15]. 

Hydrozirconation of terminal alkynes R-C=CH (R= aryl, alkyl) with 1 affords terminally ( )-Zr-substituted alkenes with high efficiency and excellent stereochemical and regiochemical control (>98%). These alkenylzirconocene complexes are of particular interest for synthetic use [136, 143, 144]. Moreover, beside the electropositive halogen sources [145] and heteroatom electrophiles [3] used in the pioneering studies to directly cleave the Zr-C bond, ( )-vinyl-Zr complexes were recently transformed into a number of other trans-functionalized alkenes such as ( )-vinyl-sul-fides[146], vinylic selenol esters [147], vinyl-sulfones [148], vinyl-iodonium [149], vinyl-(R0)2P(0) [150], and vinilic tellurides [143]. 

The mechanism for the addition of 50 to anthraquinone (Scheme 52), 1,4-diacetylbenzene (Scheme 53), and 1,2-diacetylbenzene (Scheme 54) has been proposed.326 The addition of 50 to benzophenone followed by reaction with Wilkinson s catalyst formally results in the hydrogenation of a double bond (Equation (262)).325 This species also undergoes stereospecific inertion into the vinyl chloride bond of various halogenated alkenes with high yields in most cases (Equations (263)-(267)).329... 

Reduction of carboxylic acids and esters, aldehydes, and nitriles, and the hydro-boration of alkenes with diborane in non-ethereal solvents is highly effective (Table 11.8), but reduction of nitro groups or cleavage of arena-halogen bonds does not occur [1]. However, in spite of the potential advantages, very little use appears to have been made of the procedure. 

Alcohols can react in several ways, depending on the reactants and on the conditions of the reaction. For example, alcohols can undergo substitution with halogen acids, elimination to form alkenes, and oxidation to form aldehydes, ketones, or carboxylic acids. 

Alkenes undergo addition reactions with hydrogen to form alkanes, with halogens to form dihaloalkanes, with hydrogen halides to form monohaloalkanes and with water to form alcohols. For example ... 

---