Carbon-halogen bond formation chlorine

We should point out, however, that depending on the relative importance of the various reactions, kohs may not be a simple function of pH and temperature, and that product formation may strongly depend on these two variables. Furthermore, we note that many environmentally important organic compounds exhibit halogen atoms bound to a carbon-carbon double bond, be it an olefinic (e.g., chlorinated ethenes) or an aromatic (e.g., chlorinated benzenes, PCBs) system. In many cases, under environmental conditions, these carbon-halogen bonds undergo SN or E reactions at extremely slow rates, and we therefore may consider these reactions to be unimportant. 

The electron-withdrawing effect of the halogen, coupled with that of the carbonyl oxygen, leads to a very electron-deficient carbon, and this is not effectively counteracted by the lone pairs on halogens such as chlorine. Consequently, the carbonyl carbon atom is very sensitive to nucleophilic addition to form a tetrahedral intermediate. The collapse of the tetrahedral intermediate with the expulsion of the halide ion, which is a good leaving group, enhances the reactivity of the acyl halides (Scheme 3.64a). The direct fission of the acyl halide C-X bond leads to the formation of an electrophilic acylium ion (Scheme 3.64b). 

The addition of halogens to carbon-carbon double bonds is interpreted as a stepwise addition which is initiated by a species containing a positively polarized halogen in the present case the chlorine molecule which becomes polarized in close proximity to the jr-electron cloud of ethylene. The initial step resnlts in the formation of a T-shape charge-transfer intermolecnlar complex which has been identified both theoretically [29-31] and experimentally [32,33] in the gas phase. 

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. 

The reaction of an alcohol with a hydrogen halide is a substitution. A halogen, usually chlorine or bromine, replaces a hydroxyl group as a substituent on carbon. Calling the reaction a substitution tells us the relationship between the organic reactant and product but does not reveal the mechanism. The mechanism is the step-by-step pathway of bond cleavage and bond formation that leads from reactants to products. In developing a mechanistic picture for a particular reaction, we combine some basic principles of chemical reactivity with experimental observations to deduce the most likely sequence of steps. 

Monomers Mechanistically, coupling an electron-rich organotin molecule with an electron- deficient halide/triflate molecule promotes the desired C-C formation. Therefore, in order to obtain D-A copolymers of high molecular weight, electron-rich donor moieties are usually di-stannylated, whereas the electron-deficient acceptor moieties are typically halogenated. lodinated acceptors are generally more reactive due to the labile carbon-iodine bond, which also lowers the stability of the iodinated acceptors. On the other hand, chlorinated acceptors are relatively rare because of their low reactivity. Therefore, with a good balance of reactivity and stability, brominated acceptors are the most common ones for polymerization. 

Bromination or chlorination at an a-carbon is catalyzed by both acid and base. For acid-catalyzed halogenation, acid generated by the reaction catalyzes further reaction. The slow step of acid-catalyzed halogenation is formation of an enol.This is followed by rapid reaction of the double bond with halogen to give the a-haloketone. 

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