Halogenation and hydrohalogenation

Copyright 2005 WILEY-VCH Verlag GmbH Co. KGaA, Weinheim ISBN 3-527-30991-8 

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Halogenation and Hydrohalogenation. Halogens add to the triple bond of acetylene. FeCl catalyzes the addition of CI2 to acetylene to form 1,1,2,2-tetrachloroethane which is an intermediate in the production of the industrial solvents 1,2-dichloroethylene, trichloroethylene, and perchloroethylene (see Chlorocarbons and chlorohydrocarbons). Acetylene can be chlorinated to 1,2-dichloroethylene directiy using FeCl as a catalyst... 

Besides the addition of halogens and hydrohalogens across the double bond just covered, there are many other reagents that will react similarly with unsaturated polymers by free radical, ionic, or radical-ion mechanisms. Of prime importance is the addition of ethylene derivatives to polydienes. One of the earliest reactions of natural rubber to be studied in detail was the combination with maleic anhydride (Cunneen and Porter, 1965). Depending on the reaction conditions and the presence or absence of free radical initiators, one or more of four basic reactions may take place, with the products shown (the arrows indicate where the addition has taken place and the new bonds formed). 

Halogenation and hydrohalogenation of elastomers have been reported extensively in the literature [26]. The main problems with these reactions are the cyclization and chain scission that occur parallel to the halogenation reaction. These introduce difficult problems in the characterization of the resulting products. Despite these problems, several products have been prepared and commercialized. Chlorination of poly(l, 4-butadiene) to prepare a product similar to poly(vinyl chloride) has been reported by several workers [27]. This process had extensive side reactions and chain degradation. The chlorination of butyl rubber and conjugated diene-butyl rubbers gives end products that are used in the tire industry as inner liners for air retention. 

Y. TANAKA, H. SAKURABA, Y. OKA, and H. NAKANISHI / Asymmetric Halogenation and Hydrohalogenation of Ethyl tpans-Cinnamate in Crystalline Cyclodextrin Complexes... 

ASYMMETRIC HALOGENATION AND HYDROHALOGENATION OF tranS-CINNAMIC ACID IN CRYSTALLINE CYCLODEXTRIN COMPLEXES... 

Table III. Asymmetric Halogenation and Hydrohalogenation of trans-Cinnamic Acid in Crystalline Cyclodextrin Complexes...

ABSTRACT The gas-solid halogenation and hydrohalogenation using micro-crystalline cyclodextrin complexes are found to be efficient for production of the optical active halides of ethyl trans-cinnamate in moderate optical yields On exposure to HBr at 2QOC for 15-20 hr, the cinnamate in solid a- and S-cyclodextrin complexes yields ethyl R-(+)-3-bromo-3-phenylpropanoate in 46% e.e., and S-(-)-enantiomer in 31% e.e., respectively. No addition nor substitution products are obtained with HCl vapor at 0-50°C for 15-65 hr. Bromination of the B-cyclodextrin complex results in the formation of optical active ethyl erz/t/zrc>-2,3-dibromo-3-phenylpropanoate, while chlorination gives the optical active mixture of trans and cis addition products, ethyl erythro- and threo-2,3-di-chloro-3-phenylpropanoates in 60-80% yields. Mechanism of chiral induction in the present gas-solid reaction has been proposed on the basis of the crystal structure of the complex. 

Fig. 3. Products from the gas-solid halogenation and hydrohalogenation of styrene in the crystalline cyclodextrin complexes.

In the same way halogenation and hydrohalogenation of polydienes are accompanied by side reactions thus the reaction of chlorine with natural rubber results in additions and cyclizations but in the case of polybutadiene crosslinking is prevalent. 

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