Dienes hydrohalogenation

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. 

The resonance description of the allylic cation shows the source of the two products, because chloride can add to either of the two carbons sharing the positive charge to give the two observed alkyl chloride products (Fig. 12.37). Hydrohalogenation of conjugated dienes with HBr also results in 1,2- and 1,4-addition. 

PROBLEM 12.69 Select the reaction 1,2-Hydrohalogenation of diene that is on the Semester IB panel. Observe the reaction several times. When the electrophilic HBr reacts with the nucleophilic diene, an intermediate carbocation is formed. 

PROBLEM 12.70 Chose the LUMO track of the 1,2-Hydrohalogenation of diene and stop the animation at the intermediate (there is only one intermediate in the energy diagram). Notice the nature of this molecular orbital. It is the same as the LUMO of an aUyl cation. Does there appear to be any more LUMO density on one carbon than the other ... 

PROBLEM 12.71 Look closely at the energy diagrams for the 1,2-Hydrohalogenation of diene and 1,4-Halogenation of diene reactions. Notice there is a faint pathway in the second half of the reaction in each case. Why is the product of the 1,4 addition lower in energy than the product of 1,2 addition Will the 1,4-addition always be lower Consider the HBr addition to 1,3-pentadiene. Is the product of 1,4-addition lower in this example ... 

Standard alkene reactions such as hydrogenation, halogenation, hydrohalogenation and isomerization as adapted to a diene polymer situation. 

Since 1,4-polyisoprene has a secondary carbon atom at the double bond it follows that it is generally more reactive to both free radicals and to carbonium ions than 1,4-poly butadiene. The typical addition reactions associated with the double bond suggest that the ultimate hydrogenated, halogenated, hydrohalogenated and isomerized diene polymers would have the same structure irrespective of the initial cis-ltrans- ratio. 

As in the case of hydrohalogenation the bulk of the theoretical studies on the halogenation of diene polymers has been made on natural rubber with chlorine as the halogen. As with natural rubber hydrochloride the derivative has some commercial value. 

A recent patent (US Patent 3 714 297 to Ugine Kuhlmann) has described the preparation of a diene elastomer epoxide which is subsequently reacted in solution with a hydrohalogen acid to give an oil resistant hydroxy halogenated polymer. 

Representative diene-based polymers include natural rubber (NR), polyisoprene (PIP), PBD, styrene—butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), which together compose a key class of polymers widely used in the rubber industry. These unsaturated polyolefins are ideal polymers for chemical modifications owing to the availability of parent materials with a diverse range of molecular weights and suitable catalytic transformations of the double bonds in the polymer chain. The chemical modifications of diene-based polymers can be catalytic or noncatalytic. The C=C bonds of diene-based polymers can be transformed to saturated C—C and C—H bonds (hydrogenation), carbonyls (hydrofbrmylation and hydrocarboxylation), epoxides (epoxidation), C—Si bonds (hydrosilylation), C—Ar bonds (hydroarylation), C—B bonds (hydroboration), and C—halogen bonds (hydrohalogenation). ... 

According to this mechanism, the catalytic and selective formation of 1-halo-1,3-dienes could be performed through a hydrohalogenative dimerization of alkynes in one step with the same catalytic system. Dienylchlorides could be obtained in 1,2-dichloroethane (DCE) by direct addition of HCl or with a hydrohalogenative system consisting of separate proton and halide sources [5] [Eqs. (2) and (3)]. 

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