Dehydrohalogenation competitive

Halobutyl Cures. Halogenated butyls cure faster in sulfur-accelerator systems than butyl bromobutyl is generally faster than chlorobutyl. Zinc oxide-based cure systems result in C—C bonds formed by alkylation through dehydrohalogenation of the halobutyl to form a zinc chloride catalyst (94,95). Cure rate is increased by stearic acid, but there is a competitive reaction of substitution at the halogen site. Because of this, stearic acid can reduce the overall state of cure (number of cross-links). Water is a strong retarder because it forms complexes with the reactive intermediates. Amine cure may be represented as follows ... 

Table Vm. Competition Between Dehydrohalogenation and Substitution Pathways for Reactions Between Halogenated Aliphatic Substrates and Sulfur Nucleophiles (Rate Constants Expressed in Units of M V1.)...

The intermediacy of a lactone (276) would seem reasonable, but the predominance of the tra/is-alkene is not explicable in this case. As long as the acid was completely ionised, the rate of dehalogenative decarboxylation was independent of pH (in aqueous solution). However, in more strongly alkaline solution (pH > II), the E2 dehydrohalogenation becomes competitive, but the rate coefficient for the dehalogenative decarboxylation also increases . An initial attack of hydroxide on the acid, giving (277) and a second-order term in the rate law, has been suggested. 

Alkylation of Amides, Phenols, Alcohols, and Adds. A variety of carboxamides were alkylated with primary alkyl halides using KOH in DMSO to give the fYalkyl amides (eq 1) in 54—90% yield. Most reactions were carried out at rt, but in some cases heating to 90 °C was required. Similar conditions were applied to alcohols, phenols, and acids to form ethers and esters. The procedure applies to Mel and all primary halides. Secondary alkyl halides show evidence of competitive dehydrohalogenation, while tertiary halides do not give any alkylation products. The procedure was applied to the N- and O-permethylation of peptides. It was also applied to the methylation of hydroxypyridines in 39-78% yield. In all the above cases, the substrate and alkyl halide were added to powdered KOH in DMSO and stirred at rt. It was unnecessary to use especially dry DMSO or to protect the reaction mixture from atmospheric moisture. 

Dehydrohalogenation of alkyl halides in the presence of strong base (E2) is often accompanied by the formation of substitution (Sn2) products. The extent of the competitive substitution reaction depends on the structure of the alkyl halide. Primary alkyl halides give predominantly substitution products (the corresponding ether), secondary alkyl halides give predominantly elimination products, and tertiary alkyl halides give exclusively elimination products. For example, the reaction of 2-bromopropane with sodium ethoxide proceeds as follows ... 

In practice, the scope and limitations of this reaction are governed by the limitations of the S 2 reaction and by competition between substitution and jS-elimination (Section 9.8). The reaction is most useful for preparation of thiols from primary halides. Yields are lower from secondary halides because of the competing jS-elimination (E2) reaction. V Sth tertiary halides, jS-elimination (E2) predominates, and the alkene formed by dehydrohalogenation is the major product. 

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