Isomerization organic additives, effect

Effect of Organic Additives upon the Isomerization of n-Pentane... 

Table 9.1). The rate of dehydrobromination from the intermediate bromoalkenes follows the pattern 2-bromoalkenes > Z-l-bromoalkenes > E- -bromoalkenes the corresponding chloro derivatives react more slowly. For optimum yield, the reaction temperature should be <100°C to reduce decomposition of the catalyst, and the concentration of base should be kept low to prevent isomerization of the resulting alkynes. [3-Elimination of HBr from 1,2-dibromo-1 -phenylethane can be controlled to yield 1-bromo-l-phenylethene in 83% yield [15]. The addition of alcohols and diols have a co-catalytic effect on the elimination reaction, as the alkoxide anions are transferred more effectively than the hydroxide ions into the organic phase [13]. 

Although the effect of quaternary ammonium salts on the stereochemistry of the two-phase condensation reaction of a-chloroacetonitrile with acrylonitriles to form cyclopropanes [4, 7] is not as pronounced as with the Darzens reaction, it can be rationalized in an analogous manner (Scheme 12.2). In the absence of the catalyst, the more highly stabilized anion (4a) is favoured leading to the preferential production of the cis isomer (5). As with the Darzens reaction, addition of the catalyst causes diffusion of the anions (4a) and (4b), as ion-pairs, into the bulk of the organic phase where their relative stabilities are similar and a more equal ratio of the two isomeric cyclopropanes (5) and (6) results (Table 12.2). 

Much of this chapter is concerned with the thermal, photolytic, photosensitized and radiolytic decompositions of pure organic hydrocarbons. Since the action of heat on cyclic hydrocarbons usually leads to reactions that occur by simpler mechanisms than is the case with the other hydrocarbons, these reactions are considered first (Section 2). This section deals with isomerizations as well as decompositions. Section 3 is concerned with cis-trans isomerizations, in which there is twisting about a double bond. The decompositions of non-cyclic hydrocarbons are dealt with in Sections 4-7, and a final section (8) treats the effects of additives, such as inhibitors, on the various decompositions and isomerizations. 

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