1.2- Dichloroethene, isomers

The dichloroethene isomers (compounds with the same formula but different structures) can be further degraded under reducing conditions into chloroeth-ene (vinyl chloride), and the oxidation state of the carbon is thereby reduced to (— I) ... 

Data obtained from Doughty, D.M. et al. 2005. Effects of dichloroethene isomers on the induction and activity of butane monooxygenase in the alkane-oxidizing Bacterium "Pseudomonas butanovora " Applied Environmental Microbiology. October 6054-6059. 

Figure 1.5 Geometric isomers the cis and trans isomers of 1,2-dichloroethene.

Vitamin B12 catalyzed also the dechlorination of tetrachloroethene (PCE) to tri-chloroethene (TCE) and 1,2-dichloroethene (DCE) in the presence of dithiothreitol or Ti(III) citrate [137-141], but zero-valent metals have also been used as bulk electron donors [142, 143]. With vitamin B12, carbon mass recoveries were 81-84% for PCE reduction and 89% for TCE reduction cis-l,2-DCE, ethene, and ethyne were the main products [138, 139]. Using Ni(II) humic acid complexes, TCE reduction was more rapid, leading to ethane and ethene as the primary products [144, 145]. Angst, Schwarzenbach and colleagues [140, 141] have shown that the corrinoid-catalyzed dechlorinations of the DCE isomers and vinyl chloride (VC) to ethene and ethyne were pH-dependent, and showed the reactivity order 1,1-DCE>VC> trans-DCE>cis-DCE. Similar results have been obtained by Lesage and colleagues [146]. Dror and Schlautmann [147, 148] have demonstrated the importance of specific core metals and their solubility for the reactivity of a porphyrin complex. 

When reaction occurs from the triplet manifold, the further problem arises of whether the reactive states are the spectroscopic triplets, 3cis and 3trans, or whether reaction occurs from a single phantom triplet. Direct irradiation of the S0— T1 absorption bands of cis- and trans-1,2-dichloroethene gives the same (trans/cis) quantum yield for each isomer, indicating that the phantom... 

Ring opening of ortho adducts may also have occurred in the experiments reported by Perrins and Simons [192], who irradiated benzene, toluene, and anisole in the presence of dichloroethenes (all isomers), trichloroethene, and tetrachloroethene. The products were chlorine-substituted linear tetraenes, but no ortho adducts were found. An analogous reaction occurred with benzonitrile and 2-methylbut-2-ene, but, in contrast to the results reported by Atkinson et al. [73], the ortho adduct was not detected. 

Calculations have shown that the rotational barrier of the C-O bond in methanol (1.1 kcal/mol) is significantly lower than the corresponding rotational barrier of methyl hypofluorite (MeOF, 3.7 kcal/mol) or methyl hypochlorite (MeOCl, 3.5 kcal/mol), in which a strong [Pg.19]

Two isomers of 1,2-dichloroethene are known. One has a dipole moment of 2.4 D the other has zero dipole moment. Draw the two isomers and explain why one has zero dipole moment. 

The effect of bond polarity is even more apparent in the 1,2-dichloroethenes, with their strongly polar carbon-chlorine bonds. The cis isomer has a large dipole moment (2.4 D), giving it a boiling point 12 degrees higher than the trans isomer, with zero dipole moment. 

Due to the high aetivation barrier for cisitrans isomerization reactions at carbon-carbon double bonds ca. 260-270 kJ/mol [82]), it is often impossible to measure directly the non-catalyzed thermal equilibration reaction in solution. For 1,2-dichloroethene, however, the relative stability of its cis and trans isomers in various solvents has been determined by means of calorimetric measurements of heats of solution [202]. Surprisingly, these measurements show a quite similar solvent effect on both diastereomers, even though the cis isomer is a dipolar molecule and the trans isomer is not. Therefore, the position of this cisjtrans equilibrium should not be very solvent-dependent. 

Which structural isomer of dichloroethene has a dipole Hint consider Section 13.4.)... 

The coupling of terminal acetylenes with the geometrical isomers of 1,2-dichloroethene affords the 2 1 coupled enediyne product in a stereospecific manner. Thus, the reaction of TMSA with (Z)-dichloroethene gives the symmetrically substituted (Z)-enediyne product under normal conditions. In the absence of copper(I) iodide, no reaction occurs [Eq. (43)] [64]. 

Figure 21-9 Two isomers of 1,2-dichloroethene are possible because rotation about the double bond is restricted. This is an example of geometric isomerism. A ball-and-stick model and a space-filling model are shown for each isomer, (a) The cis isomer, (b) The tram isomer.

The meta photocycloaddition of ( )-l,2-dichloroethene with benzonitrile, toluonitriles, benzene, phenol, cresols, fluorobenzene, chlorobenzene, trifluoromethylbenzene and 3-trifluoro-methylbenzonitrile shows not only the influence of the aromatic substituents on the direction of the alkene attack, but also a control of the cyclopropane closure by the vinyl chlorine to give exclusively the exo-(>, ent/o-7-dichloro adduct isomer 5 which was, in the most favorable cases, isolated in gram quantities. ... 

The apparent nonreactivity of frflMs-l,2-dichloroethene (and of Dieldrin and Aldrin) is not immediately explained by this mechanism in view of the facile reactivity of cis-l,2-dichloroethene. Steric effects may account for the differential reactivities of cis- and trfl s-l,2-dichloroethene addition of superoxide ion would be expected to relieve steric strain between the two chlorines in the cis isomer. However, this reactivity pattern also is consistent with base-catalyzed elimination reactions of these systems.32... 

Hence, the elimination reaction (dehydrohalogenation) of cis-1,2-dichloroethene [Eq. (7-9)] is much more facile than that of the trans isomer because trans elimination can occur with the former but not the latter.32 Subsequently, chloroethyne can react with superoxide ion via nucleophilic attack to give the observed products (Table 7-1). 

The two molecules shown above are cA-l,2-dichloroethene and /ra ,s-l.2-diclilorocthcnc. These two molecules are geometrical isomers because the two carbon atoms cannot be rotated relative to each other, due to the rigidity caused by the pi bond between them. Therefore, they are not superimposeable - they are not identical, and cannot take each other s place. Cis/trans isomers have different chemical and physical properties and can exhibit dramatically different biological activity. 

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