Disubstituted ethylenes trans

Chemical shifts of cis protons in substituted ethylenes and in trans-1,2-disubstituted ethylenes have been correlated with the Hammett equation in... 

We may now proceed to an analysis of the significance of the d values. If the transition state were close to the product, then the P values of the disubstituted ethylenes should be small, because in the product, the substituent is bonded to an sp hybridized carbon atom. Thus, it is incapable of resonance interaction. As the observed values of p for the trans-disubstituted dienophiles are very large, we conclude that the transition state is closer to reactants than to products. The cis-disubstituted dienophiles show a much smaller value of P than do the trans compounds. It therefore seems likely that the transition state for the CIS compounds will be closer to product than is the transition state for the trans compounds. The values of p for the reaction of the tnws-disubstituted dienophiles with 9,10-dimethylanthracene, while large, are much smaller than... 

In this section we shall determine the effect of nonbonded interactions on the physical properties of related molecules. As our model compounds we will utilize difluoroethylene, a molecule having substituents bearing only lone pairs and dicyano-ethylene, a molecule bearing substituents with both filled and unfilled MO s. In particular, we focus on the three possible isomers of a disubstituted ethylene ie. the 1,1, trans-1,2- and cis- 1,2-isomers. The three possible isomers are shown below. 

It should be pointed out that the predicted higher ionization potential and lower electron affinity of a cis 1,2-disubstituted ethylene relative to the corresponding trans isomer is based upon the assumption that substantial pi nonbonded inter-... 

Ionization Potentials (IP) for various disubstituted ethylenes are shown in Table 24. As can be seen, the IP s for 1,2 cis and trans isomers are usually very close but the IP s of 1,1-disubstituted ethylenes relative to their 1,2-disubstituted counterparts are consistently higher, in agreement with our predictions. 

We now consider the effect of nonbonded interaction on the energies of pi lone pair MO s in cis and trans disubstituted ethylenes. Our model systems are again cis and trans 1,2-difluoroethylene. The consequence of the interaction of the pi lone pair electrons in the cis and trans geometries is shown in Fig. 36. We distinguish the following three hypothetical cases and work out their consequences. 

We first consider the relative electronic transition energies in cis and trans 1,2-disubstituted ethylenes. From Fig. 28 we can clearly see that the pi HOMO-LUMO energy gap is larger for the case of the cis isomer relative to the trans isomer. Hence, the mr transition is expected to occur at shorter wavelengths in cis 1,2-disubsti-tuted ethylenes. 

Table 26. Ultraviolet spectral data for cis and trans 1,2-disubstituted ethylenes...

A comparison of the cis- and tranr-l,2-dichloroethylenes shows the trans isomer to be the more reactive by a factor of 6 [Dawson et al., 1969]. This is a general phenomenon observed in comparing the reactivities of cis- and trans-1,2-disubstituted ethylenes. The cis isomer, which is usually also the less stable isomer, is the less reactive one toward reaction with a radical. The difference in reactivity has been attributed to the inability of the cis isomer to... 

Fig. 8-14 Syn addition to a trans 1,2-disubstituted ethylene to yield the threodiisotactic polymer. After...

The later structure might explain why trans- disubstituted ethylenes form more stable complexes than their cis- isomers (see following section). 

The effects contributed by alkyl groups to the relative rate constants, kreh for the reaction of ozone with cis- and trans-1,2-disubstituted ethylenes are adequately described by Taft s equation = k °reX -f pSo-, where So- is the sum of Taft s polar substituent constants. The positive p values (3.75 for trans- and 2.60 for cis-l,2-disubstituted ethylenes) indicate that for these olefins the rate-determining step is a nucleophilic process. The results are interpreted by assuming that the electrophilic attack of ozone on the carbon-carbon double bond can result either in a 1,3-dipolar cycloaddition (in which case the over-all process appears to be electrophilic) or in the reversible formation of a complex (for which the ring closure to give the 1,2,3-trioxolane is the nucleophilic rate-determining step). 

The results obtained for the cis- and trans-l,2-disubstituted ethylenes are indicated, for the symmetrical and the unsymmetrical olefins, respectively, in Tables III and IV and in Table V for the monosubstituted ethylenes. 

Figure 3. Verification of Equation 8 for trans-1,2-disubstituted ethylenes CH3CH=CHR (1), C2H5CH=CHR (2), i-C3H7CH=CHR (3), and tert-C H9CH=CHR (4)...

Table 17). Only in tetrasubstituted olefins with tert-butyl groups and in trans-cyclooctene (37a), are the IP values slightly shifted to lower energies. With the exception of cyclopropene (59) and cyclobutene (8), the ionization potentials of the cycloolefins with ring size 5-8 fall in the range observed for disubstituted ethylenes (Table 16). 

The 1,2-disubstituted ethylenes, XHC=CHY, exist in two forms with the cis often more stable than the trans. For example, the equilibrium composition of FHC=CHF at 200° is... 

The stability gained by the molecule through the overlapping of the orbitals in the n bond locks the molecule in a planar configuration. If the plane of one CH2 group were at 90° to the plane of the other, the orbitals would not overlap the molecule would be much less stable in such a configuration. This accounts for the absence of rotation about the double bond and makes possible the existence of geometric isomers, the cis and trans forms of disubstituted ethylene. 

Comparing cis and trans isomers of propenyl propyl ether in their reactivity towards diphenylketene, a ratio kcislknans of about 180 has been found (in PhCN, 40°C) , confirmed by the value of 172 20 from experiments without solvent with dimethylketene the ratio is 60 10 . The opposite phenomenon has been observed in Diels-Alder reactions of fumarate/maleate and other 1,2-disubstituted ethylenes (Section 4.1.3). The fact that cis isomers are thermodynamically less stable than trans isomers seems to prevail in determining relative reactivities of these 1,2-cycloadditions. 

As can be seen, the ns MO of the trans isomer is lower in energy than the ns MO of the cis isomer, a result consistent only with the presence of strong through space and through bond interaction of the two Cl atoms. This is an important result insofar as it indicates that four electron overlap repulsion is greater for trans than for cis 1,2-disubstituted ethylenes. 

A more complicated picture emerges when the polymerization of 1,2-disubstituted ethylenes (CHR=CHR ) is considered because now each carbon atom in the chain becomes a chiral center. The resulting ditactic structures are illustrated in Figure 6.1(d,ed). Two isotactic structures are obtained, the erythro, in which all the carbon atoms have the same configuration, and the threo, in which the configuration alternates. Only one disyndiotactic structure is possible. The differences arise from the stereochemistry of the starting material if the monomer is cis-substimted the threo form is obtained, whereas a trans monomer leads to the erythro structure. 

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