Ethylenes substituted, ionization potential

Certain physical properties of substituted ethylenes may be correlated with the extended Hammett equation. Included in this category are dipole moments and ionization potentials. 

A number of correlations of ionization potentials for substituted benzenes (40-42), benzyl (43), phenoxy (44), and alkyl (45) radicals and substituted pyridines (46) with the simple Hammett equation have been reported. Charton (47) has studied the application of the extended Hammett equation to substituted ethylenes and carbonyl compounds. The sets studied here are reported in Table II (sets 2-10 and 2-11). Results of the correlations are set forth in Table 111. The results obtained are much improved by the exclusion of the values for X = C2 H3, Ac, F, H and OAc from set 2-10 (set 2-lOA) and the value for X = H from set 2-11 (set 2-11 A). The composition of the electrical effect corresponds to that found for the Op constants as is shown by the pR values reported in Table IV. 

The ionization potentials of substituted cyclopropanes also show a significant correlation with eq. (2). The value of pr obtained is comparable to that observed for substituted ethylenes and 1-substituted propenes (section II.A.2.) and is considerably above that found for substituted benzenes (for which a value of Pr = 59 is obtained). This result confirms the existence of a large resonance interaction between the cyclopropane ring and substituents. The magnitude of a is considerably greater for substituted cyclopropanes than it is for substituted ethylenes or benzenes. 

SiM 3 Fig. 1. Correlation diagram giving electron attachment energies (AE) and ionization potentials (IP) of a-,—SiMe3 and jS-silyl substituted ethylenes... 

In this section we shall examine the effects of n—n and n—n interactions on the ionization potentials of substituted ethylenes and benzenes. A theoretical analysis has already been given in section 1.1. In the space below we survey some pertinent data. 

From the ionization potential listed in Table 40 it can be seen that substitution of ethylene or benzene by an electron releasing group of the first period lowers the ionization potential. Also, as the energy gap between the ethylene n MO and the substituent pz AO decreases the change in ionization potential increases. Thus, the energy change for F substitution is 0.22 eV while that for OMe substitution is 1.59 eV. 

Table 40. Ionization potentials of substituted ethylenes and benzenes (eV)...

This result is similar to that obtained by Widing and Levitt for the normal alkanes and to that observed for alkyl-substituted ethylenes [171] in the latter case, linear correlations were obtained between the IPs and the sum of charges of the unsaturated carbon atoms, whereby any increase of their electron density due to substituent effects leads to a lowering of the molecular ionization potential. 

First ionization potentials of substituted ethylenes (set PP13, Table 6) were correlated with the LDRA equation (equation 38). The regression equation is equation 77 ... 

We have also employed ETS to study the effect of fluorine substitution on the ir orbitals of benzene and ethylene (10). Here we briefly discuss the results for the fluoroethylenes. Fluorine substitution is known to cause only small shifts in tv ionization potentials (IP) of unsaturated hydrocarbons (1 1). For example, the vertical iv IP s of ethylene and perfluoroethylene agree to within 0.1 eV. The reason that has been most often forwarded to explain this is that the electron withdrawing inductive effect, which stabilizes the occupied orbitals, is nearly cancelled by the destabilizing resonance mixing of the fluorine p orbitals with the ir orbitals of the ethylenic double bond. 

A more detailed analysis has to take into account the number of carbon atoms in the substituents and their branching. The a-donating and hyper-conjugative interaction of the alkyl substituents appears to play an important role in the dependency of the IP values on substitution pattern. With a few exceptions, the out-of-plane particularly twist distortions of tetrasubstituted ethylenes appear to be too small for a shift in ionization potential (213)... 

Rate constants ( 2) for cycloadditions of phenyl azide to substituted ethylenes and acetylenes stretch over seven magnirndes high values were observed for enamines, moderate k2 for the acrylic ester type, and the rate minimum was found for common alkenes and alkynes (Huisgen, Szeimies, Mobius, 1967). hi the PMO treatment of concerted cycloadditions, Reiner Sustmann found the key to the understanding of substituent effects e.g., a plot of k2 for the cycloadditions of Ph-Ns versus the ionization potential of substituted ethylenes and acetylenes furnished a degenerate U shape (1971). Such a plot is a distinguishing feature for each 1,3-dipole and reflects the specific mix of nucleophilic and electrophilic activity, modified by steric effects. 

Ortho-cycloaddition takes place with a olefin which has low ionization potential in comparison to benzene where polar nature of reaction overpowers the symmetry imposed barrier to this reaction. Polar nature of ortho-cyclo-addition is supported by the fact that in case of doner substituted ethylenes, reaction is promoted by polar solvent, but in meta-addition no solvent effect is there, o-and p-photocycloadditions are disallowed to occur as concerted addition between of benzene and Sq of alkene until mixing of charge-transfer states occurs. 

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