Substituent groups electron-contributing

Results for these CEBEs are presented in Table 1. As can be seen, for the carvone variants I-V the various substitutions have absolutely no effect at the carbonyl C=0 core, and are barely significant at the chiral center that lies between the carbonyl and substituent groups in these molecules. Only upon fluorine substitution at the tail (molecule VI) does the C=0 CEBE shift by one-half of an electronvolt the second F atom substitution adjacent to the C=0 in the difluoro derivative, VII contributes a further 0.6-eV shift. This effect can be rationalized due to the electron-withdrawing power of an F atom. Paradoxically, it is these fluorine-substituted derivatives, VI, VII, that arguably produce b curves most similar to the original carvone conformer, I, yet they are the only ones to produce a perturbation of the ground-state electron density at the C li core. This contributes further evidence to suggest that, at least for the C li... 

In particular the last observation is easily understood by the molecular orbital picture given above. The 7z>orbitals of the monomers are required for a stabilization of the otherwise anti-bonding cluster orbitals of t2 symmetry which must accept six electrons. If this mixing is prevented because these orbitals adopt the electron density of the ligands, e.g., 7z>electrons of the side-on coordinated cyclopentadienyl groups, their contribution to the cluster stability is minimized or in particular cases the formation of clusters does not occur at all. Thus, the substituents attached terminally to the clusters strongly influence their stability by the different donor or acceptor capabilities. A further effect may result from the different steric demand of the substituents which will be discussed below. 

All of these studies have used alkyl groups as the substituents on the C=C bond, which, however, differ only slightly in their polar effects. In order to find out the extent of electronic contribution to the overall reactivity, a broader range of substituents is necessary. The literature yields earlier data of this type (5) for the hydrogenation of imsaturated compounds CH2=CH X (where X = —CH NHj, —CHjCOOH, —CHjCN, —CHjOH,... 

The problem of testing the application of a linear free-energy treatment for substitution reactions was solved by reversing the usual Hammett procedure. In the conventional approach, log (kjkn) is plotted against a. It is evident that an alternative would be to examine the application of a linear correlation by maintaining the group constant and to plot log (k/ka) versus p for different reactions. A satisfactory linear relationship would indicate constancy of the electronic contribution of the substituent. The slope of the line would define a substituent constant for the substituent in question. 

The displacement of metalloid groups from naphthalene, however, is unusually slow. The rate differential between the 1- and 2-positions is little more than a factor of three for protodesilylation or protodegermyla-tion. Even more important is the failure to observe a steric acceleration for these reactions. Benkeser and Krysiak (1954) showed that the rate of protodetrimethylsilylation reaction was increased by o-methyl substituents to an extent greater than the anticipated electronic contribution. Presumably, steric strains are relieved in the transformation from trigonal to tetrahedral geometry in the transition state (de la Mare, 1958). The failure to observe this acceleration and a greater lf-N/2f-N ratio for these reactions is puzzling. 

The relative importance of a and r contributions to the overall bonding is unclear, but several different combinations of relative strengths lead to limiting case models. When there are 2 electrons in the forward (T-bond and 2 electrons in the ir-backbond, there are 2 bonding electrons for each metal-carbon bond. This is mathematically equivalent to 2tr-bonds and a metallocyclopropane structure (72). This model does not necessitate strict sp3 hybridization at the carbon atoms. Molecular orbital calculations for cyclopropane (15) indicate that the C—C bonds have higher carbon atom p character than do the C—H bonds. Thus, the metallocyclopropane model allows it interactions with substituent groups on the olefin (68). 

A methyl or a f-butyl substituent in 99 in an ortho position dramatically increases the barrier. Thus, for a given model the modification of the barriers can be used to determine a steric scale of substituents in this particular geometrical context provided the electronic contribution of the substituent on the cross-conjugation capability of the heterocycle can be determined. This approach can be applied with success to the steric effect of alkyl groups whose electronic contribution should be very similar. 

In summary the results observed in these studies [160] of poly(Sty-co-DVB) swelling in aromatic liquids serve to show that the method of measuring a is so sensitive that it can detect an effect caused by even the smallest modification in the molecular geometry of attached substituents, and that these differences correlate qualitatively with expectation based on the known principles of physico-organic chemistry of aromatic compounds. Since the observed a is the net effect of electronic attraction and steric hindrance between the sorbed molecule and the adsorption site, i.e. the monomer unit of the polymer, it would be impossible to separate quantitatively the electronic and steric contributions of a particular substituent. The ability to make such a differentiation, however, appears to be more promising with liquids that comprise homologous series of the type Z(CH2)nH (where Z is a phenyl, chloro, bromo or iodo substituent), since the added electronic contribution to Z by each additional methylene group is well known to be extremely small when n becomes >3 [165],... 

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