The Electronic Effect

In a M.o. treatment of the electronic effect of the methyl group it was found necessary to take into account both inductive and hypercon-jugative effects. This treatment is commented on in 9.3 below. 

Table 17 3 compares the equilibrium constants for hydration of some simple aldehydes and ketones The position of equilibrium depends on what groups are attached to C=0 and how they affect its steric and electronic environment Both effects con tribute but the electronic effect controls A hydr more than the steric effect... 

Consider first the electronic effect of alkyl groups versus hydrogen atoms attached to C=0 Recall from Section 17 2 that alkyl substituents stabilize C=0 making a ketone carbonyl more stable than an aldehyde carbonyl As with all equilibria factors... 

Cl Acid Blue 40 (119) has a greener and somewhat duller shade than the parent dye (ie. Cl Acid Blue 25) (118), which is considered to be due to the electronic effect of the i ra-acetylamino group. 

The ortho effect may consist of several components. The normal electronic effect may receive contributions from inductive and resonance factors, just as with tneta and para substituents. There may also be a proximity or field electronic effect that operates directly between the substituent and the reaction site. In addition there may exist a true steric effect, as a result of the space-filling nature of the substituent (itself ultimately an electronic effect). Finally it is possible that non-covalent interactions, such as hydrogen bonding or charge transfer, may take place. The role of the solvent in both the initial state and the transition state may be different in the presence of ortho substitution. Many attempts have been made to separate these several effects. For example. Farthing and Nam defined an ortho substituent constant in the usual way by = log (K/K ) for the ionization of benzoic acids, postulating that includes both electronic and steric components. They assumed that the electronic portion of the ortho effect is identical to the para effect, writing CTe = o-p, and that the steric component is equal to the difference between the total effect and the electronic effect, or cts = cr — cte- They then used a multiple LFER to correlate data for orrAo-substituted reactants. 

Within some limited series of substituents it appears that Es is correlated with a, which is not unreasonable, because the electronic effect of a group is in part related to the size of the group. DeTar has discussed this matter. Kramer has demonstrated well-defined familial relationships between E s and a and concludes that Es possesses some polar character. 

The separation of mixtures involving N-methyl-JLtetrahydropyridines into their pure components by means of gas-liquid chromatography was discussed in a report by Holik et al. (87). They found that, using tris(/3-cyanoethoxymethyl)-y-picoline as the stationary phase, the primary factors involved in the specific retention volumes of these enamines is the electronic effect of a methyl substituent and the nitrogen atom on the carbon-carbon double bond. It was observed that 1,3-dimethyl-Zl -tetrahydropyridine (141) has a smaller specific retention volume and, hence, is eluted before... 

The electronic effects (8+ on carbon and S on nitrogen) that favor the hydration of heteroaromatic molecules and of Schiff bases to give Dimroth bases are the same as those that would favor the ringopening of the hydrated heteroaromatic molecules and cleavage of the C—bond in Dimroth compounds. 

In most of the papers discussing tautomerism in dihydropyrimidines, the possibility of the existence of 4,5-dihydro isomer 47c (Scheme 19) was not even considered or was ruled out on the basis of H NMR spectra. In 1985, however, Kashima et al. (85TL5057) reported that, although dihydropyrimidines 47 with r = H or Pr (R = R = R = Me, R = H) indeed exist only as mixtures of 47a and 47b tautomers, for analogs with r = Ph, OEt, or SMe, 4,5-dihydro tautomers 47c were also observed in CDCI3 solution in relative amounts of 10%, 20%, and 31%, respectively. The proportion of this tautomer rises to 45% in the case of the 2-dimethylamino-substituted derivative. The electronic effects of a heteroatom or an aromatic group in the 2 position were proposed as an explanation for this phenomenon. No 4,5-dihydropyrimidine has ever been found in the solid state. 

Within the context of this book the quantitative relationships between structure and chemical reactivity are very informative. One of the early postulates of Ingold and his school in the 1930s (review see Ingold, 1969, p. 78) was that the electronic effects of substituents are composed of two main parts a field/inductive component and a mesomeric component. Hammett s work indicated clearly from the beginning that his substituent constants am and crp reflect Ingold s postulate in numerical terms. In particular, many observations indicated that the /7-substituent constant ap is the sum of a field/inductive component 0 and a resonance (mesomeric) component (Jr. 

The values of para positions, are thus available from pX measurements of benzoic acids. One cannot so easily extend the correlation to ortho groups, however, because steric effects are superimposed on the electronic effects under study. Table 10-1 presents a list3 of crm and ap values. Note that the groups considered as electron-withdrawing relative to H have positive <7 values, and vice versa. Thus, we have a(p-CF ) — +0.54 and cr(/n-CHi) = —0.069. 

The electronic effects of many substituents have been examined by studies of PMR118,119 sulfinyl and sulfonyl groups have been included in some of these. For example, Socrates120 measured the hydroxyl chemical shifts for 55 substituted phenols in carbon tetrachloride and in dimethyl sulfoxide at infinite dilution, and endeavored to... 

However, in more recent years it has become usual to employ ar or crR-type constants, either together in the dual substituent-parameter equation or individually in special linear regression equations which hold for particular infrared magnitudes. In this connection a long series of papers by Katritzky, Topsom and their colleagues on Infrared intensities as a quantitative measure of intramolecular interactions is of particular importance. We will sample this series of papers, insofar as they help to elucidate the electronic effects of sulfinyl and sulfonyl groups. 

In this chapter it is clearly impossible to do more than sample the extensive literature on the carbon acidity of sulfinyl and sulfonyl compounds, as it illuminates the electronic effects of these groups, particularly in connection with linear free-energy relationships. There are three main areas to cover first, as already indicated, equilibrium acidities (pKa values) second, the kinetics of ionization, usually studied through hydrogen isotopic exchange and finally, the kinetics of other reactions proceeding via carbanionic intermediates. 

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