Conjugating substituents, effect

The pK values of phenols in singlet and triplet states are valuable guide to substituent effect in the excited states, specially for the aromatic hydrocarbons. In general, the conjugation between substituents and -electron clouds is very significantly enhanced by electronic excitation without change in the direction of conjugative substituent effect. The excited state acidities frequently follow the Hammett equation fairly well if exalted substituent constants a are used. 

The attempt to correct experimental data to zero ionic strength is fundamental to the treatment, even though often this can be done only approximately. The model of non-conjugative substituent effects which is used is a combination of Lewis s model of the inductive effect as a through-bonds displacement of electrons, and the electrostatic model of the field effect as devised by Bjerrum in his treatment of the first and second ionization constants of aliphatic dicarboxylic acids in water. As Wepster acknowledges, these models have their limitations, but he claims that their combination has nevertheless led to a very successful treatment. 

The dissociation constants of eight 6-substituted spiro[3,3]heptane-2-carboxylic acids in 50% aqueous ethanol at 25 " C have been measured and used to correlate non-conjugative substituent effects. Detailed analysis of the results suggests that a field effect rather than a a-inductive effect is the more reliable model. The first and second dissociation constants of squaric acid (3,4-dihydroxycyclobut-3-ene-l,2-dione) in aqueous 3M-NaC104 have been measured by e.m.f. methods.Results were in agreement with earlier ones after correction for ionic strength. 

A familiar feature of the electronic theory is the classification of substituents, in terms of the inductive and conjugative or resonance effects, which it provides. Examples from substituents discussed in this book are given in table 7.2. The effects upon orientation and reactivity indicated are only the dominant ones, and one of our tasks is to examine in closer detail how descriptions of substituent effects of this kind meet the facts of nitration. In general, such descriptions find wide acceptance, the more so since they are now known to correspond to parallel descriptions in terms of molecular orbital theory ( 7.2.2, 7.2.3). Only in respect of the interpretation to be placed upon the inductive effect is there still serious disagreement. It will be seen that recent results of nitration studies have produced evidence on this point ( 9.1.1). 

These and other studies of the relative substituent effects of X and CH X in nitration were considered in terms of the transmission factor a of the methylene group. To avoid complications from conjugative interactions, attention was focussed mainly on substitution at the meta-position, and ct was defined in terms of partial rate factors by the equation ... 

In the second, which belongs to a systematic study of the transmission of substituent effects in heterocyclic systems, Noyce and Forsyth (384-386) showed that for thiazole, as for other simple heterocyclic systems, the rate of solvolysis of substituted hetero-arylethyl chlorides in 80% ethanol could be correlated with a constants of the substituent X only when there is mutual conjugation between X and the reaction center. In the case of thiazole this situation corresponds to l-(2-X-5-thiazolyl)ethyl chlorides (262) and l-(5-X-2-thiazolyl)ethyl chlorides (263). 

Table 36 summarizes the known annular tautomerism data for azoles. The tautomeric preferences of substituted pyrazoles and imidazoles can be rationalized in terms of the differential substituent effect on the acidity of the two NFI groups in the conjugate acid, e.g. in (138 EWS = electron-withdrawing substituent) the 2-NFI is more acidic than 1-NFI and hence for the neutral form the 3-substituted pyrazole is the more stable. 

Another example of enhanced sensitivity to substituent effects in the gas phase can be seen in a comparison of the gas-phase basicity for a series of substituted acetophenones and methyl benzoates. It was foimd that scnsitivtiy of the free energy to substituent changes was about four times that in solution, as measured by the comparison of A( for each substituent. The gas-phase data for both series were correlated by the Yukawa-Tsuno equation. For both series, the p value was about 12. However, the parameter r" ", which reflects the contribution of extra resonance effects, was greater in the acetophenone series than in the methyl benzoate series. This can be attributed to the substantial resonance stabilization provided by the methoxy group in the esters, which diminishes the extent of conjugation with the substituents. 

Conjugated substituents at C-2, C-3, C-4, or C-5 accelerate the rearrangement. Donor substituents at C-2 and C-3 have an accelerating effect. The effect of substituents can be rationalized in terms of the stabilization of the transition state by depicting their effect on two interacting allyl systems. 

The previous analysis by the dual substituent parameter equation of substituent effects in the naphthalene series provided support for the scale, especially for sets involving nonconjugating positions (2p). The available data yield six basis sets which presumably give a critical analysis and, in particular, provide distinctions between conjugative (three sets) and nonconjugative positions (three sets). The results (using the earlier symbolism (2p)) are given in Table X. 

Eq. (1) has potential application to other types of measurements of substituent effects besides those specifically considered in this paper e.g., nmr coupling constants and shifts for other nuclei, ir and uv spectral shifts and intensities. We caution (with emphasis) in these applications the needed use of data sets of high quality, both with respect to the precision of the measurement and substituents considered (i.e., a full complement of substituent o/ and Or properties must be encompassed for a meaningful correlation to be obtained). There is, of course, no requirement that all data sets will be uniquely fitted by eq. (1) using one of the four or scales of Table V. For example, the data for the ionization of the conjugate acids of pyridine-N-oxides (30), HjO, 25° is found to fit equally well the or(ba.) or Or scales (SD=. 14 /=. 072). The data (31) for the rates of alkaline ("OMe) cleavage of ArSnMea are not fitted to acceptable precision (fs >. 23) by any of the Or parameters. This data set is nevertheless indicated... 

Molecules with donor and acceptor in trans- and a s-configurations give much higher nonlinearities, due to favorable linear donor-acceptor conjugation, than those with substituents at the geminal position, where only the weaker cross-conjugation is effective (see 89 vs 86 and 92, Fig. 8). 

The effect of a conjugating substituent in the monomer may be summarized by observing that its influence is much greater in the product radical than in the monomer. In the activated complex, which is intermediate in character between reactants and product, resonance stabilization is substantially greater than in the monomer reactant, though less than in the product radical. The substituent therefore lowers the activation energy for the process, and enhances thereby the reactivity of the monomer. 

These substituent effects are due to the stabilization of the carbocations that result from protonation at the center carbon. Even if allylic conjugation is not important, the aryl and alkyl substituents make the terminal carbocation more stable than the alternative, a secondary vinyl cation. 

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