Substituent constants , definition

Taft began the LFER attack on steric effects as part of his separation of electronic and steric effects in aliphatic compounds, which is discussed in Section 7.3. For our present purposes we abstract from that treatment the portion relevant to aromatic substrates. Hammett p values for alkaline ester hydrolysis are in the range +2.2 to +2.8, whereas for acid ester hydrolysis p is close to zero (see Table 7-2). Taft, therefore, concluded that electronic effects of substituents are much greater in the alkaline than in the acid series and. in fact, that they are negligible in the acid series. This left the steric effect alone controlling relative reactivity in the acid series. A steric substituent constant was defined [by analogy with the definition of cr in Eq. (7-22)] by Eq. (7-43), where k is the rate constant for acid-catalyzed hydrolysis of an orr/to-substituted benzoate ester and k is the corresponding rate constant for the on/to-methyl ester note that CH3, not H, is the reference substituent. ... 

Many other definitions of an ortho substituent constant have been made Shorter has reviewed these. Charton analyzed Oo in terms of Oi and CTr, i.e., = a(Ti -I- fpoR, finding that the distribution of inductive and resonance effects (the ratio a/b) varies widely with the substituent and, therefore, that no general Oo scale is possible. Charton also subjected to analysis according to Eq. (7-47),... 

Probably the most important development of the past decade was the introduction by Brown and co-workers of a set of substituent constants,ct+, derived from the solvolysis of cumyl chlorides and presumably applicable to reaction series in which a delocalization of a positive charge from the reaction site into the aromatic nucleus is important in the transition state or, in other words, where the importance of resonance structures placing a positive charge on the substituent - -M effect) changes substantially between the initial and transition (or final) states. These ct+-values have found wide application, not only in the particular side-chain reactions for which they were designed, but equally in electrophilic nuclear substitution reactions. Although such a scale was first proposed by Pearson et al. under the label of and by Deno et Brown s systematic work made the scale definitive. 

In summary we think that, on a superficial basis, a comparison of the effects of different nucleophilic species added covalently at the (3-nitrogen atom of an arenedi-azonium ion yields results that are almost trivial. Of more interest are unexpected results such as those of Exner and Lakomy for the substituent -N = CHC6H5. A possible explanation for the latter results emerged when the twisted structure of the substituent became known. We emphasize, however, that definitive explanations on the basis of Hammett or related substituent constants are not found very frequently. 

A common probe of reaction mechanisms used to infer charge distribution in the transition state involves variation of substituent groups near the reaction center. From the variation in reaction rate produced by electron-donating and electron-withdrawing groups or by the steric hindrance of various sized groups, transition state characteristics can be inferred. Two empirical correlations have been proposed and refined which provide a common framework for this process. The Hammett equation is applied to aromatic systems [45]. The Taft correlation is applied to aliphatic systems [45], Definitions of terms, collections of substituent constants (steric and electronic effects for various substituents), and listings of observed reaction response parameters (for typical reaction types) have been collected [45]. 

The electrophilic substituent constants, given in Table 1, were defined by a set of apparent substituent constants, i.e. (1/p) log(A /Aro), derived from the solvolysis rates of a,a-dimethylbenzyl(a-cumyl) chlorides [2] (Scheme 1) in 90% aqueous acetone at 25°C. For the definition, the reaction constant p = -4.54, based exclusively on meta and ir-electron withdrawing (tt-EW) para substituents, was applied. 

Equation (2) for a reaction giving a set of apparent substituent constants o can be rewritten in the form, (o- t/ ) = r a - cr ), where r is constant for the reaction regardless of substituents. As the increment of any o-from (f should be a reasonable measure of the resonance capability of the respective substituents, this proportionality represents a linear resonance energy relationship. The original form (Yukawa and Tsuno, 1959, 1965) using cr instead of (f in (2) has the same significance since the proportionality relation holds for the resonance increment (a - t/ ) or (a - cr). The definition of resonance substituent constants Ao by any set of (o— o") is arbitrary, and the definition of the r scale is also arbitrary. While the definition of the r = 0 scale by [Pg.269]

The extent to which a given reaction responds to electronic perturbation constitutes a measure of the electronic demands of that reaction, which is determined by its mecha-, nism. The introduction of substituent groups into the framework and the subsequent alteration of reaction rates helps delineate the overall mechanism of reaction. Early work examining the electronic role of substituents on rate constants was first tackled by Burckhardt and firmly established by Hammett (13, 14, 77, 78). Hammett employed, as a model reaction, the ionization in water of substituted benzoic acids and determined their equilibrium constants K. See Equation 1.28. This led to an operational definition of o-, the substituent constant. It is a measure of the size of the electronic effect for a given substituent and represents a measure of electronic charge distribution in the benzene nucleus. 

This approach to separating the different types of interactions contributing to a net solvent effect has elicited much interest. Tests of the ir, a, and p scales on other solvatochromic or related processes have been made, an alternative ir scale based on chemically different solvatochromic dyes has been proposed, and the contribution of solvent polarizability to it has been studied. Opinion is not unanimous, however, that the Kamlet-Taft system constitutes the best or ultimate extrathermodynamic approach to the study of solvent effects. There are two objections One of these is to the averaging process by which many model phenomena are combined to yield a single best-fit value. We encountered this problem in Section 7.2 when we considered alternative definitions of the Hammett substituent constant, and similar comments apply here Reichardt has discussed this in the context of the Kamlet-Taft parameters. The second objection is to the claim of generality for the parameters and the correlation equation we will return to this controversy later. 

This definition is analogous to that used for a electronic substituent constants and accounts for the formation ability of a charge-transfer complex (CTC), such as the n -complex formation ability of aromatic systems. 

In the early 1950s Taft (10) outlined a sound quantitative basis for the estimation of steric effects and for separating them from polar and resonance effects. This derivation follows the standard extrathermodynamic approach and is therefore empirical. The definition of steric substituent constants is closely related to polar substituent constants, for they are obtained from the same reference system (10). The polar substituent constants, however, have been shown to arise from electronic effects, and they strongly correlate with the inductive substituent constants (10, 43, 61). 

The advantage of this definition of substituent constants is that they are free of the electronic effects that are present in E (see below, section C.2). Their use is limited to symmetrical substituents, such as CHo, CCl (i.e., XYg). In asymmetric groups the definition of the raaius rv is not possible. The similarity of these parameters to the Eg scale would suggest similar problems, but since they are not based on specific experimental measurements, improvement of the scale is more difficult. 

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