Constant inductive substituent

Charton inductive constants electronic substituent constants ( inductive electronic constants)... 

LINEAR FREE ENERGY RELATIONSHIPS Hammett substituent constants Taft substituent constants Inductive and resonance substituent constants Steric substituent constants Homolytic substituent constants Field and resonance components Aqueous solubility The parachor... 

This work already showed that substituent constants of one reaction can only be transferred to another reaction when similar effects are operating and when they are operating to the same extent. In order to find a broader basis for the transfer-ability of substituent constants, they were split into substituent constants for the resonance effect and those for the inductive effect. 

Decades of work have led to a profusion of LEERs for a variety of reactions, for both equilibrium constants and reaction rates. LEERs were also established for other observations such as spectral data. Furthermore, various different scales of substituent constants have been proposed to model these different chemical systems. Attempts were then made to come up with a few fundamental substituent constants, such as those for the inductive, resonance, steric, or field effects. These fundamental constants have then to be combined linearly to different extents to model the various real-world systems. However, for each chemical system investigated, it had to be established which effects are operative and with which weighting factors the frmdamental constants would have to be combined. Much of this work has been summarized in two books and has also been outlined in a more recent review [9-11]. 

One underlying physical basis for the failure of Hammett reaction series is that substituent interactions are some mixture of resonance, field, and inductive effects. When direct resonance interaction is possible, the extent of the resonance increases, and the substituent constants appropriate to the normal mix of resonance and field effects then fail. There have been many attempts to develop sets of a values that take into account extra resonance interactions. 

Another approach to evaluating cri was taken by Roberts and Moreland,who defined inductive substituent constants in terms of the acid dissociation constants of 4-substituted bicyclof2.2.2]octane-l-carboxylic acids, 3. 

The nonspecialist reading Table 7-7 will probably be impressed by the substantial consistency among cti values evaluated by different methods, but the specialist tends to concentrate on the differences. There is one very interesting difference in Table 7-7, that for cti of alkyl groups based on Eq. (7-33) compared with cti based on the ionization of 3, the latter values showing practically no effect of inductive electron release and certainly no trend with increased branching. (The uncertainties associated with these substituent constants can be found in the original literature.) Swain... 

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. 

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),... 

Research on the nature of substituent constants continues, with results that can bewilder the nonspecialist. The dominant approach is a statistical one, and the main goal is to dissect substituent effects into separate electronic causes. This has led to a proliferation of terms, symbols, and conclusions. A central issue is (here we change terminology somewhat from our earlier usage) to determine the balance of field and inductive effects contributing to the observed polar electronic effect. In... 

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. 

B. The Separation of Inductive and Resonance Effects Substituent Constants from Spectroscopic Studies. 

However, the duality of substituent constants and the attempt to deal with crossconjugation by selecting cr+, a or a in any given case is somewhat artificial. The contribution of the resonance effect of a substituent relative to its inductive effect must in principle vary continuously as the electron-demanding quality of the reaction center is varied, i.e. whether it is electron-rich or electron-poor. A sliding scale of substituent constants would be expected for each substituent having a resonance effect and not just a pair of discrete values a and a for — R, or o and a for + R substituents55. 

Exner devoted some attention to SOPh and S02Ph in his extensive compilation of substituent constants of 197879. For the former, however, values based directly on chemical reactivity were found to be few, as is already apparent in the present section, and so Exner considered it useful to give values based on appropriate summation of inductive and resonance constants (see Section III.B). These were considered to be normal values, i.e. 7m° and a°, of 0.51 and 0.50 respectively. Corresponding values for S02Ph were 0.59 and 0.66 respectively, both slightly smaller (for no obvious reason) than the chemically based values of am and ap (and a value supposed to be of a°) quoted earlier in this section. 

Oj and aR are respectively the inductive and resonance constants of Taft s analysis of ordinary composite Hammett constants (values obtained by Charton were used) and v is the steric substituent constant developed by Charton161,162. The intercept term, h, notionally corresponds to log k for H as an orfho-substituent, but is not found in practice always to agree closely with the observed value of log k for the parent system. 

Taft (21) has suggested that the electrical effect of a substituent is composed of localized (inductive and/or field) and delocalized (resonance) factors. Thus we may write the substituent constant of the group X as... 

Finally, in this account of multiparameter extensions of the Hammett equation, we comment briefly on the origins of the a, scale. This had its beginning around 1956 in the a scale of Roberts and Moreland for substituents X in the reactions of 4-X-bicyclo[2.2.2]octane-l derivatives. However, at that time few values of o were available. A more practical basis for a scale of inductive substituent constants lay in the o values for XCHj groups derived from Taft s analysis of the reactivities of aliphatic esters into polar, steric and resonance effects . For the few o values available it was shown that o for X was related to o for XCHj by the equation o = 0.45 <7. Thereafter the factor 0.45 was used to calculate c, values of X from o values of XCH2 . ... 

A large number of modifications and refinements have been made on this equation, the most relevant being those which attempt to separate inductive < j and mesomeric Taft equation. In addition to substituent constants based on reactivity, acidity and the like, a variety of spectroscopically derived constants such as those from... 

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