Substituent constant normal

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. 

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. 

Fujita and Nishioka have attempted to place ortho effects on the same numerical scale as meta and para effects. They assume that the normal ortho electronic effect can be represented by the standard substituent constant appropriate to the reaction (cr, cr", cr, cr°), that the steric effect is given by E , and that the proximity effect is measured by the Swain-Lupton Then a multiple LFER is written... 

Another rather extensive series of similar data, obtained using CS2 solutions and nujol mulls, has been published by Shindo (Fig. 4). His series include considerable data for jS-substituted compounds, for which the question of a choice of substituent constants does not arise. His data also show considerable scatter but seem to suggest strongly that <7+-values are indicated for + M substituents and normal <7-values for —M substituents. The conclusion is confirmed by the short series of similar data reported by Costa and Blasina and by Shupack and Orchin. The data of the latter authors for the NO frequencies in mws-ethylene pyridine N-oxide dichloroplatinum(II) complexes are also moderately well correlated with <7+-values. 

The Hammett equation is not successful for reactions of aliphatic compounds if the normal cr constants are used. A new scale of substituent constants, labeled cr, was invented to allow the extension of the method to such systems. The difference in the rates of hydrolysis of esters in basic versus acidic solutions is used to define the scale. The transition states are... 

In connection with normal substituent constants it must finally be mentioned that S02Me was included in an elaborate statistical analysis of a large body of data relating to substituent effects in aromatic systems87. What are claimed to be normal substituent constants emerge as am° = 0.69 and ap° = 0.73, not greatly different from those determined by Yukawa and coworkers84 or in the case of ap° from the ordinary ap value. 

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. 

Attempts have been made to correlate the effects of acceptor groups on the C=C barriers by Hammett substituent constants. Since the important interaction is concerned with the delocalization of a negative charge, the cr scale should be more appropriate than the normal a scale (136). However, in the ctp scale, CN is more efficient than COMe, which is contrary to experience, and therefore Shvo et al. preferred the ctr = ctp - o, scale (78 see also Sect. II-C-5). Similarly, in a series of ketene mercaptals, a reasonable correlation was found... 

The constant o° is another normal substituent constant determined by choosing only reaction series in which at least one methylene group insulates the reaction site from the aromatic ring. The resonance parameter, o°R, is determined from Equation 2.15 and is the resonance contribution of a substituent when it is not directly conjugated with the reaction site.15 Table 2.4 lists a number of an and u° values. Note the close correspondence between the two. 

Because of the way in which they were developed, only substituent constants are available. If a substituent is meta, then normal am values are used because there is no direct resonance interaction with a substituent in the meta position. 

Gas-phase basicities of several substituted benzaldehydes (62 X = o-/m-/p-Me/F, o-j 77 -Cl) have been measured, relative to benzaldehyde or mesitylene as reference bases, over a range of temperatures.101 The tolualdehydes are more basic than benzaldehyde, the halobenzaldehydes less so, following classical aromatic substituent effects. The data also correlate well with solution-based linear-free-energy substituent constants, as well as with theoretical (MNDO) calculations. Some deviations are noteworthy (i) the o-halobenzaldehydes (especially chloro) have higher basicities than predicted, but calculations tend to rale out the hydrogen-bonded isomer (63), which is also contraindicated by a normal A,S value, inconsistent with the expected restriction of— hOH rotation in such a structure (ii) anomalies in the high-temperature behaviour of m-fluorobenzaldehyde in the presence of mesitylene reference base are consistent with a specific catalysed isomerization to the ortho- or para-isomer. 

In 1959, four independent and simultaneous reports defined the problem, van Bekkum et al. (1959), not distinguishing among the ordinary and electrophilic side-chain reactions, showed the o--constants to be broadly variable. They tabulated the a-parameters for twelve groups as derived from each reaction of an extended series. Just as illustrated in Fig. 50 for the p-methoxy, p-methyl, and p-chloro substituents, they detected extensive variations in the apparent values of the constants. The authors concluded that the postulation of two or three sets of -constants could not be regarded as sound. By eliminating all those reactions involving important resonance contributions Wepster and his associates (van Bekkum et al., 1959) obtained a series of reactions similar to those presented in Table 23, and hence derived a series of normal, an, parameters for meta and para substituents. These normal values, presumably representing the reactions for which the Hammett relationship is most precise, exhibit standard deviations of 10-30%. 

Here (f is the normal substituent constant which does not involve any additional rr-electronic interaction between the substituent and the reaction centre Actr is the resonance substituent constant measuring the capability for TT-delocalization of rr-electron donor substituents and is defined by o- - a"... 

In (27), ct g) is a normal substituent constant and AaR(G) is a resonance substituent constant defined as (o- - ro is a resonance demand parameter, and po is a susceptibility parameter of the system in the gas phase. The gas-phase substituent parameter values are given in Table 15. 

Steric requirements, hydrogen and deuterium, 299 Stem-Volmer plot, 181 Stiff differential equations, 109 Stochastic simulation, 109 Stoichiometric coefficients, 11 Stokes-Einstein equation, 135 Stopped flow, 179 Stmetured water, 395 Structure-reactivity relationships, 311 Sublimation energy, 403 Substituent, 313 Substituent constant, 323 alkyl group, 341 electrophilic, 322 Hammett, 316 inductive, 325, 338 normal. 324 polar, 339 primary, 324 resonance, 325... 

Sawada, M., Tsuno, Y. and Yukawa, Y. (1972). The Substituent Effect. II. Normal Substituent Constants for Polynuclear Aryls from the Hydrolysis of Arylcarbinyl Benzoates. Bull. Chem. Soc.Jap., 45,1206-1209. 

Resonance effect in Hammett relation. IV. Linear free energy based on the normal substituent constants. Bull. Chem. Soc. Jap., 39, 2274-2286. 

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