Conjugation substituent constants

Methods have been presented, with examples, for obtaining quantitative structure-property relationships for alternating conjugated and cross-conjugated dienes and polyenes, and for adjacent dienes and polyenes. The examples include chemical reactivities, chemical properties and physical properties. A method of estimating electrical effect substituent constants for dienyl and polyenyl substituents has been described. The nature of these substituents has been discussed, but unfortunately the discussion is very largely based on estimated values. A full understanding of structural effects on dienyl and polyenyl systems awaits much further experimental study. It would be particularly useful to have more chemical reactivity studies on their substituent effects, and it would be especially helpful if chemical reactivity studies on the transmission of electrical effects in adjacent multiply doubly bonded systems were available. Only further experimental work will show how valid our estimates and predictions are. 

The Hammett substituent constant o is a measure of the electron-with-drawing capacity of substituents directly conjugated to the reaction center [84], Since the values of this parameter were obtained from the rate constants of base-catalyzed hydrolysis of other aryl esters, it can be concluded from Eqn. 8.5 that the same electronic factors are involved in both cases. 

So for each p-substituent we now have available two, alternative, substituent constants—electron-withdrawing substituents or cTp.x and o-p.x for electron-donating substituents— whose use depends on whether through-conjugation between p-substituent and reaction centre does, or does not, take place in a particular reaction. It would be nice to think that these dual substituent constant values would now take care of all eventualities, and an analysis was therefore made of no less than eighty different... 

Conversely, if a. para substituent stabilizes the conjugate base of an acid-base pair rather more than it stabilizes the benzoate ion, more positive substituent constants are required to achieve linearity in Hammett plots. Examples of this are acid dissociations of phenols and anilinium ions, where mesomerically electron-withdrawing substituents (Y = —NO2, —C N) are more effective in enhancing acid strength than they are in benzoic acid, because charge delocalization of the type [15] is not possible in the benzoate anion. 

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 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. 

A number of other diazocompounds have been studied. Warren (1961) has shown that the reaction of 9-diazofluorene in ethanol and in ethanol-water mixtmes is similar to that of diphenyldiazomethane. The rate of reaction is slower in deuteriated solvent and the operation of general acid catalysis was inferred from measurements with picric acid. As would be expected from the conjugation between the diazo and dibenzo-cyclopentadienyl groups in 13, the rate of reaction is slower than that of DDM (by a factor of about twelve) and there seems little doubt that both substrates react with a rate-determining proton transfer. Bate studies with substituted diazofluorenes yield a value of — p = l 6-l-9 depending on the substituent constants used (Warren, 1963 Warren and Yandle, 1965). This value is similar to that for the reaction of substituted diphenyldiazomethanes in toluene and is a further indication that the stability of diazoalkanes is quite sensitive to substituent effects. 

Analysis of 19F chemical shifts show that the 1-aziridinyl group is not as conjugatively electron-donating as either theNH2 orNMe2 group51. The polar and resonance substituent constants based on 19F chemical shifts of the respective p-substituted fluorobenzenes are given in Table 551. 

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