Ortho substituents, calculation equations

Using the a values for ortho substituents given in Appendix 3, Table 10 calculate the pA values of the following (experimental values given in brackets) 2-chloro-4-nitrobenzoic acid (1.96), 2,6-dimethylpyridine (6.77), 2,4-dichloroanilinium ion (2.05) and 2,3,5-trichlorophenol (6.43). Hammett equations for the dissociation of substituted phenols, pyridinium ions, anilinium ions and benzoic acids are given in Appendix 4, Table 1. 

We have stated earlier that because of proximity effects, no generally applicable aj values may be derived for ortho substitution. Nevertheless, one can determine a set of apparent 0)ortho values for a specific type of reaction, as for example, for the dissociation of substituted phenols. Table 8.7 gives such apparent O)ortho constants for estimating pKa values of substituted phenols and anilines. Of course, in cases of multiple substitution, substituents may interact with one another, thereby resulting in larger deviations of experimental from predicted pKa values. Some example calculations using the Hammett equation are given in Illustrative Example 8.2. 

The predictions reached on the basis of the induced CD data were verified through calculations conducted using the Kirkwood-Tinoco equations. In a related work, CD induced through complexation with B-cyclodextrin in a series of ortho, meta, and para-substituted benzenes was compared [33]. The magnitude of the induced CD effects revealed that strong complexation took place only for para substitution, and that the presence of substituents at the ortho or meta positions interfered with the inclusion process. 

The substituent does not enter the ortho-position for steiic reasons. The relative content of para-i omer can be calculated from the equation... 

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