Tautomerism in Symmetrical Amidines

This was also the simplest the addition of rtg, and/or attempting to distinguish between substituents able or unable to form an intramolecular hydrogen bond 

Rearrangement of Eq. (5.10) gives Eq. (5.11) whereas rearrangement of Eq. (5.6) leads from this to Eq. (5.12). The result is two equations, Eqs. (5.11) and (5.12), which spell out alternative ways of estimating /Cj for each individual compound, the first based on p/C differences while the second employs a standard Hammett relation. Since there is bound to be scatter in the first but the second is in some sense smoothed these will not always give the same result but that is inevitable and no great cause for concern. 

Returning back to Eqs. (5.2) and (5.3), is equal to 2.0. It makes sense that C=N should transmit electronic effects better than C-N, though for the ratio to be exactly 2 seems a happy coincidence. Later work on a more extended series [4b] gave -1.44 and p -3.08, again very close to p = 2. However, when in 4 [4c] simple substituent cr-values were replaced, as R, by a mixture of those for alkyl and for substituted phenyl rings [5], the best statistical fit was found when ctj was employed. Since 4, Eq. (5.4), like 1, Eq. (5.1), is a major tautomer it is reasonable that its equation should take a similar form the very large value of p for both reflects the much smaller sensitivity of CTj than of tj or to changes in while 

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