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CHAPTER 3 95 
 
shown are not meant to be precise (due to other structural factors that can affect these values), but rather, they should 
be considered as rough approximations. 
 
 
 
The proton on the nitrogen atom adjacent to the C=O group is also a good candidate for one of our four most acidic 
protons, because it is bound to an electronegative atom (nitrogen). In this case there is no obvious molecule of 
comparison in Table 3.1. However, we can still make a reasonable prediction of pKa range by using our knowledge of 
the relationship between structure and acidity. First of all, we can predict that the pKa of the indicated proton is greater 
than 4.75 by making the following comparison: 
 
 
 
A second comparison with another molecule in Table 3.1 allows us to predict that the pKa of the proton of interest is 
lower than 19.2. 
 
 
 
After determining the approximate pKa values for each of the four most acidic protons, we can now rank them in order 
of increasing acidity. Recall that the lowest pKa value is associated with the most acidic proton: 
 
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