Good Bookkeeping Practices

In general, it is a good practice to assume that a six-membered ring adopts a chair conformation if this is the case, we will want to differentiate between axial and equatorial H s. We build a model if we can—this model helps clarify the picture of the molecule we develop. 

For each column of predictions, we fill in as many guesses as we can. Making initial guesses is a good way to improve our predictive skills later we can compare the correct answers with our predictions to see where we went wrong. Without looking at any spectra or consulting any tables, 1 have filled in as many of the boxes as 1 can in columns 2, 4, and 6 of Table 9.1 and columns 2 and 4 of Table 9.2. 

From simple chemical shift arguments, we can hope to readily identify the H s on carbons 2, 6, and 8 because these H s are on carbons bound to heteroatoms. To save time and space, we write the aforementioned H s as H2 s, H6 s, and H8 s. Because oxygen is more electronegative than nitrogen, we expect to find the H8 s farther downfield than the H2 s or H6 s. We also expect to find the H2 s slightly farther downfield relative to the H6 s because the H2 s are also adjacent to a methine group (position 3) rather than a methylene group (position 5). 

The order of the chemical shifts (from left to right, from greatest chemical shift to smallest) can be written as 

HTs resonance can be almost anywhere in the spectrum because HI is on the nitrogen atom the chemical shift of the resonance of a bound to a heteroatom defies accurate prediction. Hydrogen bonding may prevent the electronegative heteroatom from withdrawing electron density. 

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The molecular orbital picture we developed in Figure 9.7 does a good job of showing the stabilization, but it is not particularly well suited for bookkeeping purposes. In practice, the ease of bookkeeping—of easily drawing the next chemical reaction—makes the resonance picture simpler to use. The price is that you have to either draw two structures (or more in other cases) or adopt some other code that indicates the presence of more than one representation for the molecule. We will see many examples in the next pages and chapters. 

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