Chemical Shifts of Hydrogens Attached to Tetrahedral Carbon

2 CHEMICAL SHIFTS OF HYDROGENS ATTACHED TO TETRAHEDRAL CARBON 

Notice from Example 6.3 that a positive value of A8X corresponds to a downfield shift in signal position (resulting from deshielding of the nucleus by the substituent) and a negative value corresponds to an upfield shift (shielding of the nucleus by the substituent), relative to the effect of X = H. 

a substituent (X) that is more strongly shielding than H (ctx cth) has a negative value of A8X (corresponding to an upfield shift), while a deshielding substituent (ctx cth) has a positive A8X (corresponding to a downfield shift).  

When H NMR spectroscopy was in its infancy, J. W. Shoolery began analyzing the spectra of literally thousands of compounds in an attempt to extract average values of A8X for a wide variety of common substituents in a number of different molecular environments. Over the years these values have been refined by him and others, and the concept has been extended to l3C chemical shifts as well as to those of many other common nuclei (Chapter 7). With these values in hand, 

Notice in Table 6.2 that the deshielding ability of substituents increases as you go down the table. Note also that the ASa x value phenyl (2.00) is slightly different from the value we calculated in Example 6.3 (2.12). This is because the latter value comes from a specific compound (toluene) while the former value represents an average value from many phenyl-containing molecules. 

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NMR was equally helpful (consult Table 10-6 for typical carbon chemical shifts), as it clearly revealed the two C=0 carbons at 6 = 171.0 and 173.4 ppm and indicated the presence of eight alkene and benzene carbons (8 > 95 ppm). The three tetrahedral carbons attached to oxygen showed up as peaks between 8 = 61 and 81 ppm, and the two remaining tetrahedral carbon signals occurred at S = 28.9 and 37.4 ppm All of these were identified further by establishing the number of attached hydrogens by techniques equivalent to DEPT NMR. 

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