Cycloalkanones shifts 218

The chemical shift dependence of the carbonyl resonances on ring size in cycloalkanones is particularly remarkable In the series of cycloalkanones, cyclopentanone is found to have the largest carbonyl shift (219.6 ppm). The CO signals of cyclobutanone and cyclohexanone are both observed at higher field (x 209 ppm). The carbonyl carbons of cy-clooctanone and cyclononanone are much more deshielded than those of cyclohexanone, cycloheptanone, cyclodecanone and cycloundecanone. The carbonyl resonances of the twelve to seventeen membered ring ketones occur at S values similar to those of acyclic ones [282, 288]. 

Of the cycloalkanones, cyclopentanone has a pronounced shift to the left. Table 4.19 presents chemical shifts of the C=0 group of some ketones and alde-... 

On the other hand, transfer of electron density from the substituent to the antibonding it orbital of the carbonyl renders the C=0 bond weaker and leads to the red-shift. Such shifts are obvious in aryl and vinyl ketones in comparison to their dialkyl counterparts (Figure 12.6). Of course, one has to be cognizant of other electronic effects that can influence the carbonyl group as well. For example, hybridization effects can mask stereoelectronic trends. In particular, strained cycles made of the p-rich banana bonds have to allocate more s-characto in the exocyclrc bonds. Increased s-character makes bonds stronger and shorter, which is reflected in the blue-shifted carbouyl stretching frequencies for the strained cycloalkanones. 

Weigert J F, Roberts J D 1970 Nuclear magnetic resonance spectroscopy. Carbon-13 chemical shifts in cycloalkanones. J Am Chem Soc 92 1347-1351... 

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