Ketones deshielding

NMR Like other carbonyl groups the carbon of the —CO2H group of a car boxylic acid is strongly deshielded (8 160-185) but not as much as that of an aldehyde or ketone (8 190-215)... 

Methyl ketones, such as 2-butanone in Figure 17.18, are characterized by sharp singlets near- 8 2 for the protons of CH3C=0. Similarly, the deshielding effect of the car bonyl causes the protons of CH2C=0 to appear- at lower field (8 2.4) than in a CH2 group of an alkane. 

Hydrogens on the carbon next to a carbonyl group are slightly deshielded and normally absorb near 2.0 to 2.3 8. The acetaldehyde methyl group in Figure 19.18, for instance, absorbs at 2.20 8. Methyl ketones are particularly distinctive because they always show a sharp three-proton singlet near 2.1 8. 

The impact of a carboxylic acid function upon the chemical shift of a CF2H group is almost indistinguishable from the impact of a ketone, whereas secondary CF2 groups next to an acid or ester function are slightly deshielded relative to those next to a ketone or aldehyde (Scheme 4.35). 

H and 13C NMR Data. The ester function of ethyl difluoroac-etate deshields the CF2H proton slightly (about 0.1 ppm), whereas as was the case for ketones and aldehydes, it shields the carbon of either a CF2H or a CF2-alkyl group significantly (by about lOppm) (Scheme 4.37). 

Intraactions between elections in three-membered rings and unsaturated groups in the same molecule have been detected via 13C chemical-shift variations in a number of instances. Thus, introduction of the carbonyl function in tricy-clo[3.2.1,02,7]decane (e.g., 274) leads to significant downfield shifts of the signals of C(l) (+8.0), C(2) (+15.5), and C(7) (+7.7) (385), whereas corresponding effects in bicyclo[3.1.0]hexan-2-one (275) are smaller (385,386). A corresponding dependence was reported for 276 and 277 and related to more effective electron withdrawal in 276 (387). An even more pronounced deshielding effect was observed by Murata and co-workers (388,389) in the ketone 278 when they compared it with 279. 

Carbonyl carbons, such as those in esters, carboxylic acids, amides, ketones and aldehydes, in which the sp carbon is bonded by a ir-bond to the highly electronegative O, have a characteristic chemical shift of S 160-220 (Box 4.13). Within this range, the sp carbon atoms of aldehydes and ketones are most deshielded and usually resonate at 8 185-220, while those of carboxylic acids and their derivatives resonate about 160-180. 

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]. 

The carbonyl carbon signal in trimethylsilyl phenyl ketone 294 (S = 238 ppm) is 42 ppm deshielded compared with that in acetophenone 295 (8 = 196 ppm)113... 

A series of thioketones have also been protonated to form carbosulfonium ions 358541,542 and studied by 13C NMR spectroscopy.665 It was found that deshielding of the thiocarbonyl carbon is much less pronounced than that of ketones. This shows that there is significant mercaptocarbenium 358b contribution to the overall protonated thioketone structure. However, this is not the case in the case of protonated ketones. 

The replacement of a tert-butyl group by SiMe3 going from di-ferf-butyl ketone to rerr-butanoyl trimethylsilane and from di-terf-butyl thioketone to ferf-thiobutanoyl trimethylsilane lg increased Xmax and deshielded the quaternary carbon. Furthermore, an inversion in polarization of the C=S bond and a decrease of the HOMO-LUMO energy differences were observed in the ferf-thiobutanoyl trimethylsilane with respect to di-rm-butyl thioketone. 

The data in Table 21 indicate that C(2) is shielded, and C(3) is deshielded, in the Z-isomer relative to the Eav-isomer. It has been found113 that in compounds of general formula XCH=CHY, where X and Y are, respectively, an electron-withdrawing and an electron-donating group, the olefinic carbon chemical shifts of isomeric E,Z pairs are simply related by an equation bz = bE + A. For enamino ketones 343 (and other structurally related compounds113), the relationship for C(2) is... 

In these condensation reactions, the relative size of macrocycles could be varied by simply changing the length of the linking chain. Specifically, the effect that the changes in the macrocycle size have upon the deshielding of the internal protons were investigated by the preparation of a series of non-conjugated ketones [150],... 

In their NMR spectra, carboxyiic acid derivatives give a highly deshielded peak at 160-180 ppm due to the carbonyl carbon. This is somewhat upheld from the carbonyl absorption of aldehydes and ketones, which occurs at 190-215 ppm. 

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