¹³C NMR of α‐haloketones

Abstract

The carbon chemical shifts of several series of a-haloketones have been measured. Whereas the carbon carrying the halogens shows a regular downfield shit with increasing electronegativity of the substituents, the adjacent carbonyl carbon is shifted upfield by chlorine, bromine and iodine, and is little affected by fluorine substitution. The conformational implications of these results are discussed.C chemical shifts of the carbonyl carbon of ahalocarbonyl compounds, probably due to lack of availability of suitable compounds, have received little systematic attention. Yet from the few available data it seems that further work on these compounds is necessary in order to understand the unusual shielding effects observed. Thus, a -chlorine substitution in acetophenones leads to a 4.5 f 1 ppm upfield shift per chlorine atom.' Similar substitution in acetic acid and acetyl chloride leads to a slightly less shielding effect.' Stothers and Lauterbur discuss these observations in terms of a decrease in the 7~ bond polarization of the carbonyl group due to the electronegativity of the chlorines.2 Having a number of a -haloketones3 available in our laboratory for another study, the I3C NMR spectra of these compounds were examined. The results for four series of haloketones are shown in Tables 1-4. In each Table the ketones are arranged in the order of increasing substituent electronegativity. (The electronegativity difference between carbon and the respective halogen(s) has been used to obtain the net effect of the halogen electronegativity).The data for the carbon carrying the substituents, with few exceptions, show a regular decrease in shielding in the order given. In fact a plot of the chemical shifts vs electronegativities of the substituents gives a near linear fit. Chlorine, bromine and iodine substitution causes an upfield shift of the carbonyl resonance. However, quantitatively, the effect does not follow the electronegativity increase as suggested by Stothers and Lauterbur.' The magnitude of this upfield shift is nearly the same for all three halogens and appears to be proportional to the number of substituents. Monofluorine substitution causes little if any (0-3 ppm) upfield shift. In the case of di-or trihalogen substitution, when one or more of the halogens are fluorines, the carbonyl carbon is less shielded than in the corresponding ketones in which all halogens are either chlorine or bromine.In the two ethyl ketone series (Tables 2 and 3 ) the methyl carbon next to the substituted carbon is shifted downfield by about 12-15 ppm by monochlorine, bromine or iodine substitution. The more electronegative fluorine causes only a 9-10ppm shift. Simi-* Author to whom correspondence should be addressed. larly, the methyl carbons of dihalo-substituted ethyl ketones are considerably less deshielded (8-9 ppm) when one of the halogens is fluorine. For the three methyl ketone series (Tables 1, 2 and 4) the methyl carbons are somewhat less shielded when the halogen three bonds away is chlorine, bromine or iodine than when it...

Carbon‐13 NMR spectra of polybromoalkanes and polychlorobromoalkanes. Structural increments of halogens in polyhalogenated groups

“…and e t h a n e~.~' Several papers have been published on monobrorno derivatives (with the halogen situated at the primary and secondary carbon atom)3.24.31 and a,o-dibr~moalkanes.~ Recently, a paper has appeared describing the 13C NMR spectra of chlorobromo aliphatic ketones. 32 Experimental data obtained in the present study I3C chemical shift of 38 ppm for CHzCl to 105 ppm for CC13), the signals of bromine-containing groups fall in a rather narrow spectral range from 25 to 50 ppm (apart from the exceptions discussed below). Thus, the signals of CBr3 found by us lie in the region from 36.6 to 41.6ppm; those of CHBrz from 39.4 to 50.2; of CH2Br from 24.6 to 39.6 and those of CHBr from 40.5 to 57.4 ppm, depending on the environment.…”

C-13 Substituent Effects in Multifunctional Marine Natural Products

“…1 This work bRef. 12 tal vs. expected 13C shift is also apparent for the C-6 methyl. Based upon analogy to the preplocamenes and arguments presented therein (10), a Z or E C-6 methyl should occur at 18 or 13 ppm, respectively, neither of which is close to its reported shift of 26.7…”