Ketones infrared frequencies

Since carbocations are structurally similar to ketones, they are discussed here. Schleyer s force field incorporates carbocation parameters, and Harris is exploring their application in conformational analysis (188). The calculated angles in a series of rigid polycyclic carbocations correlated well with ketone infrared frequencies (188a). The calculated relative stabilities among various conformers of tertiary cations of methylcyclohexane, methylcycloheptane, and methyl-cyclooctane do not contradict the limited MNR observations of these species at low temperature (188b). 

An example of degenerate 1,2-methyl shifts are arenonium ions of the type A it has been found that the electronic influence of the MCR of the carbocation on the rate of rearrangement is characterized not only by 8C but also by such structural parameters as the <7 -constants of substituents and the infrared frequencies of C = 0, V( of ketones related to the rearranging carbocations . ... 

Table 48. Infrared carbonyl stretching absorption frequencies of cyclic ketones ) ...

Figure 4 illustrates the infrared spectrum for a sample of PPE. The absorptions of the peaks at 3.4, 6.9 and 7.3 pm were assigned to C-H stretch and C-H bending frequencies in CH2 and CH3 (33). These absorptions are proportional to the surface density of deposited ethane (16). However, the absorptions at photons near 10 pm are attributable to OH deformations and CO stretchings of alcoholic groups and vibrations of alkyl ketones (22). They also indicate the existence of branches in unsaturated chain (33). 

The widespread use of infrared spectroscopy at that time was probably due to the observation that many chemical groups absorb in a very narrow range of frequency. Furthermore, within this frequency range, the observed frequency may be correlated to specific chemical structures. For example, aldehydes can be differentiated from ketones by the characteristic stretching frequency of the carbonyl group near 1700 cm-1, and the spectral pattern may be likened to a molecular fingerprint. ... 

Different types of carbonyl groups give characteristic strong absorptions at different positions in the infrared spectrum. As a result, infrared spectroscopy is often the best method to detect and differentiate these carboxylic acid derivatives. Table 21-3 summarizes the characteristic IR absorptions of carbonyl functional groups. As in Chapter 12, we are using about 1710 cm-1 for simple ketones and acids as a standard for comparison. Appendix 2 gives a more complete table of characteristic IR frequencies. 

A similar coupling may be observed for a, /J-unsaturated aldehydes and ketones. Table 4.1-3 shows that the s-cis or s-trans conformation of the C=C-C=0 systems is indicated by the difference of the frequencies of the C=0 and the C=C stretching vibration as well as by its relative intensity in the infrared and the Raman spectrum and the depolarization factor in the Raman spectrum (Oelichmann et al., 1982). 

An infrared study including 2-furyl f-butyl ketone furnished evidence for changes in both ring and carbonyl stretching bands that could well signal an increasing tendency for the bulkier substituents to rotate out-of-plane. The study is of special interest because it is based not only upon vibrational frequencies, as is common, but mainly upon intensities, which is not. In the f-butyl ketone, the carbonyl stretching frequency and its intensity (1662 cm -1 A 6,500) are both lower than in the methyl ketone (1674 cm" 1 A 10,500).357... 

Norlutin and Enovid are ketones that suppress ovulation. Consequently, they have been used clinically as contraceptives. For which of these compounds would you expect the infrared carbonyl absorption (C = 0 stretch) to be at a higher frequency Explain. 

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