Cyclopentanone spectrum

An example of Fermi resonance in an organic structure is the doublet appearance of the C=0 stretch of cyclopentanone under sufficient resolution conditions. Figure 3.2 shows the appearance of the spectrum of cyclopentanone under the usual conditions. With adequate resolution (Fig. 3.3), Fermi resonance with an overtone or combination band of an a-methylene group shows two absorptions in the carbonyl stretch region. 

FIGURE 3.3. Infrared spectrum of cyclopentanone in various media. A. Carbon tetrachloride solution (0.15 M). B. Carbon disulfide solution (0.023 M). C. Chloroform solution (0.025 M). D. Liquid state (thin films). (Computed spectral slit width 2 cm-1.)... 

The first absorption region of cyclopentanone extends from about 3500 A. to 2300 A. The absorption spectra in the vapor state and in heptane solution appear to be identical and show definite banded structure on both sides of the region of maximum absorption (24). A detailed analysis of the spectrum has been carried out (26a). 

The details of the photochemical primary process in cyclopentanone are fairly well understood (33). Since the spectrum in the first absorption region appears to be similar to that of the simple aliphatic ketones, it has been assumed that initial excitation is to the upper singlet state and that the zero-zero band is near 3600 A. 

The ultraviolet absorption spectra of cyclohexanone in the vapor phase and in heptane solution appear to be identical and are situated in the same region of the spectrum as that of cyclopentanone, although more band structure is apparent in the latter instance (24). 

The ultraviolet absorption spectrum of cyclobutanone has been reported only in heptane solution (24). The absorption is appreciably shifted towards shorter wavelengths as compared to cyclopentanone, but shows a similarly banded structure. 

Cyclic Ketones. The molecular ion peak in cyclic ketones is prominent. As with acyclic ketones, the primary cleavage of cyclic ketones is adjacent to the C=0 group, but the ion thus formed must undergo further cleavage in order to produce a fragment. The base peak in the spectrum of cyclopentanone and of cyclohexanone (Figure 1.22) is mlz 55. The mechanisms are similar in both... 

FIGURE 2.3. Infrared spectrum of cyclopentanone in various media. A. Carbon tetrachloride solution (0.15 M). 

Irradiation of the cyclohexenone (45) gave the cyclopentanone (46) with a very similar mass spectrum (Burlingame et al., 1967). It was proposed that loss of ketene from the molecular ion of (45) occurred via... 

Prasad et al. (1982) have described Raman investigations of solid state reactions. They show that it is advantageous to obtain the spectra at lower temperature, typically at 120 K. Lower frequency phonon spectra show considerable broadening at room temperature, so that details of spectral features are lost. Raman phonon spectra of 2-benzyl-5-benzilidene cyclopentanone at room temperature and at 77 K (see Fig. 6.8-18) indicate that lower temperature improve.s the resolution of the spectral features. This phenomenon can be used for a detailed analysis in order to elucidate the reactivity of a compound at room temperature. However, it is necessary to make sure that neither the reactant nor the product undergoes a structural phase transition between room temperature and the low temperature at which the spectrum is recorded. This is confirmed by studying the phonon spectra as a function of the temperature. 

Rees and Yelland have reported a fascinating elimination of CO2 from nonadjacent carbonyl groups that occurs both on electron impact and thermolysis Eq. (56). Cyclopentanone p5nrolysis is substantially complicated by radical chain reactions.ii > Nonetheless the three most abundant p5n olysis products, ethylene, carbon monoxide and 1-butene are all represented in the six most intense fragment ions in the mass spectrum of cyclopentanone. 

A broad spectrum of a-diazo-/5-ketoesters (e. g., 4), -sulfones, and -phosphonates (e. g., 6) have been converted in one-step procedures and in decent yields into cyclopentanones such as 5 and 7, respectively (eqs. (2) and (3)). 

The clues to the nature of this functionality were evident from the PMR spectrum, which showed a doublet (/ = 7.6 Hz) methyl group at 1.12 ppm and the 1750 cm-1 carbonyl absorption characteristic of a cyclopentanone. At this point structure 243 was proposed. Reduction of meloscandonine with sodium borohydride gave two isomeric alcohols [244 and 249 (M+ 322)] each of which was readily acetylated (to 246 and 247, respectively). Evaluation of the PMR spectra of these compounds defined the stereochemistry at C-20. 

The methanol adduct produced by the photoisomerization of isocolchicine in methanol exhibited a styrene chromophore and a cyclopentanone carbonyl group (38) no olefinic protons were detectable in the NMR-spectrum. Catalytic hydrogenation resulted only in reduction of the carbonyl group acid hydrolysis of the dihydro compound (with... 

Section 2.13A (p. 53). Ketene is included in Figure 2.44 because it is an extreme example of an exo double-bond absorption (see p. 38). The s character in the C=0 group increases as the ring size decreases, until it reaches a maximum value that is found in the 5p-hybridized carbonyl carbon in ketene. The spectrum of cyclopentanone (Fig. 2.41) shows how ring strain increases the frequency of the carbonyl group. 

In general, the classes of compounds Included furfurals (9.4Z), phenols (2.5Z), methoxyphenols (16.9Z), cyclic compounds such as methyl cyclopentanones (10.8Z), methoxy benzenes (3.8Z), and the substituted propane tentatively Identified for the peak at scan number 1207 (36.8Z). Although the compound eluting at scan 1207 Is by far In the highest concentration. Insufficient Information Is available from Its spectrum to allow a reasonable Identification. The base peak observed Is 75amu, and a 115 amu Ion Is also present at 40Z abundance. These results, which are preliminary, differ somewhat from other published compositions of pyrolysis oils (3. 7-8). Future work will be required to verify the compositions observed above. 

Two ions in the spectrum of cyclopentanone are m/z 55 and 56. Write reasonable mechanisms for their formation. 

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