Cyclobutanone, absorption

The purity of cyclobutanone was checked by gas chromatography on a 3.6-m. column containing 20% silicone SE 30 on chromosorb W at 65°. The infrared spectrum (neat) shows carbonyl absorption at 1779 cm. - the proton magnetic resonance spectrum (carbon tetrachloride) shows a multiplet at 8 2.00 and a triplet at S 3.05 in the ratio 1 2. 

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. 

Ketones. Normal acyclic ketones can be recognised by a strong band at 1720 cm -1 (e.g. see the spectrum of 4-methylpentan-2-one, Fig. 3.28). Branching at the a-carbon atoms results in an increase in the C—CO—C bond angle and this results in a decrease in frequency of absorption from the normal value of 1720 cm -1 to, for example, 1697 cm -1 as in di-t-butylketone. Conversely as the C—CO—C bond angle is decreased the absorption frequency rises, thus cyclo-pentanone and cyclobutanone absorb at 1750 cm-1 and 1775 cm-1 respectively. 

FIGURE 1. Ultraviolet absorption spectra of cyclobutanone and cyclopentanone in hydrocarbon solvents. 

From a comparison of the spectroscopy of cyclo-butanone to that of cyclopentanone, we see apparently similar absorption and emission predissociative behavior for cyclobutanone when Xex > 313 nm, while no such effect is found for cyclopentanone. If we suggest that three competing pathways account for the predissociative behavior, we have done little more than to offer a detailed but uninformative representation of the observations. Furthermore, we would be forced into adding more primary acts when interpreting the [1,3]-shifts and aldehydeforming reactions discussed above. 

The unknown is cyclobutanone, A. The symmetry indicated by the carbon NMR rules out structure B. The IR absorption of the carbonyl at 1790 cm" is characteristic of small ring ketones ring strain strengthens the carbon-oxygen double bond, increasing its frequency of vibration. (See Section 12-9 in the text.)... 

The high CH stretching frequencies of three-membered rings of this type reflects their well-known hmsaturated character , and for the same reason they are sometimes difficult to differentiate from aromatic or olefinic absorptions. Absorption above 3000 cm can also arise in special cases in four-membered rings which contain polar groups able to influence the hybridisation of the CH bonds. 3-butyrolactone [121] is such a case with CH bands at 3014 and 2933 cm Other examples are 3-methylene oxirane [122] and cyclobutanone [123]. 

Simple ketones absorb at 1710-1715 cm" simple aldehydes absorb at 1720-1725 cm. Aldehydes also have a characteristic absorption near 2710 cm for the aldehyde C—H bond. Cyclohexanones have carbonyl absorptions at the same position as simple acyclic ketones. However, decreased ring size results in shifts to higher wavenumber. Cyclopentanone, cyclobutanone, and cyclopropanone absorb at 1745, 1780, and 1850 cm , respectively. 

Figure IX-E-I. Approximate absorption cross sections versus wavelength for cyclopropanone vapor (adopted from Thomas and Rodriguez, 1971) cyclobutanone measured in cyclohexane solution (adopted from Carless and Lee, 1972) cyclopentanone in cyclohexane solution (adopted from Nakashima et al., 1982) cyclohexanone vapor was measured independently in two laboratories National Center for Atmospheric Research (NCAR) and Ford Motor Company (Ford), (Iwasaki et al., 2008a) acetone vapor (Martinez et al., 1992).

Figure IX-E-1 shows the approximate cross sections for cyclobutanone (in cyclohexane solution) versus wavelength as redrawn from the publication of Carless and Lee (1972). The general structure and magnitude seen in this absorption band is in qualitative agreement with that reported by Benson and Kistiakowsky (1942). Note in the figure that there is much less shift of the absorption band from that of acetone than is observed for cyclopropanone. The tabular listing of the cross sections is given in table IX-E-2.

Carless and Lee (1972) have studied the photodecomposition of 2-n-propylcyclobutanone at 325 nm. The absorption cross sections of this ketone are significantly larger than those of cyclobutanone (amax 1-07 x 10 cm molecule" ), and the maximum in the absorption band is shifted to the longer wavelengths ( 295 nm). Calvert et al. (2008) conclude that the following overall primary processes are consistent with the results of Carless and Lee (1972) ... 

The ultraviolet absorption spectrum of cyclohexanone reflects the n jt transition common to all carbonyls see figure IX-E-1. The data derived from gas-phase measurements of the cross sections for cyclohexanone from two different research groups [National Center for Atmospheric Research (NCAR) and Ford Scientific Laboratories (Ford)] are in reasonable agreement (Iwasaki et al., 2008). The cyclohexanone cross sections as measured in cyclohexane solution by Benson and Kistiakowski (1942) had indicated seemingly low values (cross sections shown here is significantly less than those observed for cyclopropanone, cyclobutanone, and cyclopentanone, and in fact, all other carbonyls considered in this work. It is not obvious why these significant differences exist in the probability for the n -> 7T transition for cyclohexanone and that of the other cyclic ketones and most other carbonyl compounds. Theoretical studies will be important in defining the reasons for these differences. 

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