Cyclohexanone carbonyl absorption

Explain why a carbonyl absorption shifts to lower frequency in an a,p-unsaturated carbonyl compound—a compound having a carbonyl group bonded directly to a carbon-carbon double bond. For example, the carbonyl absorption occurs at 1720 cm" for cyclohexanone, and at 1685 cm" for 2-cyclohexenone. 

Ring C was investigated through a-dihydrocaranone (LXX), prepared by the Oppenauer oxidation of a a-dihydrocaranine. The ketone showed carbonyl absorption at 5.87 p, characteristic of a cyclohexanone. A neutral by-product of the oxidation was shown to be LXXI, the isolation of which provided additional evidence for the 1-hydroxyl group of caranine. Finally, LXX formed an enol acetate in wMch the double bond was conjugated with the aromatic ring. 

In 1953, Corey reported that conformational equiUbrium in a-chloro and bromo-substituted cyclohexanones strongly favors (>97%, 2.3kcal/mol) the axial conformer. IR-analysis of the carbonyl stretching frequencies indicated that this conformational effect is accompanied by a red-shift in the carbonyl absorption, indicative of the C = O bond weakening due to the Jt interaction. 

By examining model compounds with fixed conformations and from earlier work, Corey determined that there were distinct differences in the carbonyl frequencies of cyclohexanones that had a bromine in the a-position, and that depended upon whether the bromine was in an axial or in an equatorial position. Model compound smdies showed that the parent cyclohexanone itself had a carbonyl stretching frequency at 1712cm in carbon tetrachloride, and cyclohexanones that contained an axial bromine in the a position showed their carbonyl absorption at very nearly the same place, about 1712-1716cm . On the other hand, when the bromine was in the equatorial position, the absorption was found about 1728-1730 cm If one had a mixture of conformations with the bromine partly equatorial and partly axial, one could usually more or less resolve these two frequencies. This information could be used to tell in an equilibrium situation whether the bromine was mostly axial or mostly equatorial, and by approximately how much. (The assumption is made here that the inherent intensity of the vibrational band is the same in both the axial and equatorial arrangements. It was later shown that this is not quite true, and the equatorial stretching band has a somewhat... 

A comparison has been made of carbonyl frequencies of cyclohexanones and their complexes with boron trifluoride. Spectra of the complexed ketones show disappearance of the free carbonyl absorption and replacement by a band at ca. 70 cm lower wavenumbers. This change is associated with a diminution of the force constant of the carbonyl bond. A u.v. spectroscopic study has also been made of cyclohexanone boron trifluoride complexes in CHjClj. For cyclohexanone a hypsochromic shift of the n ti band is noted on complexa-tion, as indicated by values of 287.3 nm, s = 17 for the free ketone and A 240.5 nm, e = 116 for the ketone boron trifluoride complex. Titanium tetrachloride also acts as a Lewis acid and forms complexes with ketones. However, the 50cm shift in the carbonyl frequency observed on complexation is taken as indicating that the oxygen-titanium bond is weaker than the oxygens boron bond. 

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. 

In a discussion of these effects, it is customary to refer to the absorption frequency of a neat sample of a saturated aliphatic ketone, 1715 cm"1, as normal. For example, acetone and cyclohexanone absorb at 1715 cm"1. Changes in the environment of the carbonyl can either lower or raise the absorption frequency from this normal value. A typical ketone spectrum is displayed in Figure 2.20. 

Prolonged thermolysis of compound 151 (R = R = Ph) results in the formation of a pair of dimeric products which have been formulated as compounds 160 (47%) and 161 (4%) and which can be regarded as being formed by addition of the indenone carbonyl group to the betaine. Precedent for this type of addition is formation of the cyclohexanone adduct 154. Evidence for structure 160 is provided by the observation that acid hydrolysis gives a monohydrate which is formulated as compound 162—this product 162 displayed. . . ultraviolent absorption Isomer 161 does not form a monohydrate. 

In order to determine the conformational equilibrium of a-halocyclohexanones, Corey used infrared spectroscopy, since the substitution of one a-hydrogen in a cyclohexanone with a halogen produced a frequency shift in the absorption of the carbonyl group, where the frequency shift magnitudes depended upon whether or not the a-halogen atom was axial or equatorial (Table 1.1). 

Aliphatic ketones such as acetone and cyclohexanone have carbonyl stretching infrared absorption at about 5.84 /x (1712 cm ). When conjugated with an aromatic ring the wavelength increases to about 5.91 fi (1692 cm ). By contrast, as indicated in Table III which lists the infrared and ultraviolet spectral properties of all known acylsilanes, and Table IV, which lists the same properties for acylgermanes and acyl-stannanes, strong bands attributed to carbonyl stretching were found for silyl alkyl ketones (Si—CO—alkyl) at about 6.08 (1645 cm ) and for... 

Explain why the carbonyl stretching absorption of cyclohexanones are shifted approximately 20 cm" to higher wavenumber when a bromine atom is substituted in the equatorial position at the a carbon atom. 

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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