Cyclohexanone is a cyclic ketone with the molecular formula C6H10O. It is a colorless, oily liquid with a smell similar to acetone. It is used as an intermediate in the production of nylon and other synthetic fibers, as well as in the synthesis of various drugs, dyes, herbicides, pesticides, plasticizers, and rubber compounds.
One of the methods to characterize the structure and properties of cyclohexanone is infrared (IR) spectroscopy. IR spectroscopy is a technique that measures the absorption of infrared radiation by molecules, which causes them to vibrate at specific frequencies. The frequencies of these vibrations depend on the type, number, and arrangement of the atoms and bonds in the molecule. By analyzing the IR spectrum of a compound, one can identify the functional groups and other structural features present in it.
Cyclohexanone IR Spectrum
The IR spectrum of cyclohexanone can be obtained by using different methods, such as gas phase IR, liquid phase IR, or attenuated total reflectance (ATR) IR. The spectrum may vary slightly depending on the method and the conditions used, but the main features are consistent. The following table summarizes the main peaks and their assignments in the IR spectrum of cyclohexanone:
| Peak (cm-1) | Assignment |
|---|---|
| 1710 | C=O stretch |
| 1450 | CH2 scissoring |
| 1375 | CH2 rocking |
| 1260 | C-O stretch |
| 1180 | C-C stretch |
| 1020 | CH2 wagging |
| 880 | Ring deformation |
The most prominent peak in the spectrum is the C=O stretch at 1710 cm-1, which is characteristic of ketones. The peak is sharp and strong, indicating a conjugated carbonyl group. The CH2 scissoring peak at 1450 cm-1 is also strong and sharp, reflecting the six equivalent CH2 groups in the ring. The CH2 rocking peak at 1375 cm-1 is weaker and broader, due to the coupling with the ring deformation mode. The C-O stretch peak at 1260 cm-1 is medium and broad, indicating a single bond between carbon and oxygen. The C-C stretch peak at 1180 cm-1 is weak and broad, due to the overlap with other modes. The CH2 wagging peak at 1020 cm-1 is medium and sharp, showing the out-of-plane bending of the CH2 groups. The ring deformation peak at 880 cm-1 is weak and broad, reflecting the distortion of the ring from a perfect hexagon.
Cyclohexanone Conformations
Cyclohexanone can exist in two main conformations: chair and boat. The chair conformation is more stable than the boat conformation, because it has less steric strain and torsional strain. The chair conformation has two types of CH2 groups: axial and equatorial. The axial CH2 groups are perpendicular to the ring plane, while the equatorial CH2 groups are parallel to the ring plane. The boat conformation has four types of CH2 groups: axial, equatorial, flagpole, and bow. The flagpole CH2 groups are above and below the ring plane, while the bow CH2 groups are on opposite sides of the ring plane.
The conformations of cyclohexanone affect its IR spectrum in subtle ways. The main difference is in the intensity and shape of the C=O stretch peak. In the chair conformation, the C=O bond is slightly tilted from the ring plane, which reduces its dipole moment and its absorption intensity. In addition, the C=O bond is coupled with the axial CH2 rocking mode, which broadens its peak shape. In contrast, in the boat conformation, the C=O bond is more aligned with the ring plane, which increases its dipole moment and its absorption intensity. Moreover, the C=O bond is coupled with the flagpole CH2 rocking mode, which sharpens its peak shape.
As can be seen from the figure, the C=O stretch peak is stronger and narrower in the boat conformation than in the chair conformation. The other peaks are similar in both conformations, except for some minor shifts and variations in intensity.
Conclusion
Cyclohexanone is a cyclic ketone that has a wide range of applications in various industries. Its structure and properties can be analyzed by using IR spectroscopy, which reveals the presence of the C=O, C-O, C-C, and CH2 groups in the molecule. The IR spectrum of cyclohexanone also depends on its conformation, which can be either chair or boat. The main difference between the two conformations is in the intensity and shape of the C=O stretch peak, which is affected by the orientation and coupling of the C=O bond with the CH2 groups.
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