A Conformational Study of 2-Halocyclohexanones*. An Advanced Laboratory Project.

Deborah L. Casher and Peter W. Samal

As part of a senior thesis project, an advanced laboratory experiment was developed for the second semester of the Chemistry Majors Organic Laboratory course. The project involved literature work, syntheses, FT-IR, and molecular modeling. Students were given a handout describing research which showed that infrared carbonyl stretching frequencies of 2-halocyclohexanones were dependent on the geometric relationship of the C=O and C-X bonds. Students were placed into "research" groups of four, and, using syntheses taken from the literature, were required to develop procedures for the synthesis of cyclohexanone by two methods and the synthesis of 2-bromocyclohexanone. A synthesis of 2-chlorocyclohexanone appropriate for this laboratory was not found, so material was purchased for the FT-IR study. Note that one requirement for the syntheses of the 2-halo derivatives was that cyclohexanone was not a starting material. Unreacted cyclohexanone that was not removed in the purification may have interfered with the FT-IR spectra of the 2-halocyclohexanone derivatives. The students then determined the FT-IR spectra of cyclohexanone, 2-chlorocyclohexanone, and 2-bromocyclohexanone. Molecular modeling was done using molecular mechanics (MM2) and semi-empirical MO calculations (AM1) to help elucidate the experimental results. The AM1 calculations were also used to predict carbonyl stretching frequencies. Although calculated frequencies did not agree well with experimental values, the direction of substituent effects was predicted correctly. Molecular mechanics was shown to fail at predicting the correct direction of conformational equilibrium in some cases, whereas semi-empirical AM1 calculations were successful for both 2-chloro and 2-bromocyclohexanone. A group discussion helped students to understand the effects of electrostatic interactions upon conformational equilibria.

* This project was inspired by the work of E.J. Corey in the 1950's (see for example, Corey and Burke, J. Am. Chem. Soc., 1955, 77, 5418.)