The acid-catalyzed dehydration of 2-methylcyclohexanol to develop a mixture of cyclic alkenes has actually become a traditional experiment in the organic chemisattempt and also upper-level laboratory curricula at a number of colleges and also colleges (1-4). Among various other points, it illustprices the energy of Zaitsev"s rule. The device is a lot of most likely that presented at the appropriate.

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The circulation of products appears to depfinish on (a) the relative rates of reactivity of the cis and trans isomers of the beginning alcohol, (b) the relative stabilities of the second and also tertiary carbocations, and also (c) the length of time over which the reactivity is lugged out. These phenomena reflect the loved one impacts of thermodynamic vs. kinetic manage over the development of the reactivity. Kinetically, the reaction of the alcohol cis isomer has a price continuous at least eight times that of the trans isomer (3). Thermodynamically, the development of 1-methylcyclohexene is desired bereason of its lower enthalpy of formation; the next the majority of exothermic enthalpy of formation is that of 3-methylcyclohexene, and the leastern negative enthalpy of formation is that of methylenecyclohexane. In addition, the greater stability of the tertiary carbocation over the second cation makes the last step of the reactivity thermodynamically rather reversible, so that if the reaction is lugged out over much longer times, a relatively larger fractivity of methylenecyclohexane (compared to 1-methylcyclohexene) is created, also though methylenecyclohexane has the least exothermic enthalpy of development of the assets.

Molecular modeling is a tool that can enable students to predict the relative thermodynamic stabilities of the intermediates and products of this reaction, and also additionally to create three-dimensional frameworks which display the lowest-power geometries of these intermediates and also commodities.


This experiment therefore provides a useful illustration of the adhering to concepts and speculative techniques:

dehydration of a second alcohol; usage of gas chromatography to separate and also quantify a mixture of products; use of mass-spectrometric detection as a tool to determine the compounds providing climb to gas-chromatographic peaks; principle of kinetic vs. thermodynamic control; use of modern computational methods to predict experimental results.

Experimental

The speculative details for carrying out the dehydration are provided in a number of recommendations (1, 3, 4).


Molecular Modeling

The organic molecules or molecule-ions involved in this reactivity were created making use of both software applications CACheTM and also MacSpartanTM, and also their geometries optimized making use of semiempirical molecular orbital calculations. When CAChe was used, the PM3 calculation was provided to optimize geomeattempt, while AM1 was provided as the optimization routine on MacSpartan. The following results were obtained:

Compound PM3 Enthalpy of Formation (kcal/mol) AM1 Enthalpy of Formation (kcal/mol)
2-methylcyclohexanol -75.783 -88.559
1-methylcyclohexene -14.426 -17.832
3-methylcyclohexene -10.293 -14.846
methylenecyclohexane -9.809 -12.686
tertiary carbocation 168.28 158.06
secondary carbocation 181.02 169.04

Both semiempirical methods predict the intended order of enthalpies of formation of the 3 hydrocarbon products, and also likewise predict the expected order of thermodynamic stcapability of the carbocation intermediates.

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Students deserve to watch that the geomeattempt around the carbon atom moving the positive charge is trigonal planar, as expected from sp2 hybridization of the carbon atom.

References

Taber, Ricdifficult L.; Champion, William C., J. Chem Educ, 1967, 44(10), 620. Todd, David, J. Chem Educ, 1994, 71(5), 440. Cawley, John J.; Lindner, Patrick E., J. Chem Educ, 1997, 74(1), 102. Poon, Thomas; Mundy, Bradford P.; McIntyre, Jean; Woods, Lesley, J. Chem Educ, embraced for publication.