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The most energetically favorable conformation for a monosubstituted cyclohexane is the chair conformation with the non-hydrogen substituent in the equatorial position because it prevents high steric strain from 1,3 diaxial interactions. [11] In methylcyclohexane the two chair conformers are not isoenergetic. The methyl group prefers the ...
A-values help predict the conformation of cyclohexane rings. The most stable conformation will be the one which has the substituent or substituents equatorial. When multiple substituents are taken into consideration, the conformation where the substituent with the largest A-value is equatorial is favored.
Most methylcyclohexane is extracted from petroleum but it can be also produced by catalytic hydrogenation of toluene: CH 3 C 6 H 5 + 3 H 2 → CH 3 C 6 H 11. The hydrocarbon is a minor component of automobile fuel, with its share in US gasoline varying between 0.3 and 1.7% in early 1990s [10] and 0.1 to 1% in 2011. [11]
The molecular motions involved in a chair flip are detailed in the figure on the right: The half-chair conformation (D, 10.8 kcal/mol, C 2 symmetry) is the energy maximum when proceeding from the chair conformer (A, 0 kcal/mol reference, D 3d symmetry) to the higher energy twist-boat conformer (B, 5.5 kcal/mol, D 2 symmetry).
The chair and twist-boat are energy minima and are therefore conformers, while the half-chair and the boat are transition states and represent energy maxima. The idea that the chair conformation is the most stable structure for cyclohexane was first proposed as early as 1890 by Hermann Sachse, but only gained widespread acceptance much later.
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In the most stable chair form of cyclohexane, axial hydrogens on adjacent carbon atoms are pointed in opposite directions, virtually eliminating eclipsing strain. In medium-sized rings (7 to 13 carbon atoms) conformations in which the angle strain is minimised create transannular strain or Pitzer strain .
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