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[2] [3] [4] The ring-flip of substituted cyclohexanes constitutes a common form of conformers. [5] The study of the energetics of bond rotation is referred to as conformational analysis. [6] In some cases, conformational analysis can be used to predict and explain product selectivity, mechanisms, and rates of reactions. [7]
The chair conformation minimizes both angle strain and torsional strain by having all carbon-carbon bonds at 110.9° and all hydrogens staggered from one another. [2] The conformational changes that occur in a cyclohexane ring flip take place over several stages. Structure D (10.8 kcal/mol) is the highest energy transition state of the process.
The gauche effect is very sensitive to solvent effects, due to the large difference in polarity between the two conformers.For example, 2,3-dinitro-2,3-dimethylbutane, which in the solid state exists only in the gauche conformation, prefers the gauche conformer in benzene solution by a ratio of 79:21, but in carbon tetrachloride, it prefers the anti conformer by a ratio of 58:42. [9]
For 1,2- and 1,4-disubstituted cyclohexanes, a cis configuration leads to one axial and one equatorial group. Such species undergo rapid, degenerate chair flipping. For 1,2- and 1,4-disubstituted cyclohexane, a trans configuration, the diaxial conformation is
The Curtin–Hammett principle is a principle in chemical kinetics proposed by David Yarrow Curtin and Louis Plack Hammett.It states that, for a reaction that has a pair of reactive intermediates or reactants that interconvert rapidly (as is usually the case for conformational isomers), each going irreversibly to a different product, the product ratio will depend both on the difference in ...
Juxtaposing the designations produces the following terms for the conformers of butane (see Alkane stereochemistry for an explanation of conformation nomenclature): gauche butane is syn-clinal (+sc or −sc, depending on the enantiomer), anti butane is anti-periplanar, and eclipsed butane is syn-periplanar. [2]
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.
The anti conformation is more stable by 0.9 kcal mol −1. [1] We would expect that butane is roughly 82% anti and 18% gauche at room temperature. However, there are two possible gauche conformations and only one anti conformation. Therefore, entropy makes a contribution of 0.4 kcal in favor of the gauche conformation. [2]
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