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Organic Chemistry: Carbocycles

Problems: Cycloalkanes

Conformational Analysis of Cycloalkanes

Polycyclic Compounds

Problem : Please draw the most stable conformations of the following molecules as 3D chair representations. When relevant, please indicate side chain conformations as well.

Problem : The isomers of dimethylcyclohexane present an interesting opportunity to examine steric interactions in cyclic compounds. Please rationalize the following thermodynamic data by (i) drawing out both chair conformations of each compound, and (ii) indicating the presence of any unfavorable steric interactions.

Problem : Please circle the more stable compound in each of the following pairs and give a brief rationale for your choice:

Problem : Monosubstituted cyclohexanes always prefer to orient their substituent in an equatorial position in order to minimize 1,3-diaxial interactions. Consider the following free energy data:

Notice that there is relatively little change in going from Me to Et to I-Pr, but that there is a large increase in free energy in going to t-Bu. Draw 3D conformations of all four compounds in axial conformations (including side-chain conformations) and rationalize this trend. (Optional: What is the name given to the "extra" interaction in the t-Bu compound?)

Consider the 1,3-diaxial interactions in the axial conformers:
Although the Me, Et, and i-Pr groups can avoid direct steric hindrance with the 1,3-axial hydrogens by directing their methyl groups away, t-Bu is forced to place a methyl group in close proximity to these hydrogens. Such interactions are called syn-pentane (there are two of them here) and are severely destabilizing.

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