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1. The C-C sigma bonds of alkanes rotate rapidly at room temperature, giving rise to conformational isomers. The two extreme conformations in ethane are staggered (more stable) and eclipsed (less stable). Staggered is more stable due to minimized torsional strain (bonding electron pair repulsion).
2. The preferred staggered conformation of butane is the 'anti' configuration, rather than the 'gauche' conformations (the other staggered conformations of butane). Staggered ethane is 12 kJ/mol (3 kcal/mol) more stable than eclipsed ethane due to torsional strain. Gauche butane is less stable than anti butane (both are staggered conformations ) entirely as a result of steric strain in the gauche conformation.
3. Averaged over time, alkanes exist in the preferred 'zig-zag' conformation, but at any given instant at room temperature, the molecule is unlikely to be linear because of bond rotations and vibrations.
See pages on alkane conformations
4. Unsubstituted cycloalkane rings pucker to minimize angle and torsional strain. Cyclopropane, bent cyclobutane and envelope cyclopentane all have some angle and/or torsional strain, but chair cyclohexane does not.
5. There are two kinds of hydrogens on chair cyclohexane, axial and equatorial.
5. Groups larger than H generally prefer to be equatorial because when they are axial there is steric strain (1,3 diaxial interactions) with the other axial hydrogens. Larger substituents have a greater preference for being equatorial.
6. Substituted cyclohexanes prefer to have as many substituents equatorial as possible, with the larger substituents dominating.
See pages on cycloalkane conformations.