Section 4.1     Rotations about single bonds in molecules result in different interconverting shapes or conformations. Specific conformations (also called conformers) may be different in shape and energy but their interconversion does not involve breaking bonds, only rotations about bonds.

Section 4.2     Newman projections are a convenient way to visualize rotations about specific bonds and the different conformations that result. The angle formed between two specific bonds on adjacent atoms, as viewed in Newman projection, is called a dihedral angle.

Section 4.3     Rotation about the C–C bond in ethane produces two extreme conformations, staggered and eclipsed. Staggered conformations (60° dihedral) are preferred over eclipsed conformations (0° dihedral). Torsional strain refers to the excess energy required to adopt an eclipsed conformation.

Section 4.4     Larger acyclic molecules like n-butane have more complex potential energy diagrams than ethane due to the effects of steric strain combined with torsional strain. Steric strain results from unfavorable through-space interactions between atoms.

Section 4.5     Angle strain is the excess energy resulting from the adoption of bond angles smaller or larger than the preferred values. Small rings of three to four atoms experience severe angle strain and torsional strain due to their necessarily flat or nearly flat conformations. Five-membered rings have much less angle strain and can alleviate torsional strain by forming puckered conformations.

Section 4.6     Cyclohexane and related six-membered heterocyclic rings exist in an especially stable conformation known as the chair, in which torsional strain and angle strain are both negligible. Other conformations available to these rings include the boat and twist-boat which are significantly higher in energy but are intermediates in the interconversion of chair conformations.

Section 4.7     The introduction of non-hydrogen substituents on six-membered ring atoms introduces steric strain that in turn impacts the conformational preferences of the ring system. Typically one chair form is preferred over the other and the low-energy conformer can be predicted using tables of conformational free energies for axial substituents.

Section 4.8     The presence of ring fusions or bridged ring systems typically introduces constraints on the conformational flexibility of molecules. Free rotation about single bonds can be constrained by the introduction of bulky substituents proximal to the bond undergoing rotation.

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