Conformational isomerism

"The different arrangements of atoms that result from bond rotation are called conformations, and molecules that have different arrangements are called conformational isomers, or conformers. Unlike constitutional isomers, however, different conformers often can’t be isolated because they interconvert too rapidly. Conformational isomers are represented in two ways, … A sawhorse representation views the carbon–carbon bond from an oblique angle and indicates spatial orientation by showing all C - H bonds. A Newman projection views the carbon–carbon bond directly end-on and represents the two carbon atoms by a circle. Bonds attached to the front carbon are represented by lines to the center of the circle, and bonds attached to the rear carbon are represented by lines to the edge of the circle."

"Molecules that differ from each other by rotation about single bonds are called conformational isomers or conformers. Derek H. R. Barton (…) showed that the chemical and physical properties of complicated molecules can be interpreted in terms of their specific or preferred rotational arrangements and that a knowledge of the conformations of molecules is crucial to understanding the stereochemical basis of many reaction."

"The preferred conformations of 2-bromo- and 2-chlorocyclohexanones depend on the polarity of the solvent. In the diequatorial conformer there is considerable electrostatic repulsion. Note that parallel dipoles are destabilizing in a nonpolar solvent. … Intramolecular hydrogen bonding between 1,3-diaxial OH groups in nonpolar sol- vents confers appreciable stability to a conformer. In polar solvents, however, the solvent competes for intermolecular H-bond formation, resulting in normal steric effects dominating the equilibrium."

"Theoretical and computational chemistry contribute greatly to our understanding of conformational preferences of both stable molecules and transition states. Molecular mechanics methods (classical or force field) help define the conformational preferences of reactants and products. These methods are empirical, having been parameterized to reproduce experimental structures and energies and/or data provided by high-level quantum mechanical calculations."

"The effect of conformation on reactivity is intimately associated with the details of the mechanism of a reaction. ... As a warning against predicting product stereochemistry based on reactant conformation, the Curtin-Hammett principle states that the rate of reaction of a molecule is a function not only of the concentration of any reacting conformation but also of its transition state energy. … The conformational barrier (A <=> B) is substantially higher than the reaction barriers TSA and TSB. This case is known as the conformational equilibrium control, where the ratio of products is equal to the ratio of the population of the starting states."