"Cyclopropane is the most strained of all rings, primarily because of the angle strain caused by its 60°C C-C bond angles. In addition, cyclopropane has considerable torsional strain because the C-H bonds on neighboring carbon atoms are eclipsed … Cyclobutane has less angle strain than cyclopropane but has more torsional strain because of its larger number of ring hydrogens. … Cyclopentane was predicted by Baeyer to be nearly strain-free, but it actually has a total strain energy of 26 kJ/mol (6.2 kcal/mol). Although planar cyclopentane has practically no angle strain, it has a large amount of torsional strain. Cyclopentane therefore twists to adopt a puckered, nonplanar conformation that strikes a balance between increased angle strain and decreased torsional strain."

"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."

"To measure the amount of strain in a compound, we have to measure the total energy of the compound and then subtract the energy of a strain-free reference compound. The difference between the two values should represent the amount of extra energy in the molecule due to strain. The simplest experimental way to do this for a cycloalkane is to measure its heat of combustion, the amount of heat released when the compound burns completely with oxygen. … Thus, three kinds of strain contribute to the overall energy of a cycloalkane. … Angle strain—the strain due to expansion or compression of bond angles … Torsional strain—the strain due to eclipsing of bonds on neighboring atoms … Steric strain—the strain due to repulsive interactions when atoms approach each other too closely."

"When speaking of an atom’s ability to polarize a bond, we often use the term inductive effect. An inductive effect is simply the shifting of electrons in a σ bond in response to the electronegativity of nearby atoms. Metals, such as lithium and magnesium, inductively donate electrons, whereas reactive nonmetals, such as oxygen and nitrogen, inductively withdraw electrons. Inductive effects play a major role in understanding chemical reactivity, and we’ll use them many times throughout this text to explain a variety of chemical observations."

"Simple aromatic hydrocarbons come from two main sources: coal and petroleum. Coal is an enormously complex mixture made up primarily of large arrays of benzene-like rings joined together. Thermal breakdown of coal occurs when it is heated to 1000 °C in the absence of air, and a mixture of volatile products called coal tar boils off. Fractional distillation of coal tar yields benzene, toluene, xylene (dimethylbenzene), naphthalene, and a host of other aromatic compounds. Unlike coal, petroleum contains few aromatic compounds and consists largely of alkanes. (...) During petroleum refining, however, aromatic molecules are formed when alkanes are passed over a catalyst at about 500 °C under high pressure."

"Dienes are named according to the rules formulated for ordinary alkenes. Conjugated dienes are more stable than dienes containing two isolated double bonds, as measured by their heats of hydrogenation. (...) The molecular-orbital picture of the π system in 1,3-butadiene shows two bonding and two antibonding orbitals. The four electrons are placed in the first two bonding levels."

"Conjugated dienes are electron rich and are attacked by electrophiles to give intermediate allylic cations on the way to 1,2- and 1,4-addition products. These reactions may be subject to kinetic control at relatively low temperatures. At relatively higher temperatures, the kinetic product ratios may change to thermodynamic product ratios, when such product formation is reversible."

"Conjugated dienes and hexatrienes are capable of (reversible) electrocyclic ring closures to cyclobutenes and 1,3-cyclohexadienes, respectively. The diene – cyclobutene system prefers thermal conrotatory and photochemical disrotatory modes. The triene – cyclohexadiene system reacts in the opposite way, proceeding through thermal disrotatory and photochemical conrotatory rearrangements. The stereochemistry of such electrocyclic reactions is governed by the Woodward-Hoffmann rules."

"Cyclic conjugated polyenes are aromatic if their p electron count is 4n + 2. This number corresponds to a completely filled set of bonding molecular orbitals. Conversely, 4n p systems have open-shell, antiaromatic structures that are unstable, are reactive, and lack aromatic ring-current effects in 1H NMR. Finally, when steric constraints impose nonplanarity, cyclic polyenes behave as nonaromatic alkenes."

"The stability of a conjugated diene arises because of an interaction between the p orbitals of the two double bonds. To review briefly, when two p atomic orbitals combine to form a π bond, two π molecular orbitals (MOs) result. One is lower in energy than the starting p orbitals and is therefore bonding; the other is higher in energy, has a node between nuclei, and is antibonding. The two p electrons occupy the low-energy, bonding orbital, resulting in formation of a stable bond between atoms."