Hydrocarbons

"Even though alkanes are relatively unreactive and rarely involved in chemical reactions, they nevertheless provide a useful vehicle for introducing some important general ideas."

"Cyclic molecules are so commonly encountered throughout organic and biological chemistry that it’s important to understand the consequences of their cyclic structures. ... A cycloalkane is a saturated cyclic hydrocarbon with the general formula CnH2n. In contrast to open-chain alkanes, where nearly free rotation occurs around C-C bonds, rotation is greatly reduced in cycloalkanes."

"Bond homolysis in alkanes yields radicals and free atoms. The heat required to do so is called the bond-dissociation energy ... Its value is characteristic only for the bond between the two participating elements. Bond breaking that results in tertiary radicals demands less energy than that furnishing secondary radicals; in turn, secondary radicals are formed more readily than primary radicals. The methyl radical is the most difficult to obtain in this way."

"When alkanes are heated to a high temperature, both C–H bonds and C–C bonds rupture, a process called pyrolysis. In the absence of oxygen, the resulting radicals can combine to form new higher- or lower-molecular-weight alkanes. Radicals can also remove hydrogen atoms from the carbon atom adjacent to another radical center to give alkenes, a process called hydrogen abstraction."

"Breaking an alkane down into smaller fragments is also known as cracking. Such processes are important in the oil-refining industry for the production of gasoline and other liquid fuels from petroleum."

"Alkanes lack functional groups, so they do not undergo the kinds of electrophile – nucleophile reactions typical of functionalized molecules. In fact, alkanes are pretty unreactive. However, under appropriate conditions, they undergo homolytic bond cleavage to form radicals, which are reactive species containing odd numbers of electrons. This is another situation in which the structure of a class of compounds determines their function."

"The relative reactivities of the various types of alkane C–H bonds in halogenations can be estimated by factoring out statistical contributions. They are roughly constant under identical conditions and follow the order CH4 < Primary CH < Secondary CH < Tertiary CH ... The reactivity differences between these types of CH bonds are greatest for bromination, making it the most selective radical halogenation process. Chlorination is much less selective, and fluorination shows very little selectivity."

"The discrepancy between calculated and measured heats of combustion in the cycloalkanes can be largely attributed to three forms of strain: bond angle (deformation of tetrahedral carbon), eclipsing (torsional), and transannular (across the ring)."

"The conformational analysis of cyclohexane enables us to predict the relative stability of its various conformers and even to approximate the energy differences between two chair conformations."

"An alkene, sometimes called an olefin, is a hydrocarbon that contains a carbon– carbon double bond. Alkenes occur abundantly in nature. … Ethylene and propylene, the simplest alkenes, are the two most important organic chemicals produced industrially. Approximately 127 million metric tons of ethylene and 54 million metric tons of propylene are produced worldwide each year for use in the synthesis of polyethylene, polypropylene, ethylene glycol, acetic acid, acetaldehyde, and a host of other substances."

"Because of its double bond, an alkene has fewer hydrogens than an alkane with the same number of carbons … and is therefore referred to as unsaturated. Knowing this relationship, it’s possible to work backward from a molecular formula to calculate a molecule’s degree of unsaturation — the number of rings and/or multiple bonds present in the molecule. … The unknown therefore contains two double bonds, one ring and one double bond, two rings, or one triple bond. There’s still a long way to go to establish structure, but the simple calculation has told us a lot about the molecule. … Add the number of halogens to the number of hydrogens. … Ignore the number of oxygens. … Subtract the number of nitrogens from the number of hydrogens."

"The double bond in alkenes, such as ethene (ethylene), and the triple bond in alkynes, such as ethyne (acetylene), are the result of the ability of the atomic orbitals of carbon to adopt sp2 and sp hybridization, respectively."

"The carbon–carbon double bond in alkenes has special electronic and structural features. This section reviews the hybridization of the carbon atoms in this functional group, the nature of its two bonds (s and p), and their relative strengths."

"Alkenes possessing allylic C-H bonds are oxidized by SeO2 either to allylic alcohols or esters or to α,β-unsaturated aldehydes or ketones, depending on the experimental conditions. … Reaction of chromic anhydride (CrO2) with t-butanol yields t-butyl hydrogen chro- mate, a powerful oxidant suitable for allylic oxidation of electron-deficient alkenene. Copper(I) salts catalyze thc allylic oxidation of alkenes in the presence of peresters, such as tert-BuO2COPh, to afford the corresponding allylic benzoate esters."

"Substituted cyclohexanes are the most common cycloalkanes and occur widely in nature. A large number of compounds, including steroids and many pharma ceutical agents, have cyclohexane rings. The flavoring agent menthol, for instance, has three substituents on a six-membered ring. … Cyclohexane adopts a strain-free, three-dimensional shape that is called a chair conformation because of its similarity to a lounge chair, with a back, seat, and footrest … The easiest way to visualize chair cyclohexane is to build a molecular model. … In addition to the chair conformation of cyclohexane, an alternative called the twist-boat conformation is also nearly free of angle strain."

"The chair conformation of cyclohexane leads to many consequences. … the chemical behavior of many substituted cyclohexanes is influenced by their conformation. … Another consequence of the chair conformation is that there are two kinds of positions for substituents on the cyclohexane ring: axial positions and equatorial positions … each carbon atom in chair cyclohexane has one axial and one equatorial hydrogen. Furthermore, each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement."