Hydrogen compounds are only required to contain carbon. However, most of them also contain hydrogen, and because it is the carbon-hydrogen bond which gives this class of compounds most of its particular chemical characteristics, carbon-hydrogen bonds are required in some definitions of the word "organic" in chemistry. Millions of hydrocarbons are known, and they are usually formed by complicated synthetic pathways that seldom involve elementary hydrogen. Compounds of hydrogen are often called hydrides, a term that is used fairly loosely. The term "hydride" suggests that the H atom has acquired a negative or anionic character, denoted H, and is used when
Carbon–hydrogen bond activation substitution at the carbon also greatly affects selectivity; the secondary methylene position is preferred over the terminal methylene position because the former has a secondary carbon while the latter has a primary one. In the oxidation of cyclohexane compound, the tertiary site on the ring is preferential over the tertiary site of the isopropyl substituent. The two methyl groups hinder the oxidation on the isopropyl group, making the less hindered cyclohexyl site more favorable. In contrast to innate selectivity, guided selectivity results from external reagents or directing groups affecting the nature of specific C-H bonds. The figure above demonstrates the