Carbon–hydrogen bond in the context of "Electron shell"

Organic compounds are a subclass of chemical compounds of carbon. Little consensus exists among chemists on the exact definition of organic compound; the only universally accepted definition is the quasi-tautological "organic compounds are the subject matter of organic chemistry".

Generally, any large chemical compound containing a carbon–hydrogen or carbon–carbon bond is accepted as an organic compound. Thus alkanes (e.g. ethane, CH₃−CH₃) and their derivatives are typically considered organic. For historical and disciplinary reasons, small molecules containing carbon are generally not accepted: cyanide ion (CN), hydrogen cyanide (HCN), chloroformic acid (ClCO₂H), carbon dioxide (CO₂), and carbonate ion (CO2−3) may all be excluded.

An inorganic compound is typically a chemical compound that lacks carbon–hydrogen bonds⁠‍—‍that is, a compound that is not an organic compound. The study of inorganic compounds is a subfield of chemistry known as inorganic chemistry.

Inorganic compounds comprise most of the Earth's crust, although the compositions of the deep mantle remain active areas of investigation.

A substitution reaction (also known as single displacement reaction or single substitution reaction) is a chemical reaction during which one functional group in a chemical compound is replaced by another functional group. Substitution reactions are of prime importance in organic chemistry. Substitution reactions in organic chemistry are classified either as electrophilic or nucleophilic depending upon the reagent involved, whether a reactive intermediate involved in the reaction is a carbocation, a carbanion or a free radical, and whether the substrate is aliphatic or aromatic. Detailed understanding of a reaction type helps to predict the product outcome in a reaction. It also is helpful for optimizing a reaction with regard to variables such as temperature and choice of solvent.

A good example of a substitution reaction is halogenation. When chlorine gas (Cl₂) is irradiated, some of the molecules are split into two chlorine radicals (Cl•), whose free electrons are strongly nucleophilic. One of them breaks a C–H covalent bond in CH₄ and grabs the hydrogen atom to form the electrically neutral HCl. The other radical reforms a covalent bond with the CH₃• to form CH₃Cl (methyl chloride).