Organic chemistry is the study of compounds of carbon called organic compounds. Organic compounds either originate from living organisms or are synthesized by chemists. A defining trait of these compounds is the presence of carbon as the principal element, which is bonded to other carbon atoms and other elements such as hydrogen, oxygen, nitrogen, and sulfur. The existence of a wide array of organic molecules is a consequence of carbon atoms’ ability to form up to four strong bonds to other carbon atoms, resulting in chains and rings of many different sizes, shapes, and complexities. However, some carbon-containing compounds, such as carbonates, cyanides, and simple oxides (CO and CO₂), are not typically classified as organic compounds.

The simplest organic compounds contain only the elements carbon and hydrogen and are called hydrocarbons. Hydrocarbons may differ in the types of carbon–carbon bonds present in their molecules. These may be alkanes (containing only single bonds), alkenes (containing one or more double bonds), or alkynes (containing one or more triple bonds). Although all hydrocarbons are composed of only two types of atoms (carbon and hydrogen), there is a wide variety of hydrocarbons owing to the varying lengths of chains, branched chains, and rings of carbon atoms, or to combinations of these structures. Properties such as the melting point and the boiling point usually change predictably as the number of carbon and hydrogen atoms in the molecules change.

The incorporation of a functional group into carbon- and hydrogen-containing molecules leads to new families of compounds called functionalized hydrocarbons. The functional group is a characteristic atom or group of atoms that primarily determines the properties of hydrocarbon derivatives. For example, compounds that have an –OH functional group bonded to the hydrocarbon portion are alcohols. By convention, the general formula of an alcohol is R–OH, where R is the hydrocarbon portion of the molecule. A group of compounds containing the same functional group forms a family.

Organic compounds are key components of plastics, soaps, perfumes, sweeteners, fabrics, pharmaceuticals, and many other substances used daily. Therefore, studying their structures and reactions is critical to understand their functions in real life.