What Are Organic Compounds?

Organic compounds are chemical substances whose molecules contain carbon atoms bonded to one another or to hydrogen, and often also to oxygen, nitrogen, sulfur, halogens, or phosphorus. Carbon's four valence electrons allow it to form stable covalent bonds with itself and with a wide range of other elements, generating an enormous structural diversity that underlies the chemistry of living systems, synthetic materials, pharmaceuticals, and fuels. The field of organic chemistry is the systematic study of these compounds: their structures, properties, synthesis, and reactions.

The modern definition of organic compounds is structural rather than biological: synthetic compounds such as polyethylene and aspirin are organic by virtue of their carbon backbone, not by origin from living matter. The number of known organic compounds exceeds 100 million distinct structures, a figure that grows continuously as synthesis and natural product isolation add new entries. Carbon's ability to form chains, rings, and branched networks of any length, combined with the modulating influence of functional groups, gives organic compounds their unmatched chemical variety.

Hydrocarbons

Hydrocarbons, compounds consisting exclusively of carbon and hydrogen, are the structural foundation of organic chemistry and the primary component of petroleum and natural gas feedstocks. Alkanes (saturated acyclic hydrocarbons with the general formula CnH2n+2) are chemically unreactive under ordinary conditions due to strong C-H sigma bonds, making them useful fuels and lubricants. Alkenes and alkynes contain carbon-carbon double or triple bonds respectively and are far more reactive; ethylene and propylene, the two simplest alkenes, are among the highest-volume industrial chemicals produced globally. Aromatic hydrocarbons, including benzene and its derivatives, are characterized by delocalized pi-electron systems that confer unusual stability and distinct substitution reactivity. The Chemistry LibreTexts resource on functional groups and classes of organic compounds covers the classification of hydrocarbons and their heteroatom-containing derivatives in detail.

Functional Group Classes

Functional groups are specific atomic arrangements within an organic molecule that determine its characteristic chemical behavior. The hydroxyl group (-OH) defines alcohols and phenols, conferring polarity and hydrogen-bonding capability. Carbonyl groups (C=O) appear in aldehydes, ketones, carboxylic acids, esters, and amides, each reacting through characteristic nucleophilic addition or substitution pathways. Amines carry the -NH2 group and are basic and nucleophilic; their nitrogen-containing analogs in rings include the aromatic heterocycles pyridine and imidazole, which appear widely in pharmaceutical compounds. Halogenated organic compounds, in which one or more hydrogens are replaced by fluorine, chlorine, bromine, or iodine, have enhanced reactivity at the substituted carbon and find applications as intermediates, solvents, and agrochemicals. A systematic account of functional groups in organic chemistry explains how each group's electronic character governs its reactivity pattern.

Biomolecules and Natural Organic Compounds

The most structurally complex organic compounds are biosynthesized by living organisms: proteins are polypeptide chains of amino acids; nucleic acids are polymers of nucleotide units carrying the genetic code; polysaccharides are carbohydrate polymers serving structural and energy-storage roles; and lipids form the bilayer membranes of cells. Natural product chemistry studies secondary metabolites, organic compounds that organisms produce for functions beyond primary metabolism, including antibiotics, alkaloids, terpenoids, and polyketides, which are major sources of pharmaceutical leads. Emerging approaches to sustainable production of chemicals via algorithm-assisted biosynthesis are expanding the scope of what can be made from bio-based feedstocks, blending synthetic and biosynthetic routes.

Applications

Organic compounds has applications in a wide range of fields, including:

• Pharmaceutical development, where the majority of approved drugs are small organic molecules
• Polymer and materials science, encompassing plastics, rubbers, fibers, and adhesives
• Electronic and photonic devices based on conjugated organic semiconductors and display materials
• Agricultural chemistry, including pesticides, fertilizers, and plant growth regulators
• Energy storage and conversion, including organic electrolytes for batteries and dye-sensitized solar cells