Organic chemistry is the branch of chemistry concerned primarily with carbon containing compounds, especially their structures, properties, reactions, synthesis and applications. Carbon is uniquely suited to forming an enormous variety of compounds because its atoms can form four strong covalent bonds and can connect to other carbon atoms in chains, branches, rings and three dimensional frameworks.
Organic chemistry includes hydrocarbons containing only carbon and hydrogen, as well as compounds containing elements such as oxygen, nitrogen, sulfur, phosphorus, halogens and many metals. The field encompasses simple molecules such as methane and ethanol, complex pharmaceuticals, polymers, dyes, fragrances, carbohydrates, lipids, many natural products and numerous synthetic materials. The term organic originally referred to substances associated with living organisms, but modern organic chemistry does not require a compound to come from biological material. Many organic substances are synthesised entirely in laboratories or industrial plants. A major characteristic of organic chemistry is its emphasis on molecular structure and how changes in bonding and functional groups affect chemical behaviour.
Carbon's bonding versatility is the fundamental reason organic chemistry contains such an enormous number of compounds. Carbon can form single, double and triple bonds and can bond repeatedly to other carbon atoms, producing chains, branched structures, rings and extended networks. This property is called catenation. Carbon also forms stable covalent bonds with hydrogen, oxygen, nitrogen, sulfur, phosphorus and halogens, allowing a vast range of functional groups.
Functional groups are specific arrangements of atoms that give organic molecules characteristic chemical properties and reaction patterns. Alcohols contain hydroxyl groups, aldehydes and ketones contain carbonyl groups, carboxylic acids contain carboxyl groups, amines contain nitrogen based functional groups, and alkenes contain carbon carbon double bonds. Molecules with the same molecular formula can have different structures and therefore different properties, a phenomenon known as isomerism. Structural isomers differ in connectivity, while stereoisomers have the same connectivity but differ in three dimensional arrangement. This structural diversity is central to understanding why apparently small molecular changes can produce large differences in chemical behaviour.
Organic chemistry uses systematic nomenclature to identify compounds unambiguously. The International Union of Pure and Applied Chemistry provides internationally recognised naming conventions based on molecular structure. Common names remain widely used for substances such as acetone, benzene and acetic acid, while systematic names provide a more general framework. The molecular formula gives the numbers of each element but does not necessarily specify the connectivity of atoms. Structural formulas and line angle representations provide more information about molecular architecture.
Stereochemical notation can specify three dimensional arrangements around chiral centres and double bonds. Chirality is particularly important because two molecules that are mirror images can interact differently with biological systems. Enantiomers have non superimposable mirror image structures, while diastereomers are stereoisomers that are not mirror images. These distinctions are crucial in medicinal chemistry because biological receptors, enzymes and transport systems are themselves three dimensional and can distinguish between molecular stereoisomers. Organic chemistry therefore treats molecular structure as a major determinant of physical properties, reactivity, biological activity and synthetic behaviour.
Organic reactions involve systematic transformations of functional groups and molecular frameworks. Important reaction classes include substitution, addition, elimination, oxidation, reduction, hydrolysis, condensation, rearrangement and coupling reactions. Reaction mechanisms describe how electrons move and how bonds are broken and formed through elementary steps. Curved arrow notation is commonly used to represent the movement of electron pairs in organic reaction mechanisms. Nucleophiles donate electron density to electrophilic centres, while electrophiles accept electron density.
Acids and bases can participate through proton transfer, and radicals can undergo reactions involving unpaired electrons. Reaction conditions such as solvent, temperature, catalyst, concentration and steric environment influence which pathway occurs. Organic synthesis involves deliberately selecting sequences of reactions to convert accessible starting materials into target molecules. Chemists may use protecting groups, selective reagents and stereochemical control to prevent unwanted reactions. Retrosynthetic analysis works backwards from a desired target to identify simpler precursor structures. These methods make organic chemistry both a descriptive science of existing molecules and a creative discipline for constructing new compounds.
Organic chemistry has major biological applications. The molecules responsible for life include proteins, nucleic acids, carbohydrates, lipids, metabolites and numerous signalling molecules, most of which are organic compounds. Biochemistry examines their roles in living systems, while organic chemistry provides the molecular framework for understanding their structures and transformations. Pharmaceutical chemistry relies heavily on organic synthesis to create and modify molecules that interact with biological targets. Changes to functional groups can alter potency, selectivity, solubility, metabolic stability and toxicity.
Medicinal chemists often construct families of structurally related compounds and test how systematic modifications affect biological activity. Natural product chemistry studies complex organic molecules produced by organisms, including alkaloids, terpenes, polyketides and many other compound classes. Total synthesis can reproduce complex natural molecules from simpler starting materials and can also provide access to analogues that do not occur naturally. Organic chemistry therefore provides essential tools for drug discovery, biological investigation and the development of new therapeutic substances.
Organic chemistry is also central to materials and industrial chemistry. Polymers are large molecules composed of repeating structural units and can be produced through addition polymerisation, condensation polymerisation and other mechanisms. Plastics, synthetic fibres, elastomers, coatings, adhesives and many advanced materials depend on organic molecular design. Petrochemical industries transform hydrocarbons into fuels, solvents, monomers and chemical feedstocks. Surfactants contain structural regions that interact differently with water and nonpolar substances, making them useful in detergents and emulsions.
Dyes and pigments depend on molecular structures that absorb particular wavelengths of light. Organic electronic materials can be designed to conduct or transport charge in specialised devices. Modern sustainable chemistry increasingly investigates renewable feedstocks, catalytic processes, recyclable polymers and methods that minimise waste. Organic chemistry therefore contributes both to conventional industrial manufacturing and to the development of alternative materials and processes intended to reduce environmental impact.
The historical development of organic chemistry was strongly influenced by the nineteenth century distinction between organic and inorganic substances. Early chemists believed that compounds associated with living organisms might require a special vital force and could not be produced from inorganic materials. Friedrich Wohler's synthesis of urea from ammonium cyanate in 1828 helped undermine this vitalist interpretation because urea, historically considered an organic substance, was produced from materials not obtained directly from an organism.
Subsequent developments in structural theory, valence, stereochemistry and spectroscopy transformed the field. August Kekule and other chemists contributed to structural representations and theories of carbon bonding, while later quantum mechanics provided a deeper explanation of bonding and molecular orbitals. Modern organic chemistry is therefore not defined by a belief in a special life force. The historical term organic remains because of the field's origins, but its scientific basis is molecular structure and carbon chemistry. This history illustrates how chemical classifications can change as experimental evidence and theoretical frameworks develop.
Organic chemistry is ultimately a highly interconnected field linking molecular structure, reactivity, synthesis, materials and biological function. Its central concepts include carbon bonding, functional groups, hydrocarbons, aromaticity, stereochemistry, reaction mechanisms, spectroscopy and retrosynthetic planning. Organic chemistry is not simply the chemistry of substances found in living organisms, nor does it exclude synthetic substances.
Carbon dioxide, carbon monoxide, carbonates and certain other carbon compounds are commonly treated within inorganic chemistry despite containing carbon, demonstrating that the boundary between organic and inorganic chemistry is based on chemical traditions and structural considerations rather than an absolute rule that every carbon compound is organic. Organic chemistry also overlaps with physical chemistry when studying reaction kinetics and thermodynamics, with analytical chemistry when identifying molecular structures, and with biochemistry when studying biological molecules.
Its breadth explains its importance across medicine, agriculture, materials science, energy, environmental science and manufacturing. The ability to predict and manipulate carbon based molecular structures makes organic chemistry one of the central disciplines through which chemists design substances with specific physical, chemical and biological properties.
