An organic compound is any member of a large class of gaseous, liquid, or solid chemical compounds whose molecules contain carbon. For historical reasons discussed below, a few types of carbon-containing compounds such as carbides, carbonates, simple oxides of carbon (such as CO and CO2), and cyanides are considered inorganic. The distinction between organic and inorganic carbon compounds, while "useful in organizing the vast subject of chemistry... is somewhat arbitrary."
The word organic is historical, dating to the 1st century. For many centuries, Western alchemists believed in vitalism. This is the theory that certain compounds could be synthesized only from their classical elements—earth, water, air, and fire—by the action of a "life-force" (vis vitalis) that only organisms possessed. Vitalism taught that these "organic" compounds were fundamentally different from the "inorganic" compounds that could be obtained from the elements by chemical manipulation.
Vitalism survived for a while even after the rise of modern atomic theory and the replacement of the Aristotelian elements by those we know today. It first came under question in 1804, when Friedrich Wöhler synthesized oxalic acid, a compound known to occur only in living organisms, from cyanogen. A more decisive experiment was Wöhler's 1828 synthesis of urea from the inorganic salts potassium cyanate and ammonium sulfate. Urea had long been considered an "organic" compound, as it was known to occur only in the urine of living organisms. Wöhler's experiments were followed by many others, where increasingly complex "organic" substances were produced from "inorganic" ones without the involvement of any living organism.
Even though vitalism has been discredited, scientific nomenclature retains the distinction between organic and inorganic compounds. The modern meaning of organic compound is any compound that contains a significant amount of carbon—even though many of the organic compounds known today have no connection to any substance found in living organisms.
There is no single "official" definition of an organic compound. Some textbooks define an organic compound as one that contains one or more C-H bonds. Others include C-C bonds in the definition. Others state that if a molecule contains carbon―it is organic.
Even the broader definition of "carbon-containing molecules" requires the exclusion of carbon-containing alloys (including steel), a relatively small number of carbon-containing compounds such as metal carbonates and carbonyls, simple oxides of carbon and cyanides, as well as the allotropes of carbon and simple carbon halides and sulfides, which are usually considered inorganic.
The "C-H" definition excludes compounds that are historically and practically considered organic. Neither urea nor oxalic acid is organic by this definition, yet they were two key compounds in the vitalism debate. The IUPAC Blue Book on organic nomenclature specifically mentions urea and oxalic acid. Other compounds lacking C-H bonds that are also traditionally considered organic include benzenehexol, mesoxalic acid, and carbon tetrachloride. Mellitic acid, which contains no C-H bonds, is considered a possible organic substance in Martian soil. C-C bond is found in most organic compounds, except some small molecules like methane and methanol, which have only one carbon atom in their structure.
The "C-H bond-only" rule also leads to somewhat arbitrary divisions in sets of carbon-fluorine compounds, as, for example, Teflon is considered by this rule "inorganic" but Tefzel organic. Likewise, many Halons are considered inorganic, whereas the rest are considered organic. For these and other reasons, most sources believe C-H compounds are only a subset of "organic" compounds.
In summary, most carbon-containing compounds are organic, and almost all organic compounds contain at least a C-H bond or a C-C bond. A compound does not need to contain C-H bonds to be considered organic (e.g., urea), but many organic compounds do.
Organic compounds may be classified in a variety of ways. One major distinction is between natural and synthetic compounds. Organic compounds can also be classified or subdivided by the presence of heteroatoms, e.g., organometallic compounds, which feature bonds between carbon and a metal, and organophosphorus compounds, which feature bonds between carbon and a phosphorus.
Natural compounds refer to those that are produced by plants or animals. Many of these are still extracted from natural sources because they would be far too expensive to produce artificially. Examples include most sugars, some alkaloids and terpenoids, certain nutrients such as vitamin B12, and, in general, those natural products with large or stereoisometrically complicated molecules present in reasonable concentrations in living organisms.
Further compounds of prime importance in biochemistry are antigens, carbohydrates, enzymes, hormones, lipids and fatty acids, neurotransmitters, nucleic acids, proteins, peptides and amino acids, lectins, vitamins, and fats and oils.
Compounds that are prepared by reaction of other compounds are referred to as "synthetic". They may be either compounds that already are found in plants or animals or those that do not occur naturally.
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- The CAS database is the most comprehensive repository for data on organic compounds. The search tool SciFinder is offered.
- The Beilstein database contains information on 9.8 million substances, covers the scientific literature from 1771 to the present, and is today accessible via Reaxys. Structures and a large diversity of physical and chemical properties is available for each substance, with reference to original literature.
- PubChem contains 18.4 million entries on compounds and especially covers the field of medicinal chemistry.
There is a great number of more specialized databases for diverse branches of organic chemistry.
- From the definition of "organic compounds" are also excluded automatically the allotropes of carbon such as diamond and graphite, because they are formed by atoms of the same element, so they are simple substances, not compounds.
- Spencer L. Seager, Michael R. Slabaugh. Chemistry for Today: general, organic, and biochemistry. Thomson Brooks/Cole, 2004, p. 342. ISBN 0-534-39969-X
- Robert T. Morrison, Robert N. Boyd, and Robert K. Boyd, Organic Chemistry, 6th edition (Benjamin Cummings, 1992, ISBN 0-13-643669-2
- "IUPAC Blue Book, Urea and Its Derivatives Rule C-971". Retrieved 2009-11-22.
- "IUPAC Blue Book, Table 28(a) Carboxylic acids and related groups. Unsubstituted parent structures". Retrieved 2009-11-22.
- S. A. Benner, K. G. Devine, L. N. Matveeva, D. H. Powell (2000). "The missing organic molecules on Mars". Proceedings of the National Academy of Sciences 97 (6): 2425–2430. Bibcode:2000PNAS...97.2425B. doi:10.1073/pnas.040539497. PMC 15945. PMID 10706606.