Organic reactions arechemical reactions involvingorganic compounds.[1][2][3] The basicorganic chemistry reaction types areaddition reactions,elimination reactions,substitution reactions,pericyclic reactions,rearrangement reactions,photochemical reactions andredox reactions. Inorganic synthesis, organic reactions are used in the construction of new organic molecules. The production of many man-made chemicals such as drugs,plastics,food additives,fabrics depend on organic reactions.
The oldest organic reactions arecombustion of organic fuels andsaponification of fats to make soap. Modernorganic chemistry starts with theWöhler synthesis in 1828. In the history of theNobel Prize in Chemistry awards have been given for the invention of specific organic reactions such as theGrignard reaction in 1912, theDiels–Alder reaction in 1950, theWittig reaction in 1979 andolefin metathesis in 2005.
Organic chemistry has a strong tradition of naming a specific reaction to its inventor or inventors and a longlist of so-callednamed reactions exists, conservatively estimated at 1000. A very old named reaction is theClaisen rearrangement (1912) and a recent named reaction is theBingel reaction (1993). When the named reaction is difficult to pronounce or very long as in theCorey–House–Posner–Whitesides reaction it helps to use the abbreviation as in theCBS reduction. The number of reactions hinting at the actual process taking place is much smaller, for example theene reaction oraldol reaction.
Another approach to organic reactions is by type oforganic reagent, many of theminorganic, required in a specific transformation. The major types areoxidizing agents such asosmium tetroxide,reducing agents such aslithium aluminium hydride,bases such aslithium diisopropylamide andacids such assulfuric acid.
Finally, reactions are also classified by mechanistic class. Commonly these classes are (1) polar, (2) radical, and (3) pericyclic. Polar reactions are characterized by the movement of electron pairs from a well-defined source (anucleophilic bond or lone pair) to a well-defined sink (anelectrophilic center with a low-lying antibonding orbital). Participating atoms undergo changes in charge, both in the formal sense as well as in terms of the actual electron density. The vast majority of organic reactions fall under this category. Radical reactions are characterized by species with unpaired electrons (radicals) and the movement of single electrons. Radical reactions are further divided intochain and nonchain processes. Finally,pericyclic reactions involve the redistribution of chemical bonds along a cyclictransition state. Although electron pairs are formally involved, they move around in a cycle without a true source or sink. These reactions require the continuous overlap of participating orbitals and are governed byorbital symmetry considerations. Of course, some chemical processes may involve steps from two (or even all three) of these categories, so this classification scheme is not necessarily straightforward or clear in all cases. Beyond these classes, transition-metal mediated reactions are often considered to form a fourth category of reactions, although this category encompasses a broad range of elementary organometallic processes, many of which have little in common and very specific.
Factors governing organic reactions are essentially the same as that of anychemical reaction. Factors specific to organic reactions are those that determine the stability of reactants and products such asconjugation,hyperconjugation andaromaticity and the presence and stability ofreactive intermediates such asfree radicals,carbocations andcarbanions.
An organic compound may consist of manyisomers. Selectivity in terms ofregioselectivity,diastereoselectivity andenantioselectivity is therefore an important criterion for many organic reactions. Thestereochemistry ofpericyclic reactions is governed by theWoodward–Hoffmann rules and that of manyelimination reactions byZaitsev's rule.
Organic reactions are important in the production ofpharmaceuticals. In a 2006 review,[4] it was estimated that 20% of chemical conversions involvedalkylations on nitrogen and oxygen atoms, another 20% involved placement and removal ofprotective groups, 11% involved formation of newcarbon–carbon bond and 10% involvedfunctional group interconversions.
There is no limit to the number of possible organic reactions and mechanisms.[5][6] However, certain general patterns are observed that can be used to describe many common or useful reactions. Each reaction has a stepwisereaction mechanism that explains how it happens, although this detailed description of steps is not always clear from a list of reactants alone. Organic reactions can be organized into several basic types. Some reactions fit into more than one category. For example, some substitution reactions follow an addition-elimination pathway. This overview isn't intended to include every single organic reaction. Rather, it is intended to cover the basic reactions.
| Reaction type | Subtype | Comment |
|---|---|---|
| Addition reactions | electrophilic addition | include such reactions ashalogenation,hydrohalogenation andhydration. |
| nucleophilic addition | ||
| radical addition | ||
| Elimination reaction | include processes such asdehydration and are found to follow an E1, E2 orE1cBreaction mechanism | |
| Substitution reactions | nucleophilic aliphatic substitution | withSN1,SN2 andSNireaction mechanisms |
| nucleophilic aromatic substitution | ||
| nucleophilic acyl substitution | ||
| electrophilic substitution | ||
| electrophilic aromatic substitution | ||
| radical substitution | ||
| Organic redox reactions | areredox reactions specific toorganic compounds and are very common. | |
| Rearrangement reactions | 1,2-rearrangements | |
| pericyclic reactions | ||
| metathesis |
Incondensation reactions a small molecule, usually water, is split off when tworeactants combine in a chemical reaction. The opposite reaction, when water is consumed in a reaction, is calledhydrolysis. Manypolymerization reactions are derived from organic reactions. They are divided intoaddition polymerizations andstep-growth polymerizations.
In general the stepwise progression of reaction mechanisms can be represented usingarrow pushing techniques in which curved arrows are used to track the movement of electrons as starting materials transition to intermediates and products.
Organic reactions can be categorized based on the type offunctional group involved in the reaction as a reactant and the functional group that is formed as a result of this reaction. For example, in theFries rearrangement the reactant is anester and the reaction product analcohol.
An overview of functional groups with their preparation and reactivity is presented below:
Inheterocyclic chemistry, organic reactions are classified by the type of heterocycle formed with respect to ring-size and type of heteroatom. See for instance the chemistry ofindoles. Reactions are also categorized by the change in the carbon framework. Examples arering expansion and ring contraction,homologation reactions,polymerization reactions,insertion reactions,ring-opening reactions andring-closing reactions.
Organic reactions can also be classified by the type of bond to carbon with respect to the element involved. More reactions are found inorganosilicon chemistry,organosulfur chemistry,organophosphorus chemistry andorganofluorine chemistry. With the introduction of carbon-metal bonds the field crosses over toorganometallic chemistry.
Compounds ofcarbon with other elements in the periodic table | |
|---|---|
| Legend |
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Branches ofchemistry | |
|---|---|
| Analytical | |
| Theoretical | |
| Physical | |
| Inorganic | |
| Organic | |
| Biological | |
| Interdisciplinarity | |
| See also | |
