Editing Chemical reaction (section)

==Reactions in organic chemistry==
{{Main|Organic reaction}}
In [[organic chemistry]], in addition to oxidation, reduction or acid-base reactions, a number of other reactions can take place which involves [[covalent bond]]s between carbon atoms or carbon and [[heteroatom]]s (such as oxygen, nitrogen, [[halogen]]s, etc.). Many specific reactions in organic chemistry are [[name reaction]]s designated after their discoverers.

One of the most industrially important reactions is the [[Cracking (chemistry)|cracking]] of heavy [[hydrocarbon]]s at [[Oil refinery|oil refineries]] to create smaller, simpler molecules. This process is used to manufacture [[gasoline]]. Specific types of organic reactions may be grouped by their reaction mechanisms (particularly substitution, addition and elimination) or by the types of products they produce (for example, [[methylation]], [[Polymerization|polymerisation]] and [[halogenation]]).

===Substitution===
In a [[substitution reaction]], a [[functional group]] in a particular [[chemical compound]] is replaced by another group.<ref name=jerry>{{JerryMarch}}</ref> These reactions can be distinguished by the type of substituting species into a [[nucleophilic substitution|nucleophilic]], [[electrophilic substitution|electrophilic]] or [[radical substitution]].
{{multiple image | direction = vertical | image1 = SN1 reaction mechanism.png|width1 = 300|image2 = SN2 reaction mechanism.png|width2 = 300| caption1 = S<sub>N</sub>1 mechanism| caption2 = S<sub>N</sub>2 mechanism}}

In the first type, a [[nucleophile]], an atom or molecule with an excess of electrons and thus a negative charge or [[partial charge]], replaces another atom or part of the "substrate" molecule. The electron pair from the nucleophile attacks the substrate forming a new bond, while the [[leaving group]] departs with an electron pair. The nucleophile may be electrically neutral or negatively charged, whereas the substrate is typically neutral or positively charged. Examples of nucleophiles are [[hydroxide]] ion, [[alkoxide]]s, [[amine]]s and [[halide]]s. This type of reaction is found mainly in [[aliphatic hydrocarbon]]s, and rarely in [[aromatic hydrocarbon]]. The latter have high electron density and enter [[nucleophilic aromatic substitution]] only with very strong [[Polar effect|electron withdrawing groups]]. Nucleophilic substitution can take place by two different mechanisms, [[SN1 reaction|S<sub>N</sub>1]] and [[SN2 reaction|S<sub>N</sub>2]]. In their names, S stands for substitution, N for nucleophilic, and the number represents the [[order (chemistry)|kinetic order]] of the reaction, unimolecular or bimolecular.<ref>{{cite book | author = Hartshorn, S.R. | url = https://books.google.com/books?id=bAo4AAAAIAAJ | title = Aliphatic Nucleophilic Substitution | publisher = [[Cambridge University Press]] | location = London | year = 1973 | isbn = 978-0-521-09801-4 | page = 1}}</ref>
{{multiple image | direction = vertical
| align = right
| width = 120
| image1= Walden-inversion-3D-balls.png
|caption1=The three steps of an [[SN2 reaction|S<sub>N</sub>2 reaction]]. The nucleophile is green and the leaving group is red
|image2=SN2-Walden-before-and-after-horizontal-3D-balls.png
|caption2=S<sub>N</sub>2 reaction causes stereo inversion (Walden inversion)
}}

The S<sub>N</sub>1 reaction proceeds in two steps. First, the [[leaving group]] is eliminated creating a [[carbocation]]. This is followed by a rapid reaction with the nucleophile.<ref>{{Cite journal|author =  Bateman, Leslie C. | author2 = Church, Mervyn G. | author3 = Hughes, Edward D. | author4 = Ingold, Christopher K. | author5 = Taher, Nazeer Ahmed |doi = 10.1039/JR9400000979|title =  188. Mechanism of substitution at a saturated carbon atom. Part XXIII. A kinetic demonstration of the unimolecular solvolysis of alkyl halides. (Section E) a general discussion|year =  1940|journal =  Journal of the Chemical Society|page =  979}}</ref>

In the S<sub>N</sub>2 mechanisms, the nucleophile forms a transition state with the attacked molecule, and only then the leaving group is cleaved. These two mechanisms differ in the [[stereochemistry]] of the products. S<sub>N</sub>1 leads to the non-stereospecific addition and does not result in a chiral center, but rather in a set of [[Cis–trans isomerism|geometric isomers]] (''cis/trans''). In contrast, a reversal ([[Walden inversion]]) of the previously existing stereochemistry is observed in the S<sub>N</sub>2 mechanism.<ref>[[#Bruckner|Brückner]], pp. 63–77</ref>

[[Electrophilic substitution]] is the counterpart of the nucleophilic substitution in that the attacking atom or molecule, an [[electrophile]], has low electron density and thus a positive charge. Typical electrophiles are the carbon atom of [[carbonyl group]]s, carbocations or [[sulfur]] or [[nitronium]] cations. This reaction takes place almost exclusively in aromatic hydrocarbons, where it is called [[electrophilic aromatic substitution]]. The electrophile attack results in the so-called σ-complex, a transition state in which the aromatic system is abolished. Then, the leaving group, usually a proton, is split off and the aromaticity is restored. An alternative to aromatic substitution is electrophilic aliphatic substitution. It is similar to the nucleophilic aliphatic substitution and also has two major types, S<sub>E</sub>1 and S<sub>E</sub>2<ref>[[#Bruckner|Brückner]], pp. 203–206</ref>

[[File:Electrophilic aromatic substitution.svg|center|thumb|648px|Mechanism of electrophilic aromatic substitution]]
{{clear}}
In the third type of substitution reaction, radical substitution, the attacking particle is a [[Radical (chemistry)|radical]].<ref name=jerry/> This process usually takes the form of a [[chain reaction]], for example in the reaction of alkanes with halogens. In the first step, light or heat disintegrates the halogen-containing molecules producing radicals. Then the reaction proceeds as an avalanche until two radicals meet and recombine.<ref>[[#Bruckner|Brückner]], p. 16</ref>
:;<chem>X. + R-H -> X-H + R.</chem>
:;<chem>R. + X2 -> R-X + X.</chem>
::<small> Reactions during the chain reaction of radical substitution </small>

===Addition and elimination===
The [[Addition reaction|addition]] and its counterpart, the [[elimination reaction|elimination]], are reactions that change the number of substituents on the carbon atom, and form or cleave [[covalent bond|multiple bonds]]. [[Double bond|Double]] and [[triple bond]]s can be produced by eliminating a suitable leaving group. Similar to the nucleophilic substitution, there are several possible reaction mechanisms that are named after the respective reaction order. In the E1 mechanism, the leaving group is ejected first, forming a carbocation. The next step, the formation of the double bond, takes place with the elimination of a proton ([[deprotonation]]). The leaving order is reversed in the E1cb mechanism, that is the proton is split off first. This mechanism requires the participation of a base.<ref>[[#Bruckner|Brückner]], p. 192</ref> Because of the similar conditions, both reactions in the E1 or E1cb elimination always compete with the S<sub>N</sub>1 substitution.<ref>[[#Bruckner|Brückner]], p. 183</ref>

{{multiple image
| align = center
| image1 = E1-mechanism.svg
| width1 = 400
| alt1 =
| caption1 = E1 elimination
| image2 = E1cb-mechanism.svg
| width2 = 400
| alt2 =
| caption2 = E1cb elimination
| footer =
}}
{{clear}}
[[File:E2-mechanism.svg|thumb|300px|E2 elimination]]

The E2 mechanism also requires a base, but there the attack of the base and the elimination of the leaving group proceed simultaneously and produce no ionic intermediate. In contrast to the E1 eliminations, different stereochemical configurations are possible for the reaction product in the E2 mechanism, because the attack of the base preferentially occurs in the anti-position with respect to the leaving group. Because of the similar conditions and reagents, the E2 elimination is always in competition with the S<sub>N</sub>2-substitution.<ref>[[#Bruckner|Brückner]], p. 172</ref>

[[File:HBr-addition.svg|thumb|300px|Electrophilic addition of hydrogen bromide]]
The counterpart of elimination is an addition where double or triple bonds are converted into single bonds. Similar to substitution reactions, there are several types of additions distinguished by the type of the attacking particle. For example, in the [[electrophilic addition]] of [[hydrogen bromide]], an electrophile (proton) attacks the double bond forming a [[carbocation]], which then reacts with the nucleophile (bromine). The carbocation can be formed on either side of the double bond depending on the groups attached to its ends, and the preferred configuration can be predicted with the [[Markovnikov's rule]].<ref>[[#Wiberg|Wiberg]], pp. 950, 1602</ref> This rule states that "In the heterolytic addition of a polar molecule to an alkene or alkyne, the more electronegative (nucleophilic) atom (or part) of the polar molecule becomes attached to the carbon atom bearing the smaller number of hydrogen atoms."<ref>{{GoldBookRef|title=Markownikoff rule|file=M03707}}</ref>

If the addition of a functional group takes place at the less substituted carbon atom of the double bond, then the electrophilic substitution with acids is not possible. In this case, one has to use the [[hydroboration–oxidation reaction]], wherein the first step, the [[boron]] atom acts as electrophile and adds to the less substituted carbon atom. In the second step, the nucleophilic [[hydroperoxide]] or halogen [[anion]] attacks the boron atom.<ref>[[#Bruckner|Brückner]], p. 125</ref>

While the addition to the electron-rich alkenes and alkynes is mainly electrophilic, the [[nucleophilic addition]] plays an important role in the carbon-heteroatom multiple bonds, and especially its most important representative, the carbonyl group. This process is often associated with elimination so that after the reaction the carbonyl group is present again. It is, therefore, called an addition-elimination reaction and may occur in carboxylic acid derivatives such as chlorides, esters or anhydrides. This reaction is often catalyzed by acids or bases, where the acids increase the electrophilicity of the carbonyl group by binding to the oxygen atom, whereas the bases enhance the nucleophilicity of the attacking nucleophile.<ref>{{cite book | author = Latscha, Hans Peter | author2 = Kazmaier, Uli | author3 = Klein, Helmut Alfons | title = Organische Chemie: Chemie-basiswissen II | volume = 2 | edition = 6th | language = de | publisher = [[Springer Science+Business Media|Springer]] | year = 2008 | isbn = 978-3-540-77106-7 | page = 273}}</ref>

[[File:H-Add-El.Mechanismus.PNG|thumb|center|500px|Acid-catalyzed addition-elimination mechanism]]
{{clear}}

[[Nucleophilic addition]] of a [[carbanion]] or another [[nucleophile]] to the double bond of an [[Α,β-unsaturated carbonyl compound|alpha, beta-unsaturated carbonyl compound]] can proceed via the [[Michael reaction]], which belongs to the larger class of [[conjugate addition]]s. This is one of the most useful methods for the mild formation of C–C bonds.<ref>{{cite book|year=2004|doi=10.1002/0471264180|title=Organic Reactions|isbn=978-0-471-26418-7 |editor-last1=Denmark |editor-first1=Scott E. }}</ref><ref>{{cite web|title = Chapter 18: Enols and Enolates — The Michael Addition reaction|author = Hunt, Ian |publisher = University of Calgary|url = http://www.chem.ucalgary.ca/courses/351/Carey5th/Ch18/ch18-4-3.html}}</ref><ref>[[#Bruckner|Brückner]], p. 580</ref>

Some additions which can not be executed with nucleophiles and electrophiles can be succeeded with free radicals. As with the free-radical substitution, the [[radical addition]] proceeds as a chain reaction, and such reactions are the basis of the [[Radical polymerization|free-radical polymerization]].<ref>{{cite book | author = Lechner, Manfred | author2 = Gehrke, Klaus | author3 = Nordmeier, Eckhard | title = Macromolecular Chemistry | edition = 3rd | publisher = [[Birkhäuser]] | location =  Basel | year = 2003 | isbn = 978-3-7643-6952-1 | pages = 53–65}}</ref>

===Other organic reaction mechanisms===
[[File:Cope Rearrangement Scheme.png|left|thumb|The Cope rearrangement of 3-methyl-1,5-hexadiene]]
{{multiple image | direction = vertical
| align = right
| width = 220
| image1= Diels Alder Mechanismus.svg
|caption1=Mechanism of a Diels-Alder reaction
| image2= Diels Alder Orbitale.svg
|caption2=Orbital overlap in a Diels-Alder reaction}}

In a [[rearrangement reaction]], the carbon skeleton of a [[molecule]] is rearranged to give a [[structural isomer]] of the original molecule. These include [[Sigmatropic reaction|hydride shift]] reactions such as the [[Wagner-Meerwein rearrangement]], where a [[hydrogen]], [[alkyl]] or [[aryl]] group migrates from one carbon to a neighboring carbon. Most rearrangements are associated with the breaking and formation of new carbon-carbon bonds. Other examples are [[sigmatropic reaction]] such as the [[Cope rearrangement]].<ref>{{cite book | author = Fox, Marye Anne | author2 = Whitesell, James K. | url = https://books.google.com/books?id=xx_uIP5LgO8C&pg=PA699 | title = Organic chemistry | edition = Third | publisher = [[Jones & Bartlett Learning|Jones & Bartlett]] | year = 2004 | isbn = 978-0-7637-2197-8 | page = 699}}</ref>

Cyclic rearrangements include [[cycloaddition]]s and, more generally, [[pericyclic reaction]]s, wherein two or more double bond-containing molecules form a cyclic molecule. An important example of cycloaddition reaction is the [[Diels–Alder reaction]] (the so-called [4+2] cycloaddition) between a conjugated [[diene]] and a substituted [[alkene]] to form a substituted [[cyclohexene]] system.<ref>{{Cite journal| volume = 460| pages = 98–122| year = 1928 | doi = 10.1002/jlac.19284600106| journal = Justus Liebig's Annalen der Chemie| title = Synthesen in der hydroaromatischen Reihe| first2 = K.| last2 = Alder| last1 = Diels| first1 = O.}}</ref>

Whether a certain cycloaddition would proceed depends on the electronic orbitals of the participating species, as only orbitals with the same sign of [[wave function]] will overlap and interact constructively to form new bonds. Cycloaddition is usually assisted by light or heat. These perturbations result in a different arrangement of electrons in the excited state of the involved molecules and therefore in different effects. For example, the [4+2] Diels-Alder reactions can be assisted by heat whereas the [2+2] cycloaddition is selectively induced by light.<ref>[[#Bruckner|Brückner]], pp. 637–647</ref> Because of the orbital character, the potential for developing [[stereochemistry|stereoisomeric]] products upon cycloaddition is limited, as described by the [[Woodward–Hoffmann rules]].<ref>{{Cite journal | doi = 10.1021/ja01080a054| title = Stereochemistry of Electrocyclic Reactions| journal = Journal of the American Chemical Society| volume = 87| issue = 2| pages = 395–397| year = 1965| last1 = Woodward | first1 = R.B.| last2 = Hoffmann | first2 = R. | bibcode = 1965JAChS..87..395W}}</ref>