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| In general, electrophiles are positively charged species that are attracted to an electron rich centre. In [[chemistry]], an '''electrophile''' (literally ''electron-lover'') is a [[reagent]] attracted to [[electron]]s that participates in a chemical reaction by accepting an [[electron pair]] in order to [[Chemical bond|bond]] to a [[nucleophile]]. Because electrophiles accept electrons, they are [[Lewis acid]]s (see [[acid-base reaction theories]]). Most electrophiles are positively [[Electric charge|charge]]d, have an atom that carries a partial positive charge, or have an atom that does not have an octet of electrons.
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| The electrophiles are attacked by the most electron-populated part of one [[nucleophile]]. The electrophiles frequently seen in the [[Organic synthesis|organic syntheses]] are [[cations]] such as [[Hydrogen ion|H<sup>+</sup>]] and [[nitrosonium|NO<sup>+</sup>]], polarized neutral molecules such as [[hydrogen chloride|HCl]], [[alkyl halide]]s, [[acyl halide]]s, and [[carbonyl compound]]s, polarizable neutral molecules such as [[chlorine|Cl<sub>2</sub>]] and [[bromine|Br<sub>2</sub>]], [[oxidizing agent]]s such as organic [[peracid]]s, chemical species that do not satisfy the [[octet rule]] such as [[carbene]]s and [[Radical (chemistry)|radicals]], and some Lewis acids such as [[Borane|BH<sub>3</sub>]] and [[Diisobutylaluminium hydride|DIBAL]].
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| == Organic chemistry ==
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| === Alkenes ===
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| [[Electrophilic addition]] is one of the three main forms of reaction concerning [[alkenes]].
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| *[[Hydrogenation]] by the addition of hydrogen over the double bond
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| *[[Electrophilic addition]] reactions with halogens and sulfuric acid
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| *[[Hydration reaction|Hydration]] to form alcohols.
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| === Addition of halogens ===
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| These occur between alkenes and electrophiles, often halogens as in [[halogen addition reaction]]s. Common reactions include use of bromine water to [[titration|titrate]] against a sample to deduce the number of double bonds present. For example, [[ethene]] + [[bromine]] → [[1,2-Dibromoethane|1,2-dibromoethane]]:
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| :C<sub>2</sub>H<sub>4</sub> + Br<sub>2</sub> → BrCH<sub>2</sub>CH<sub>2</sub>Br
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| This takes the form of 3 main steps shown below;<ref>Lenoir, D.; Chiappe, C. ''Chem. Eur. J.'' '''2003''', ''9'', 1036.</ref>
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| :[[Image:Electrophilic addition of Br2.png|410px]]
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| #'''Forming of a π-complex'''
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| #:The electrophilic Br-Br molecule interacts with electron-rich alkene molecule to form a [[pi-complex|π-complex]] '''1'''.
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| #'''Forming of a three-membered bromonium ion'''
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| #:The alkene is working as an electron donor and bromine as an electrophile. The three-membered [[bromonium ion]] '''2''' consisted with two carbon atoms and a bromine atom forms with a release of Br<sup>−</sup>.
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| #'''Attacking of bromide ion'''
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| #:The bromonium ion is opened by the attack of Br<sup>−</sup> from the back side. This yields the [[Vicinal (chemistry)|vicinal]] dibromide with an [[antiperiplanar]] configuration. When other nucleophiles such as water or alcohol are existing, these may attack '''2''' to give an alcohol or an ether.
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| This process is called '''Ad<sub>E</sub>2 mechanism'''. [[Iodine]] (I<sub>2</sub>), [[chlorine]] (Cl<sub>2</sub>), [[sulfenyl]] ion (RS<sup>+</sup>), [[Mercury (element)|mercury]] cation (Hg<sup>2+</sup>), and [[dichlorocarbene]] (:CCl<sub>2</sup>) also react through similar pathways. The direct conversion of '''1''' to '''3''' will appear when the Br<sup>−</sup> is large excess in the reaction medium. A β-bromo [[carbenium ion]] intermediate may be predominant instead of '''3''' if the alkene has a cation-stabilizing substituent like phenyl group. There is an example of the isolation of the bromonium ion '''2'''.<ref>Brown, R. S. Acc. Chem. Res. 1997, 30, 131.</ref>
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| === Addition of hydrogen halides=== | |
| Hydrogen halides such as hydrogen chloride (HCl) adds to alkenes to give alkyl halide in [[hydrohalogenation]]. For example, the reaction of HCl with ethylene furnishes chloroethane. The reaction proceeds with a cation intermediate, being different from the above halogen addition. An example is shown below:
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| :[[Image:Electrophilic addition of HCl.png|390px]]
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| #Proton (H<sup>+</sup>) adds (by working as an electrophile) to one of the carbon atoms on the alkene to form cation '''1'''.
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| #Chloride ion (Cl<sup>−</sup>) combines with the cation '''1''' to form the adducts '''2''' and '''3'''.
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| In this manner, the [[stereoselectivity]] of the product, that is, from which side Cl<sup>−</sup> will attack relies on the types of alkenes applied and conditions of the reaction. At least, which of the two carbon atoms will be attacked by H<sup>+</sup> is usually decided by [[Markovnikov's rule]]. Thus, H<sup>+</sup> attacks the carbon atom that carries fewer substituents so as the more stabilized carbocation (with the more stabilizing substituents) will form.
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| This process is called '''A-S<sub>E</sub>2 mechanism'''. [[Hydrogen fluoride]] (HF) and hydrogen iodide (HI) react with alkenes in a similar manner, and Markovnikov-type products will be given. Hydrogen bromide (HBr) also takes this pathway, but sometimes a radical process competes and a mixture of isomers may form.
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| === Hydration ===
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| One of the more complex [[hydration reaction]]s utilises [[sulfuric acid]] as a [[catalyst]]. This reaction occurs in a similar way to the addition reaction but has an extra step in which the OSO<sub>3</sub>H group is replaced by an OH group, forming an alcohol:
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| :C<sub>2</sub>H<sub>4</sub> + H<sub>2</sub>O → C<sub>2</sub>H<sub>5</sub>OH
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| As can be seen, the H<sub>2</sub>SO<sub>4</sub> does take part in the overall reaction, however it remains unchanged so is classified as a catalyst.
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| This is the reaction in more detail:
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| :[[Image:Electrophilic reaction of sulfuric acid with ethene.png|580px]]
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| #The H-OSO<sub>3</sub>H molecule has a δ+ charge on the initial H atom. This is attracted to and reacts with the double bond in the same way as before.
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| #The remaining (negatively charged) <sup>−</sup>OSO<sub>3</sub>H ion then attaches to the [[carbocation]], forming ethyl hydrogensulphate (upper way on the above scheme).
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| #When water (H<sub>2</sub>O) is added and the mixture heated, ethanol (C<sub>2</sub>H<sub>5</sub>OH) is produced. The "spare" hydrogen atom from the water goes into "replacing" the "lost" hydrogen and, thus, reproduces sulfuric acid. Another pathway in which water molecule combines directly to the intermediate carbocation (lower way) is also possible. This pathway become predominant when aqueous sulfuric acid is used.
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| Overall, this process adds a molecule of water to a molecule of ethene.
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| This is an important reaction in industry, as it produces [[ethanol]], whose purposes include fuels and starting material for other chemicals.
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| ==Electrophilicity scale== | |
| {|align="right" class="wikitable"
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| |colspan=2 align="center"|'''Electrophilicity index'''
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| |-
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| || [[Fluorine]]||3.86
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| |-
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| ||[[Chlorine]]||3.67
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| |-
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| || [[Bromine]]||3.40
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| |-
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| || [[Iodine]]||3.09
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| || [[Hypochlorite]]||2.52
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| |-
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| || [[Sulfur dioxide]]||2.01
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| || [[Carbon disulfide]]||1.64
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| |-
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| || [[Benzene]]||1.45
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| |-
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| || [[Sodium]]||0.88
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| |-
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| |Colspan=2|Some selected values <ref name=Parr/> (no dimensions)
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| |}
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| Several methods exist to rank electrophiles in order of reactivity <ref>Electrophilicity Index Chattaraj, P. K.; Sarkar, U.; Roy, D. R. [[Chem. Rev.]]; (Review); '''2006'''; 106(6); 2065-2091. {{DOI|10.1021/cr040109f}}</ref> and one of them is devised by [[Robert Parr]] <ref name=Parr>''Electrophilicity Index''
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| Parr, R. G.; Szentpaly, L. v.; Liu, S. [[J. Am. Chem. Soc.]]; (Article); '''1999'''; 121(9); 1922-1924. {{DOI|10.1021/ja983494x}}</ref> with the '''electrophilicity index''' [[omega|ω]] given as:
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| :<math>\omega = \frac{\chi^2}{2\eta}\,</math>
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| with <math>\chi\,</math> the [[electronegativity]] and <math>\eta\,</math> [[chemical hardness]]. This equation is related to classical equation for [[electric power|electrical power]]:
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| :<math>P = \frac{V^2}{R}\,</math>
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| where <math>R\,</math> is the [[Electrical resistance|resistance]] ([[Ohm]] or Ω) and <math>V\,</math> is [[voltage]]. In this sense the electrophilicity index is a kind of electrophilic power. Correlations have been found between electrophilicity of various chemical compounds and reaction rates in biochemical systems and such phenomena as allergic contact dermititis.
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| An electrophilicity index also exists for [[free radical]]s.<ref>''Electrophilicity and Nucleophilicity Index for Radicals'' Freija De Vleeschouwer, Veronique Van Speybroeck, Michel Waroquier, Paul Geerlings, and Frank De Proft Org. Lett.; 2007; 9(14) pp 2721 - 2724; (Letter) {{doi|10.1021/ol071038k}}</ref> Strongly electrophilic radicals such as the halogens react with electron-rich reaction sites, and strongly nucleophilic radicals such as the 2-hydroxypropyl-2-yl and [[tert-butyl]] radical react with a preference for electron-poor reaction sites.
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| ==Superelectrophiles==
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| '''Superelectrophiles''' are defined as cationic electrophilic reagents with greatly enhanced reactivities in the presence of [[superacid]]s. These compounds were first described by [[George A. Olah]].<ref>''Electrophilic reactions at single bonds. XVIII. Indication of protosolvated de facto substituting agents in the reactions of alkanes with acetylium and nitronium ions in superacidic media'' George A. Olah, Alain Germain, Henry C. Lin, David A. Forsyth [[J. Am. Chem. Soc.]]; '''1975'''; 97(10); 2928-2929. {{DOI| 10.1021/ja00843a067}}</ref> Superelectrophiles form as a doubly electron deficient superelectrophile by protosolvation of a cationic electrophile. As observed by Olah, a mixture of [[acetic acid]] and [[boron trifluoride]] is able to remove a hydride ion from [[isobutane]] when combined with [[hydrofluoric acid]] via the formation of a [[superacid]] from BF<sub>3</sub> and HF. The responsible [[reactive intermediate]] is the CH<sub>3</sub>CO<sub>2</sub>H<sub>3</sub> dication. Likewise, [[methane]] can be nitrated to [[nitromethane]] with [[nitronium tetrafluoroborate]] NO{{su|b=2|p=+}}BF{{su|b=4|p=−}} only in presence of a strong acid like [[fluorosulfuric acid]].
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| In '''gitionic''' ('''gitonic''') superelectrophiles, charged centers are separated by no more than one atom, for example, the protonitronium ion O=N<sup>+</sup>=O<sup>+</sup>—H (a protonated [[nitronium ion]]). And, in '''distonic''' superelectrophiles, they are separated by 2 or more atoms, for example, in the fluorination reagent [[Selectfluor|F-TEDA-BF<sub>4</sub>]]<ref>''Knorr Cyclizations and Distonic Superelectrophiles'' Kiran Kumar Solingapuram Sai, Thomas M. Gilbert, and Douglas A. Klumpp [[J. Org. Chem.]] '''2007''', 72, 9761-9764 {{DOI|10.1021/jo7013092}}</ref>
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| ==See also==
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| *[[Nucleophile]]
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| ==References==
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| {{reflist}}
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| [[Category:Physical organic chemistry]]
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