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Electrophile

In chemistry, an electrophile (literally electron-lover) is a reagent attracted to electrons that participates in a chemical reaction by accepting an electron pair in order to bond to a nucleophile. Because electrophiles accept electrons, they are Lewis acids (see acid-base reaction theories). Most electrophiles are positively charged, have an atom which carries a partial positive charge, or have an atom which does not have an octet of electrons.

The electrophiles attack the most electron-populated part of a nucleophile. The electrophiles frequently seen in the organic syntheses are cations such as H⁺ and NO⁺, polarized neutral molecules such as HCl, alkyl halides, acyl halides, and carbonyl compounds, polarizable neutral molecules such as Cl₂ and Br₂, oxidizing agents such as organic peracids, chemical species that do not satisfy the octet rule such as carbenes and radicals, and some of lewis acids such as BH₃ and DIBAL.

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1 Electrophiles in organic chemistry
2 Electrophilicity scale
3 Superelectrophiles
4 See also
5 References

Electrophiles in organic chemistry

Alkenes

Electrophilic addition is one of the three main forms of reaction concerning alkenes. They consist of:

Hydrogenation by the addition of hydrogen over the double bond.
Electrophilic addition reactions with halogens and sulfuric acid.
Hydration to form alcohols.

Addition of halogens

These occur between alkenes and electrophiles, often halogens as in halogen addition reactions. Common reactions include use of bromine water to titrate against a sample to deduce the number of double bonds present. For example, ethylene + bromine → 1,2-dibromoethane:

C₂H₄ + Br₂ → BrCH₂CH₂Br

This takes the form of 3 main steps shown below^[1];

Forming of a π-complex
The electrophilic Br-Br molecule interacts with electron-rich alkene molecure to form a π-complex 1.
Forming of a three-membered bromonium ion
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⁻.
Attacking of bromide ion
The bromonium ion is opened by the attack of Br⁻ from the back side. This yields the 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.

This process is called Ad_E2 mechanism. Iodine (I₂), chlorine (Cl₂), sulfenyl ion (RS⁺), mercury cation (Hg²⁺), and dichlorocarbene (:CCl_{2) also react through similar pathways. The direct conversion of 1 to 3 will appear when the Br⁻ 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.^[2]}

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:

Proton (H⁺) adds (by working as an electrophile) to one of the carbon atoms on the alkene to form cation 1.
Chloride ion (Cl⁻) combines with the cation 1 to form the adducts 2 and 3.

In this manner, the stereoselectivity of the product, that is, from which side Cl⁻ 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⁺ is usually decided by Markovnikov's rule. Thus, H⁺ attacks the carbon atom which carries the less number of substituents so as to the more stabilized carbocation (with the more stabilizing substituents) will form.

This process is called A-S_E2 mechanism. Hydrogen fluoride (HF) and hydrogen iodide (HI) react with alkenes similarly 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.

Hydration

One of the more complex hydration reactions 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 OSO₃H group is replaced by an OH group, forming an alcohol:

C₂H₄ + H₂O → C₂H₅OH

As you can see the H₂SO₄ does not take part in the overall reaction, however it does take part but remains unchanged so is classified as a catalyst.

This is the reaction in more detail:

The H-OSO₃H 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.
The remaining (negatively charged) ⁻OSO₃H ion then attaches to the carbocation. Forming ethyl hydrogensulphate (upper way on the above scheme).
When water (H₂O) is added and the mixture headed ethanol (C₂H₅OH) 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.

Overall this process adds a molecule of water to a molecule of ethene.

This is an important reaction in industry as it produces ethanol, which is the alcohol having various purposes including fuels and starting material for other chemicals.

Electrophilicity scale

Electrophilicity index
Fluorine	3.86
Chlorine	3.67
Bromine	3.40
Iodine	3.09
Hypochlorite	2.52
sulfur dioxide	2.01
Carbon disulfide	1.64
Benzene	1.45
Sodium	0.88
Some selected values ^[3] (no dimensions)

Several methods exist to rank electrophiles in order of reactivity ^[4] and one of them is devised by Robert Parr ^[3] with the electrophilicity index ω given as:

$\omega = \frac{\chi^2}{2\eta}\,$

with $\chi\,$ the electronegativity and $\eta\,$ chemical hardness. This equation is related to classical equation for electrical power:

$P = \frac{V^2}{R}\,$

where $R\,$ is the resistance (Ohm or Ω) and $V\,$ 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.

A electrophilicity index also exists for free radicals ^[5]. 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.

Superelectrophiles

Superelectrophiles are defined as cationic electrophilic reagents with greatly enhanced reactivities in the presence of superacids. These compounds were first described by George A. Olah ^[6]. 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 deprotonate isobutane only in combination with superacid hydrofluoric acid. The responsible reactive intermediate is the CH₃CO_H dication. Likewise methane can be nitrated to nitromethane with nitronium tetrafluoroborate NO₂⁺BF₄^- only in presence of a strong acid like fluorosulfuric acid.

In gitionic superelectrophiles charged centers are separated by no more than one atom, for example the protonitronium ion O=N⁺=O⁺-H (a protonated nitronium ion) and in distonic superelectrophiles they are separated by 2 or more atoms for example in the fluorination reagent F-TEDA-BF₄ ^[7]

References

^ Lenoir, D.; Chiappe, C. Chem. Eur. J. 2003, 9, 1036.
^ Brown, R. S. Acc. Chem. Res. 1997, 30, 131.
^ ^a ^b Electrophilicity Index Parr, R. G.; Szentpaly, L. v.; Liu, S. J. Am. Chem. Soc.; (Article); 1999; 121(9); 1922-1924. doi:10.1021/ja983494x
^ Electrophilicity Index Chattaraj, P. K.; Sarkar, U.; Roy, D. R. Chem. Rev.; (Review); 2006; 106(6); 2065-2091. doi:10.1021/cr040109f
^ 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
^ 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 10.1021/ja00843a067
^ 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

Category: Physical organic chemistry

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