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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.
Additional recommended knowledge
Electrophiles in organic chemistry
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:
This takes the form of 3 main steps shown below;
This process is called AdE2 mechanism. Iodine (I2), chlorine (Cl2), sulfenyl ion (RS+), mercury cation (Hg2+), and dichlorocarbene (:CCl2) 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.
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:
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-SE2 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.
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:
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:
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.
where is the resistance (Ohm or Ω) and 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 . 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 are defined as cationic electrophilic reagents with greatly enhanced reactivities in the presence of superacids. These compounds were first described by George A. Olah . 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 CH3COH 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 
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Electrophile". A list of authors is available in Wikipedia.|