Electrophile
An electrophile is any compound or reaction intermediate that is electron-deficient and thus accepts electrons from another source. The word electrophile is derived from the Greek words electros meaning electron, and philos meaning loving. Coined by Sir Christopher Ingold, the common notation used by chemists to represent an electrophile is E+, where E stand for the electrophile and the positive sign emphasizes its electron deficiency.
However, electrophiles can be neutral, partially positively charged, or have a full positive charge. Some examples of positively charged electrophiles are the hydronium ion, nitronium ions, metal ions, and carbocations. Some examples of neutral electrophiles are ozone (O3), Lewis acids (such as BF3), sulfonating agents, halogenating agents (such as Cl2), and peracids. Electrophiles can also be reaction intermediates that do not have an octet of valence electrons, such as carbenes, nitrenium ions, and halogen atoms. Ingold was among the earliest chemists to recognize that many chemical mechanisms may be understood in terms of electrophiles reacting with nucleophiles.
Electrophiles are encountered most often in organic chemistry, and they are involved in many general reaction groups. Among the reactions involving electrophiles are electrophilic addition to multiple bonds (alkenes and alkynes), reactions with alkanes (as rearrangements, free radical halogenation, and reactions with peracids), and electrophilic aromatic reactions (such as Friedel-Crafts alkylation and acylation).
The most fundamental electrophile in organic chemistry is the proton written as H+ or H3O+ (the hydronium ion). The addition of the proton to a double bond creates a carbocation that, in turn, is subject to nucleophilic attack. Electrophilic addition to a multiple bond is one example of the use of the proton as an electrophile. The first step is the addition of the proton to the pi bond of the alkene or alkyne, acting as the electron source. This leads to an electron- deficient carbocationic intermediate. In the second step, this carbocation acts as the electrophile and bonds with a nucleophile.
Rearrangements of hydrocarbons are often found in the presence of Lewis acids. One example is the complete skeletal rearrangement of endo- tetrahydrodicyclopentadiene to adamantane in the presence of AlCl3. Although the mechanism is uncertain, it has been found that this reaction is initiated by electrophiles that generate carbocationic intermediates. Hydride abstraction processes initiated by these electrophiles are the origin of these carbocation intermediates.
Carbocations make excellent electrophiles, and in the presence of Lewis acids they are able to substitute onto an aromatic ring. Two men discovered that in the presence of Lewis acids (such as aluminum chloride or ferric chloride), alkyl halides will alkylate benzenes. French chemist Charles Friedel (1832-1899) and American chemist James Crafts (1839-1917) were the first scientists to study this aromatic addition in 1877. In this electrophilic aromatic substitution, the first electrophile is the Lewis acid that accepts the halogen from an alkyl halide (the nucleophile). This produces the second electrophile, the positively charged alkyl group. This carbocation attacks the pi system of the benzene ring leading to an electron-deficient arenium ion. The arenium intermediate loses a proton to reestablish the resonance of the 6e- pi system.
There are many important industrial processes that use electrophilic reactions. One important reaction is the production of isooctane (2,2,4- trimethylpentane) for gasoline. This reaction is initiated by the electrophilic protonation of isobutylene. Electrophilic addition of the t- butyl cation to isobutylene, followed by hydride transfer from isobutane completes the process and regenerates the t-butyl cation. The end result is the addition of isobutane to isobutylene to form isooctane, the molecule that is the basis of octane ratings seen at the gas pumps.
In the mining industry, the production of TNT (trinitrotoluene) is made possible by the electrophilic nitration of toluene (methyl benzene). A nitronium ion, made by treating nitric acid with sulfuric acid, is the electrophile. The nitronium ion attacks the pi electrons of the benzene ring to give--by electrophilic aromatic substitution--nitrotoluene. This nitration is repeated an additional two times (with each nitration requiring increasingly rigorous reaction conditions) to yield the 1- methyl-2,4,6-trinitrobenzene as the final product. The presence of the three stongly electrophilic nitro substituents in the TNT molecule makes the molecule reactive (in this case, as an explosive).
The concept of an electrophile is very general one. There are literally hundreds of reagents not discussed here that can be categorized as electrophiles. In each case, electrophiles are simply molecules or ions in search of electrons.
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