The electrophiles are either a positive ion (E+) or some other electron-deficient species with a large partial positive charge. For example, benzene can be brominated when it reacts with bromine in the presence of FeBr3. Bromine and FeBr3 reacts to produce positive bromonium ions, Br+. These positive bromonium ions act as electrophiles and attack the benzene ring replacing one of the hydrogen atoms in a reaction that is called an electrophilic aromatic substitution (EAS).Electrophilic aromatic substitutions allow the direct introduction of a wide variety of groups into an aromatic ring and because of this they provide synthetic routes to many important compounds. The five electrophilic aromatic substitutions that we shall study in this package are outlined in fig. All of these reactions involve the attack on the benzene ring by an electron-deficient species – (by an electrophile). Later we shall learn what the electrophile is in each instance.
The species which are electron deficient and have tendency to form bond with carbon are good Electrophiles. Few example of Electrophiles are.
H+, C+, Br+, Cl+,AlCl3,….
These acidic reagents (Lewis acids) that are requiring a pair of electrons are called electrophilic reagents (Greek: electrophiles, electron-loving).Thus the characteristic reaction of an alkene is electrophilic addition (i.e. addition of acidic reagents). Alkenes also undergo free-radical addition where the reagents, the free radicals, need just an electron.Except ethene, alkenes contain not only the carboncarbon double bond but also alkyl groups having essentially the alkane structure. Therefore, alkenes under special conditions may undergo the free-radical substitution reactions typical of alkanes.When addition occurs, the trigonal planar arrangement in the alkene changes into tetrahedral. In the first step, alkene’s addition reactions involve the attack of an electrophile. This is the ratecontrolling step. In the second step, the intermediate, a carbonium ion, reacts rapidly with a nucleophile. If E and Nu, describe the electrophilic and nucleophilic fragments of the addendum E+Nu, the addition.