Electrophiles

Literally, species seeking electrons.

This means that an electrophile is either a positively charged species, a partially positive species, or is electron deficient, i.e. does not have a full outer shell of electrons.

Electrophilic behaviour can also be induced during the process of a collision, in which charge clouds (electrons) repel causing exposure of the positive nucleus that can then attract electrons pairs.

Positively-charged species

Positively charged species are cations. These may be formed in the process of a reaction, such as the nitronium ion, NO₂⁺. In the reaction between aromatic compounds and nitrating agents, the nitrating mixture, consisting of sulfuric and nitric acid react together to form the nitronium ion.

H₂SO₄ + HNO₃ → HSO₄^- + H₂O + NO₂⁺

The nitronium ion can then behave as an electrophile in the electrophilic substitution of benzene.

NO₂⁺ + C₆H₆ → C₆H₅NO₂ + H⁺

Electron deficient species

Certain molecules, such as aluminium chloride do not have a full outer shell of electrons, making them electron deficient. This allows them to easily accept an electron pair from another species. They are electrophiles - Lewis acids.

Partially positively-charged species

Effectively, this refers to polarised species that contain a partial positive "end". This can also be achieved by using molecules that polarise other molecules, such as Friedel Crafts catalysts.

Aluminium chloride is an electron deficient species that behaves as a Lewis acid catalyst, accepting electron pairs from other species forming polarised molecules that are able to act as Lewis acids themselves. The aluminium atom can accept an electron pair from a chlorine molecule, effectively polarising the molecule leaving a partially positive chlorine atom, that can then act as a Lewis acid.

AlCl₃ + Cl₂ → AlCl₄----Cl^δ+

These reactions were studied by Friedel and Crafts and are very useful in the field of organic synthesis.

R-Cl + C₆H₆ → C₆H₅-R + HCl

---

^ top