Electrophilicity

A chemical species, or a region of a molecule, is ELECTROPHILIC when it is poor in electron density and thus tends to attack chemical species or regions of a molecule with an excess of electron density.

Video: Nucleophilicity: hydroxide anion and methyl chloride

Reactions between molecules can be understood in terms of their electron distribution. Electron density is to molecules what atmospheric pressure is to the Earth: weather is explained by low-pressure (storm) and high-pressure (anticyclone) zones. Electronegative atoms concentrate electron density and leave their surroundings depleted of it.

An ELECTROPHILIC species or region of a molecule will attack species or molecular regions with an excess of electron density.

ELECTROPHILE literally means 'electron-loving', i.e. attracted to negative charges.

Does this reaction make sense to you?

Addition of HX to an alkene: electrophilic attack of H⁺ on the double bond and nucleophilic attack of X⁻ on the carbocation

Electrostatic potential maps of ethene (electron-rich double bond) and HCl (positive H, negative Cl)

Electron density distribution in ethene (red = high; blue = low)

Animation of the addition of HX to propene

The relatively high electron density of the double bond is electrophilically attacked by the proton of HCl. A carbocation is thus formed, which electrophilically attacks the chloride anion.

The same can be described in a complementary way:

The relatively high electron density at the double bond nucleophilically attacks the proton of HCl. A carbocation is thus produced that is nucleophilically attacked by the chloride anion.

Both ways of describing the reaction are correct. Can you appreciate the subtlety of the language when using the pair of terms electrophile-nucleophile?