How does the mechanism for electrophilic addition work?

First we must consider what an elecrophile is. An electrophile is a molecule or ion which is attracted to (and accepts electrons from) electron dense regions in other molecules, because it is positively charged or has a partial positive charge within it (i.e. a dipole). An electrophile with a permeant or full positive charge is usually stronger. An example of an electrophile would be the hydrogen chloride gas, which can react with electron-rich, unsaturated ethene to form 1-chloroethane in an addition reaction. This addition reaction is "so-called" because 2 smaller reactants have reacted together to form 1 larger product.

The mechanism for a reaction like this has 2 steps. It happens due to the elctrophile having a permenant dipole (H is delta positive; Cl is delta negative), due to the greater electronegativity of the cholrine atom. In the first step (illustrated by a diagram), the positive dipole of the hydrogen gets attracted to the electron-rich C=C double bond in ethene. The electrons in ethene start to repel the shared electrons in HCl further away from the hydrogen, weakening the bond. Eventually the hydrogen accepts a pair of electrons from a carbon in the C=C bond, to form a covalent C-H bond, breaking the H-Cl bond and transferring the electron pair to chlorine. As a result for the second step (illustrated) there is now a negative chloride ion and a carbocation, with one of its carbon atoms only bonded to two hydrogens and therfefore positively charged. The chloride ion is then attracted to the fully positive carbon, and donates a lone pair of electrons to the carbon to form a dative covalent bond. Overall this two-step reaction produces 1-chloroethane (no double bond) with no other product. This electron transfer mechanism is common throughout all addition reactions with all electrophiles, regardless of strength (e.g. H+, NO2, Br2).