Supplementary info on atom states in Lewis structures Changes in a reaction are represented by using arrows to show electron movements . The single electron movements are represented by fish-hook arrows . Homolytic processes are typically associated with free radical reactions. Heterolytic fission results in both electrons of the pair moving in the same direction, represented by a full-headed arrow. See Supplementary info on atom states in Lewis structures .
Heterolytic reactions are common in the aqueous environment of biochemical reactions.
They are initiated by electronic imbalances in reactants, i.e. the presence of electron rich and electron deficient species. Nucleophiles are atoms with a lone pair available to contribute to a new bond. Electrophiles are species which are electron deficient or electron seeking, such as the carbonium ion with its empty orbital. The presence of a neighbouring electron withdrawing atom or group can also leave an atom in an electron deficient state, and receptive to attack by a nucleophile. N, O or S have lone pairs in their neutral state, but generally neutral O is too electronegative to share easily.