Nucleophilic attack (or more accurately a nucleophilic substitution), is where a nucleophile[note 1] 'attacks' a molecule with a positive or partial positive charge and replaces the negative or partial negative part of that molecule (which is called the 'leaving group') causing that leaving group to do what its name suggests and leave the molecule. Reaction wise it can be represented as:
Nuc: + R-LG → R-Nuc + LG:
Nuc: is the nucleophile,
R is the positive/partial positive part of the molecule undergoing attack,
LG is the negative/partial negative part of the molecule (i.e., the Leaving Group). It does not matter if you do or do not understand this concept, it's more a curiosity here than it is a prerequisite.
- ↑ A species (an ion or a molecule) which is strongly attracted to a region of positive charge in something else.