A nucleophile provides a pair of electrons to form a new
covalent bond. It can be also seen as a Broensted base. Stronger
the base (higher pKa for the acid/base couple), stronger the
nucleophile, e.g. this series of nucleophilicity
–NH2 > HO- > F-,
but it not true for this series Cl- > F-
or HS- > HO-.
In an acid/base couple, the base is always a stronger
nucleophile.
For strong bases/nucleophiles, pKa>14.
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An electrophile accepts a pair of electrons to form a
new covalent bond. It can’t be identified as Broensted acid or
base.
