Respectively, nucleophiles and electrophiles
are designated Nu– and E+. But, bear in
mind, not all nucleophiles or all electrophiles
are always negatively and positively charged,
but it’s useful way of defining their identity
to understanding how they react.
In organic chemistry, reactions happen between
electrophiles and nucleophiles. A nucleophile
is defined as an atom, ions or a molecule
that have an electron pair that may be donated
to form a covalent bond to an electrophile.
Electrophiles, on the other hand, accept electrons.
Nucleophiles can have a negative charge, a
partial negative charge as we saw there, delta-
or an electron pair or pi electrons available
to them. Electron pairs being, for example,
on nitrogen atoms, in the case of ammonia.
Some examples of these nucleophilic species
are shown below. C = C, C ≡ C, H2O, OH,
CH3OH, CH3O-, CN-, NH3, NH2- and as
we have seen here, a dicarbide.
Electrophiles, on the other hand, are atoms,
ions or molecules which are deficient in electrons
and will have the affinity for an electron
pair bringing electrons to them and will bond
to a base or a nucleophile accordingly. They
can have a positive charge. So, for example,
in the case of our HBr; H, delta H, delta+ or
they have an empty orbital available. And
one of the ones that springs to mind when
looking at this would be the element boron,
which is capable of accepting a lone pair
of electrons. Examples of this would be: H+,
BF3, SO3, Nitrite (NO2+), CH3+
and also, CH3CO+.
Organic ions which have a charge, which are
localised on the carbon atom, they are called
carbocations, if they have a positive charge
and carboanions, if they have a negative charge.
And we’ve come across the idea of carbocations
and carboanions before.