NEUTRAL INORGANIC SPECIES
Polar
bonds are polarized such that the more electronegative atom has the greater
share of the bonding electrons. As a result, it will be electron rich and will
be a nucleophilic center. The less electronegative atom will be elec- tron
deficient and will be an electrophilic center. Electronegative atoms are to the
right of the periodic table and have lone pairs of electrons.
The
relative strengths of neutral nucleophilic centers are determined by how well
they can accommodate a positive charge. The more electronega- tive the atom,
the less nucleophilic it will be. Therefore, nitrogen is more nucleophilic than
oxygen, and oxygen is more nucleophilic than fluorine.
The
relative electrophilic strengths of hydrogen atoms in different mole- cules is
determined by the stability of the ions formed. Hydrogen atoms attached to
nitrogen are only
weakly electrophilic, whereas
hydrogens attached to halogen
atoms are strongly
electrophilic. Therefore, theelectrophilic strength of hydrogen atoms
depends on the electronegativity of the neighboring atom.
It is
possible to predict whether a molecule is likely to react as an elec- trophile
or as a nucleophile, based on the strength of the nucleophilic or electrophilic
centers present.
If two atoms of quite different electronegativities
are linked together, then the bond connecting them will be polar covalent such
that the bonding electrons are biased towards the more electronegative atom.
This will give the latter a slightly negative charge and make it a nucleophilic
center. Conversely, the less electronegative atom will gain a slightly positive
charge and be an electrophilic center (Fig.
1). The further to the right an element is in the periodic table, the more
electronegative it is. Thus, fluorine is more electronegative than oxygen,
which in turn is more electronegative than nitrogen. Note also that all the
nucleophilic atoms identified above have lone pairs of electrons. This is
another way of identifying nucleophilic atoms.
The molecules above have both nucleophilic and
electrophilic centers and could react as nucleophiles or as electrophiles.
However, it is usually found that there is a preference to react as one rather
than the other. This is explained by considering the relative strengths of
nucleophilic and electrophilic centers. First of all, let us consider the
relative strengths of nucleophilic centers by comparing N, O, and F. If we
compare the relative positions of these atoms in the periodic table, we find
that fluorine is more electronegative than oxygen, which in turn is more
electro-negative than nitrogen. However, when we compare the nucleophilic
strengths of these atoms, we find that the nitrogen is more nucleophilic than
oxygen, which in turn is more nucleophilic than fluorine.
The relative nucleophilic strengths of these
atoms is explained by looking at the products which would be formed if these
atoms were to act as nucleophiles.
Let us compare the three molecules HF, H2O, and NH3 and
see what happens if they were to form a bond to a proton (Fig. 2). Since the proton has no electrons, both electrons for the
new bond must come from the nucleophilic centers (i.e. the F, O, and N). As a
result, these atoms will gain a positive charge. If hydrogen fluoride acts as a
nucleophile, then the fluorine atom gains a positive charge. Since the fluorine
atom is strongly electronegative, it does not tolerate a positive charge.
Therefore, this reaction does not take place. Oxygen is less electronegative
and is able to tolerate the positive charge slightly better, such that an
equilibrium is pos-sible between the charged and uncharged species. Nitrogen is
the least elec-tronegative of the three atoms and tolerates the positive charge
so well that the reaction is irreversible and a salt is formed.
Thus, nitrogen is strongly nucleophilic and
will usually react as such, whereas halogens are weakly nucleophilic and will
rarely react as such.
Lastly, it is worth noting that all these
molecules are weaker nucleophiles than their corresponding anions, i.e. HF, H2O,
and NH3 are weaker nucleophiles than F -, OH-
and NH-2 respectively.
The same argument can be used in reverse when looking at the relative electrophilic strengths of atoms in different molecules. Let us compare the elec-trophilic strengths of the hydrogens in HF, H2O, and NH3. In this case, reaction with a strong nucleophile or base would generate anions (Fig. 3). Fluorine being the most electronegative atom is best able to stabilize a negative charge and so the fluoride ion is the most stable ion of the three. Oxygen is also able to stabilize a negative charge, though not as well as fluorine.
Nitrogen is the least electronega-tive of the three atoms and has the
least stabilizing influence on a negative charge and so the NH2 ion is
unstable. The more stable the anion, the more easily it is formed and hence the
hydrogen which is lost will be strongly electrophilic. This is the case for HF.
In contrast, the hydrogen in ammonia is a very weak electrophilic center since
the anion formed is unstable. As a result, nitrogen anions are only formed with
very strong bases.
It is possible to predict whether molecules are
more likely to react as nucleophiles or electrophiles depending on the strength
of the nucleophilic and electrophilic centers present. For example, ammonia has
both electrophilic and nucleophilic centers. However, it usually reacts as a
nucleophile since the nitrogen atom is a strong nucleophilic center and the
hydrogen atom is a weak electrophilic center. By contrast, molecules such as
hydrogen fluoride or aluminum chloride prefer to react as electrophiles. This
is because the nucleophilic centers in both these molecules (halogen atoms) are
weak, whereas the electrophilic centers (H or Al) are strong. Water is a
molecule which can react equally well as a nucleophile or as an electrophile.
For example, water reacts as a nucleophile with a proton and as an electrophile
with an anion (Fig. 4).
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