A hydrogen of sulfuric acid (H2SO4) is the acid, and the nitrogen of
ammonia (NH3) is the base. They react to form the hydrogen sulfate
anion (HSO4c- ) and the ammonium ion (NH4⊕). The ammonium ion is
the conjugate acid of ammonia. The bisulfate ion is the conjugate base
of the sulfuric acid
Like Brønsted and Lowry, G. N. Lewis defined acids and bases
in a broader scheme than Arrhenius did. Lewis noted that there are a
number of reactions that look like acid-base reactions but do not
involve the transfer of a proton. Instead, they involve the interaction
of a pair of nonbonding electrons. From that observation, he defined
an acid as a molecule that forms a covalent bond by accepting a pair of
electrons and a base as a molecule that forms a covalent bond by
donating a pair of electrons. Below is a simplified statement of the
Lewis definition of acids and bases.
A Lewis acid is an electron-pair acceptor.
A Lewis base is an electron-pair donor.
Reconciling the Acid-Base Theories
To prevent confusion over the terms acceptor and donor, stop and look at the
three definitions of acids and bases. Keep in mind that although all three
definitions consider the same concept, they do so from different viewpoints.
Arrhenius and Brønsted-Lowry look at acids and bases from the viewpoint ofproton transfers. Lewis looks at them from the viewpoint of electron pairs. The two viewpoints mesh when you remember that a proton is a positive hydrogen
ion that has no electron, and is thus capable of accepting a pair of electrons.
Solved Exercise 5.1
The following compounds can act either as a Brønsted-Lowry acid or a Lewis
acid. Show the reactive site in each compound and the structure of the
conjugate base that results from a reaction with base Ac- . Determine
whether the compound is a Brønsted-Lowry acid or a Lewis acid.
a) CH3OH
Solution
Both the oxygen and the carbon have full valence shells and both have at
least one hydrogen as a source of protons. However, oxygen is much more
electronegative than carbon, so a negative charge on oxygen is more stable
than a negative charge on carbon. Thus, the O—H bond is the reactive site
and a stronger Brønsted-Lowry acid than is the C—H bond.
b) CH3NH2
Solution
Nitrogen is much more electronegative than carbon, so a negative charge on
nitrogen is more stable than a negative charge on carbon. Thus, the N—H
bond is a stronger Brønsted-Lowry acid than is
c) CH3BH2
Solution
Because boron is electron deficient with only six electrons in its valence shell,
it will react before any bonds to hydrogen are broken. Thus, the boron is the
reactive site, and it acts as a Lewis acid. the C—H bond.
Because a Lewis acid accepts a pair of electrons, chemists call
it an electrophile, from the Greek meaning "lover of electrons." They
call the base a nucleophile, or "lover of nuclei," because it donates
the electrons to a nucleus with an empty orbital. In a chemical
reaction, a nucleophile seeks a nucleus, or a positive charge, and an
electrophile seeks electrons, or a negative charge. Fundamental to
organic chemistry is the fact that nearly all the reactions that you will
study are reactions of an acid with a base or, more commonly, of an
electrophile with a nucleophile.
Chemists use a curved arrow ( ) to show electron
movement. A curved arrow points from the electron-rich reactant, the
base or nucleophile, toward the electron-poor reactant, the acid or
electrophile. Rewriting the previous two reactions using a curved
arrow shows the movement of electrons. In each reaction, a pair of
nonbonding electrons from a nucleophile reacts with an electrophile to
form a bond
Exercise 5.1
Use curved arrows to write the acid-base reaction of a hydrogen ion
with a hydroxide ion.