Brønsted–Lowry Theory for Acids and Bases
Acid- base reactions
We begin our
study of chemical reactions and mechanisms by examining some of the basic principles of acid–base chemistry. There are several reasons for doing this:
(1) Many of the
reactions that occur in organic chemistry are either acid–base reactions themselves
or they involve an acid–base reaction at some stage.
(2) Acid–base
reactions are simple fundamental reactions that will enable you to see how
chemists use curved arrows to represent mechanisms of reactions and how they depict
the processes of bond breaking and bond making that occur as molecules react.
Brønsted–Lowry Acids and Bases
** Two classes of
acid–base reactions are fundamental in organic chemistry: Brønsted–Lowry and
Lewis acid–base reactions.
** We start our
discussion with Brønsted–Lowry acid–base reactions.
(a) Brønsted–Lowry acid–base reactions involve the
transfer of protons.
(b) A
Brønsted–Lowry acid is a substance that can donate (or lose) a proton.
(c)
Brønsted–Lowry base is a substance that can accept (or remove) a proton.
** Let us consider
some examples:
Reaction between HCl (gas) with water
** Hydrogen
chloride (HCl), in its pure form, is a gas. When HCl gas is bubbled into water,
the following reaction occurs.
** In this
reaction hydrogen chloride donates a proton; therefore it acts as a
Brønsted–Lowry acid. Water accepts a proton from hydrogen chloride; thus water
serves as a Brønsted– Lowry base. The products are a hydronium ion (H3O+)
and chloride ion (Cl-).
Important Notes
(1) Just as we
classified the reactants as either an acid or a base, we also classify the products
in a specific way:
(a) The
molecule or ion that forms when an acid loses its proton is called the conjugate
base of that acid. In the above example, chloride ion is the conjugate base.
(b) The
molecule or ion that forms when a base accepts a proton is called the conjugate
acid. Hydronium ion is the conjugate acid of water.
(2) Hydrogen
chloride is considered a strong acid because transfer of its proton in water
proceeds essentially to completion. Other strong acids that completely transfer
a proton when dissolved in water are hydrogen iodide, hydrogen bromide, and
sulfuric acid.
(3) The extent
to which an acid transfers protons to a base, such as water, is a measure of
its strength as an acid. Acid strength is therefore a measure of the percentage
of ionization and not of concentration.
(4) Sulfuric
acid is called a diprotic acid because it can transfer two protons. Transfer of
the first proton occurs completely, while the second is transferred only to the
extent of about 10% (hence the equilibrium arrows in the equation for the
second proton transfer).
Acids and Bases in Water
** Hydronium ion is the strongest acid
that can exist in water to any significant extent. Any acid stronger than
hydronium ion will simply transfer its proton to a water molecule to form
hydronium ions.
** Hydroxide ion is the strongest base
that can exist in water to any significant extent. Any base stronger than
hydroxide will remove a proton from water to form hydroxide ions.
** When an
ionic compound dissolves in water the ions are solvated. With sodium hydroxide,
for example, the positive sodium ions are stabilized by interaction with
unshared electron pairs of water molecules, and the hydroxide ions are
stabilized by hydrogen bonding of their unshared electron pairs with the
partially positive hydrogens of water molecules.
** When an
aqueous solution of sodium hydroxide is mixed with an aqueous solution of hydrogen
chloride (hydrochloric acid), the reaction that occurs is between hydronium and
hydroxide ions. The sodium and chloride ions are called spectator ions because
they play no part in the acid–base reaction:
** What we have
just said about hydrochloric acid and aqueous sodium hydroxide is true when solutions
of all aqueous strong acids and bases are mixed. The net ionic reaction is
simply.
Reference: Organic chemistry / T.W. Graham Solomons , Craig B.Fryhle , Scott A.snyder , / ( eleventh edition) / 2014.
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