Chemistry 2213a Fall 2012 Western University
Topic 1. Acid-Base Reactions
A. Brønsted-Lowry Acid-Base Reactions
One of the most important classes of reactions in chemistry and biology is the
Brønsted-Lowry acid-base reaction, which is a proton-transfer reaction. For
example, caffeine can be protonated by the acid in the stomach.
N N N N
+ H 3 + H 2
O N O N H
According to the Brønsted-Lowry definition of acids and bases, acids are
proton donors, and bases are proton acceptors. Generically:
HA H + A B + H BH
acid conjugate base conjugate
base acid Acid-Base Reactions 2
Every proton-transfer reaction must have an acid and a base. If one of these is
water (an amphiprotic species), we would more properly write:
For an acid: HA + H O 2 H 3 + A
For a base: B + H O2 BH + OH
In the above, the acid and the base are shown as neutrally charged species,
but they do not need to be neutrally charged.
1. Use of arrows
Proton-transfer reactions involve the movement of pairs of electrons. In organic
chemistry, the movement of a pair of electrons is shown using a curved arrow
that depicts the origin and the destination of the electrons. For example, the
deprotonation of NH4 by water can be shown as: Acid-Base Reactions 3
Other arrows that are used in chemistry are:
o Curved fishhooks, , which depict the movement of single electrons
and are used for reactions that form, change, or destroy radicals.
o Straight arrows, and , which are respectively used to show
one-way reactions or reactions that are in equilibrium.
o Double-headed arrows, , which are used exclusively to show
resonance. Acid-Base Reactions 4
2. Measuring the strength of an acid in water
Most organic acids are weak acids. When placed in water, they only ionize to a
small extent. In this ionization, water acts as the base that deprotonates the
acid. The extent of the ionization (i.e. the thermodynamic favourability of the
ionization) can be described by the acid dissociation constant, K .a
[H3O ][A ]
HA + H O 2 H3O + A Ka= pK a logK a
Stronger acids are those that ionize to a greater extent, so they have higher K a
and lower pK values. Their equilibria lie further to the right, so their ionizations
are thermodynamically more favourable (recall the relationship G = −RTlnK).
Note the use of relative terms in this paragraph.
Acid Name K a pKa
CH C3OH acetic acid 1.8 × 10 4.76
H 2 water 2.0 × 10 15.7
NH 3 ammonia 1.0 × 10 38.0
CH C3 3 ethane 1.0 × 10–51 51.0 Acid-Base Reactions 5
3. Predicting the favourability of acid-base reactions
K aor pK ) aeasures the favourability of a reaction between an acid and water.
What if the base were something else besides water? We would still use K or
pK aalues to predict the position of the equilibrium.
Consider the example from page 54 of the textbook, and notice the
stronger/weaker relationships between the conjugate pairs:
A proton-transfer reaction will favour the side with the weaker acid and the
weaker base. Be sure to understand why this “rule” works. Acid-Base Reactions 6
4. Effect of structure on the strength of an acid
Chemists are interested in the structural features of molecules that influence
acid or base strength. Consider again the equilibrium:
HA + H O 2 H 3 + A
The strength of an acid is mainly dependent on the stability of the products. In
water, one of the products formed is always H O 3egardless of the acid. The
only product that differs from one acid to another is the identity of A .
Recall from year-1 chem that i− is the relative thermodynamic stabilities of the
different conjugate bases (A ) that allow us to assess the relative strengths of
different acids (HA). Six features affecting the stability of A are:
a.Electronegativity of the atom bearing the negative charge
b.Atomic size of the atom bearing the negative charge
c. Hybridization of the atom bearing the negative charge
d.Inductive effect of other atoms/groups present in the conjugate base
e.Resonance stabilization of A relative to resonance stabilization of HA
f. Solvation of the conjugate base Acid-Base Reactions 7
Within the same period of the periodic table, acidity increases from left to right.
– – – –
Stabilities of conjugate bases: H C < 3 N < HO 2 F
When the atom bearing the hydrogen becomes more electronegative, the
hydrogen becomes more acidic. The more-electronegative atom is best able to
accommodate the negative charge.
Note that every molecule that has a hydrogen atom can act as an acid and will
have a pK value for each of the hydrogens. Methane and ammonia can act as
acids, although the pK vaaues show that they are very weak acids. Acid-Base Reactions 8
b. Atomic size
Going down the same group of the periodic table, acidity increases.
For example, acids of group 17:
HF < HCl < HBr < HI
pK a 3 −7 −9 −10
Another example would be group 16:
H 2 < H2S
pK 16 7
The larger is the atom bearing the hydrogen, the more acidic is that hydrogen.
This is because a larger atom is better able to stabilize the negative charge by
spreading it out over its larger surface area.
When going down the same group of the periodic table, the size effect is
greater than that of electronegativity. Do not look at electronegativity when
going down the same group. Acid-Base Reactions 9
The effective electronegativity of an atom depends on the identity of the atom
as well as its hybridization.
s orbitals are lower in energy and are closer to the nucleus. They can bind
electrons more strongly, and this occurs when a hybrid orbital has more s
character (the percentage of a hybrid orbital that is s).
sp has 25% s character, sp has 33%, and sp has 50%.
CH -3H 3 < CH =C2 < C2≡CH
pK 50 36 25
Essentially Not acidic Acidic enough to
not an acid react with strong
bases (e.g. NaNH ) 2
How many times more acidic is acetylene compared to ethylene? Acid-Base Reactions 10
d. Inductive effects
As seen in year-1 chem, the inductive effect is the stabilization of a charge by
an atom or group that is bonded elsewhere in the molecule.
The presence of an electron-withdrawing group (EWG) increases the acidity of
a molecule by increasing the stability of the conjugate base. This is
accomplished by removing electron density from the negatively charged area.
In that sense, electron density has been delocalized (spread out).
carbon that is two
bonds away from the
stabilizes it by drawing
electrons away from
the carboxylate group
through sigma bonds. Acid-Base Reactions 11
As seen below, different groups (X) lead to different in tae pK values.
O CH3= 4.87 HO = 3.87
H = 4.75 Cl = 2.87
X C CH2=CH = 4.35 (CH ) N = 1.83
C OH Ph = 4.31 3 3
H2 NO 2 1.68
CH 3 H < (CH =2H or Ph) < (HO or CH O)3< halogen < [NO or 2CH ) N3 3
Why is the nitro (NO2) group such a great withdrawing group?
Electron-withdrawing ability is roughly based on
O I = 3.12
electronegativity, as seen in the haloacetic acids Br = 2.86
on the right. Note that we are not looking at the C Cl = 2.81
effect of atomic size because it is not the C OH F = 2.66
halogen that bears the negative charge in the
conjugate base. Acid-Base Reactions 12
EWG effects are cumulative:
O O O
Cl C C OH Cl HCC OH Cl C C OH
2 Cl Cl
pKa= 2.81 pKa= 1.29 pKa= 0.64
Because the inductive effect is caused by a group that is bonded elsewhere in
the molecule, inductive effects fall of