Chemistry Reference
In-Depth Information
substitution reactions in Chapter 8, and this is why
we have purposely discussed the acidity of aromatic
derivatives in some detail.
•
a weak base has a large
K
a
and thus a small p
K
a
,
i.e. B is favoured over BH
+
.
Or, put another way:
4.4
Basicity
•
the larger the value of p
K
a
, the stronger is the
base;
We have already defined a base as a substance that
will accept a proton by donating a pair of electrons.
Just as we have used p
K
a
to measure the strength of
an acid, we need a system to measure the strength of
a base. Accordingly, a basicity scale based on p
K
b
was developed in a similar way to p
K
a
.
For the ionization of the base B in water
•
the smaller the value of p
K
a
, the weaker is the
base.
The relationship between p
K
a
and p
K
b
can be
deduced as follows:
[HO
−
][BH
+
]
[B]
K
b
=
K
B
H
2
O
BH
HO
+
+
[B][H
3
O
+
]
[BH
+
]
K
a
=
the equilibrium constant
K
is given by the formula
[B][H
3
O
+
]
[BH
+
]
[HO
−
][BH
+
]
[B]
[HO
−
][BH
+
]
[B][H
2
O]
K
a
×
K
b
=
×
K
=
[H
3
O
+
][HO
−
]
=
and since the concentration of water will be essen-
tially constant, the equilibrium constant
K
b
and the
logarithmic p
K
b
may be defined as
Thus,
K
a
×
K
b
reduces to the ionization constant for
water
K
w
.
[HO
−
][BH
+
]
[B]
K
K
b
=
H
2
O
H
2
O
H
3
O
+
+
HO
base
accepts
proton
acid
donates
proton
with
p
K
b
=−
log
10
K
b
This system has been almost completely dropped in
favour of using p
K
a
throughout the
acidity - basicity
scale
. To measure the strength of a base, we use
the p
K
a
of its conjugate acid, i.e. we consider the
equilibrium
In this reaction, one molecule of water is acting as
a base and accepts a proton from a second water
molecule. This second water molecule, therefore,
is acting as an acid and donates a proton. The
equilibrium constant
K
for this reaction is given by
the formula
K
BH
+
H
2
O
H
3
O+
B
[H
3
O
+
][HO
−
]
[H
2
O][H
2
O]
K
=
conjugate
acid
and because the concentration of water is essentially
constant in aqueous solution, the new equilibrium
constant
K
w
is defined as
for which
[B][H
3
O
+
]
[BH
+
]
K
a
=
[HO
−
][H
3
O
+
]
K
w
=
It follows that
•
a strong base has a small
K
a
and thus a large p
K
a
,
i.e. BH
+
is favoured over B;
For every hydronium ion produced, a hydroxide anion
must also be formed, so that the concentrations of
