4.6
Acids and Bases: General Principles
4.6
Acids and Bases: General Principles
Brønsted-Lowry definition
an acid is a proton donor
a base is a proton acceptor
Proton Transfer from HBr to Water
hydronium ion
H
.. O ..
H
.. .
.
H Br
.
.
base
acid
H
.. O+
H
H
.. . –
.. Br
.
.
.
conjugate conjugate
acid
base
Acid Strength
A strong acid is one that is completely
ionized in water.
A weak acid is one that ionizes in water
to the extent of less than 100%.
Acid strength is measured by Ka or pKa
Equilibrium constant for proton transfer
H
.. . –
.
.. O+
.
H + . Br
.
.
H
H
.. .
.. O .. + H Br .
.
.
H
K=
[H3O+][Br–]
[HBr]
[H+][Br–]
Ka =
[HBr]
pK a = – log Ka
Table 4.2 Strength of Some Brønsted Acids
Acid
Ka
pKa
Conj. Base
10
–10
I
9
–9
Br
7
–7
Cl
–4.8
HSO4
–1.7
H 2O
HI
~10
HBr
~10
HCl
~10
H2SO4
1.6 x 10
+
H 3O
55.5
5
–
–
–
–
strong acids are stronger than hydronium ion
Table 4.2 Strength of Some Brønsted Acids
Acid
H 3O
+
HF
CH3CO2H
+
NH4
H2O
Ka
pKa
Conj. Base
55.5
–1.7
H 2O
3.5 x 10
1.8 x 10
5.6 x 10
1.8 x 10
–4
–5
–10
–16
–
3.5
F
4.6
–
CH3CO2
9.2
NH3
15.7
–
HO
weak acids are weaker than hydronium ion
Table 4.2 Strength of Some Brønsted Acids
Acid
Ka
H 2O
1.8 x 10
CH3OH
~10
CH3CH2OH
~10
(CH3)2CHOH
~10
(CH3)3COH
~10
pKa
-16
Conj. Base
–
15.7
HO
-16
~16
CH3O
-16
~16
CH3CH2O
-17
~17
(CH3)2CHO
-18
~18
(CH3)3CO
–
–
–
–
alcohols resemble water in acidity; their conjugate
bases are comparable to hydroxide ion in basicity
Table 4.2 Strength of Some Brønsted Acids
Acid
NH3
(CH3)2NH
Ka
-36
~10
-36
~10
pKa
~36
~36
Conj. Base
NH2
–
(CH3)2N
–
ammonia and amines are very weak acids;
their conjugate bases are very strong bases
Proton Transfer to Alcohols
alkyloxonium ion
R
.. O ..
H
.. .
.
H Br
.
.
base
acid
R
.. O+
H
H
.. . –
.. Br
.
.
.
conjugate conjugate
acid
base
4.7
Acid-Base Reactions:
A Mechanism for Proton Transfer
Proton Transfer from HBr to Water
H
.. O ..
H
.. .
.
H Br
.
.
base
acid
H
.. O+
H
H
.. . –
.. Br
. .
.
conjugate conjugate
acid
base
Mechanism:
single step
transition
state
Potential
energy
activation energy
reactants
products
Reaction coordinate
Mechanism:
single step
d+
H2O
Potential
energy
H2O + H—Br
Reaction coordinate
d-
H
Br
bimolecular:
both H2O and
HBr involved at
transition state
+
H2O—H + Br –