Chemistry Reference
In-Depth Information
Me
NH
4
H+
NH
3
H
N
H
H
ammonium ion
N
H
H
H
H
ammonium
p
K
a
9.2
methylammonium
H
3
O
H+
H
2
O
p
K
a
10.6
hydronium ion
Oxygen is more electronegative than nitrogen,
so its electrons are less likely to be donated to a
proton. Neutral oxygen bases are generally very much
weaker than nitrogen bases, but as we shall see later,
protonation of an oxygen atom is important and the
first step in many acid-catalysed reactions, especially
carbonyl compounds.
Me
Me
H
H
N
N
Me
H
Me
Me
dimethylammonium
trimethylammonium
p
K
a
10.7
p
K
a
9.8
electron-donating effects of alkyl
groups stabilize positive charge
4.5.2 Inductive effects
This apparent anomaly is a consequence of
measuring p
K
a
values in aqueous solution, where
there is more than ample opportunity for hydro-
gen bonding with water molecules. Hydrogen bond-
ing helps to stabilize a positive charge on nitrogen,
and this effect will decrease as the number of
alkyl groups increases. Therefore, the observed
p
K
a
values are a combination of increased basic-
ity with increasing alkyl groups (as predicted via
electron-donating effects) countered by a stabiliza-
tion of the cation through hydrogen bonding, which
decreases with increasing alkyl groups. Note that
we saw solvent molecules influencing the acidity
of alcohols by stabilizing the conjugate base (see
Section 4.3.3).
Electron-donating groups
on nitrogen are going to
increase the likelihood of protonation, and help to
stabilize the conjugate acid. They thus increase the
basic strength.
The p
K
a
values for the
amines
ammonia, methy-
lamine, dimethylamine, and trimethylamine are 9.2,
10.6, 10.7, and 9.8 respectively. The electron-
donating effect of the methyl substituents increases
the basic strength of methylamine over ammonia by
about 1.4 p
K
a
units, i.e. by a factor of over 25
(10
1
.
4
25
.
1). However, the introduction of a sec-
ond methyl substituent has a relatively small effect,
and the introduction of a third methyl group, as in
trimethylamine, actually reduces the basic strength to
nearer that of methylamine.
=
H
O
H
O
H-bonding
stabilizes cations
H
O
Me
Me
Me
H
H
H
H
H
H
N
>
N
>
N
H
H
Me
H
Me
Me
H
H
H
O
H
O
O
H
H
When p
K
a
values are measured in the gas phase,
where there are no hydrogen bonding effects, they
are found to follow the predictions based solely on
electron-donating effects. In water, mono-, di-, and
tri-alkylated amines all tend to have rather similar
p
K
a
values, typically in the range 10 - 11.
Electron-withdrawing groups
will have the oppo-
site effect. They will decrease electron density on
the nitrogen, destabilize the conjugate acid, and thus
make it less likely to pick up a proton, so pro-
ducing a weaker base. Refer back to Table 4.7 for
a summary list of inductive effects from various
groups.
For example, groups with a strong electron-
withdrawing inductive effect, such as trichloromethyl,
decrease basicity significantly.
