Chemistry Reference
In-Depth Information
4-amino substituent. We do not need to consider any further resonance structures. Effects involving the 2-amino
substituent will be the same whether N-1 or N-3 is protonated. We can predict that protonation occurs on
N-1.
Quinine
has
two
potentially
basic
centres,
a
cyclic
tertiary
amine
at
a
ring
junction,
and
one
in
a
quinoline
ring
system.
p
K
a
8.5
is
reasonably
basic,
and
this
is
most
likely
from
the
aliphatic
tertiary
amine.
We
need
to
convince
ourselves
that
the
quinoline
nitrogen
is
less
basic.
This
is
true.
As
far
as
reactivity
is
concerned,
a
quinoline
ring
system
behaves
as
two
separate
parts,
either
pyridine
or
benzene,
depending
upon
the
reagent.
Thus,
quinoline
has
p
K
a
very
similar
to
that
of
pyridine,
i.e.
around 5.
Chloroquinine
is more basic than quinine, and the p
K
a
suggests we must be looking at an aliphatic amine.
There are two; so which? Note though that one of the amine functions is
para
to the quinoline nitrogen, so we
may expect resonance stabilization when the quinoline nitrogen is protonated.
NH
2
NH
2
resonance effect stabilizes
conjugate acid and provides
maximum charge distribution
N
N
H
H
This will increase the basicity of the quinoline system from about p
K
a
5, but almost certainly not as far as
represented by p
K
a
10.8. However, it solves our problem, since it means the 4-amino substituent is donating its
lone pair into the aromatic ring system and is not, therefore, available for bonding to a proton. This site is going
to be less basic than a tertiary amine. p
K
a
10.8 must represent the terminal -NEt
2
.
Problem 16.23 (level 2; chapters 11 and 7)
(a) During porphyrin biosynthesis, porphobilinogen is produced from two molecules of 5-aminolaevulinic acid
and is subsequently converted into uroporphyrinogen I. Formulate a chemical mechanism to account for the
transformations.
P
A
P
A
CO
2
H
HO
2
C
CO
2
H
A
P
A
P
CO
2
H
NH
HN
NH
HN
H
2
N
O
H
NH
HN
NH
HN
NH
2
NH
2
O
A
A
P
P
NH
2
5-aminolaevulinic acid
P
P
A
A
porphobilinogen
aminomethylbilane
uroporphyrinogen I
CO
2
H
P =
CO
2
H
A =
