Chemistry Reference
In-Depth Information
N
O
H
H
H
H
H
pyrrolidine
piperidine
morpholine
piperazine
diethylamine
p
K
a
(conjugate acid)
11.3
11.1
8.5
9.7, 5.3
10.8
From the p
K
a
values shown, there is relatively little
difference in basicities for diethylamine, pyrrolidine,
or piperidine. Note, however, that morpholine and
piperazine are weaker bases than piperidine. This is
the result of an electron-withdrawing inductive effect
from the second heteroatom, making the nitrogen
atom both less basic and also less nucleophilic.
This makes morpholine a useful base with basicity
between that of piperidine and pyridine (p
K
a
5.2) (see
Section 4.6). The second p
K
a
value for the diamine
piperazine is substantially lower than the first, since
the inductive effect from the protonated amine will
withdraw
detail, we shall see even closer similarity. Thus,
we have seen that the ring atoms in benzene are
sp
2
hybridized (see Section 2.9.1). The remaining
singly
occupied
p
orbitals
are
oriented
at
right
angles
to
the
plane
of
the
ring,
and
overlap
to
form
system, extending to form
a closed loop above and below the ring (see
Section 2.9.1). Compared with what we might expect
for the hypothetical cyclohexatriene, this results in a
considerable stabilization, with significantly modified
structure and reactivity in benzene. We termed this
aromaticity
(see Section 2.9).
Benzene conforms to
H uckel's rule
, which pre-
dicts that planar cyclic polyenes containing 4
n
+
2
π
a
delocalized
π
electrons
away
from
the
unprotonated
amine (see Section 4.7).
electrons show enhanced stability associated with
aromaticity (see Section 2.9.3). Pyridine is also aro-
matic: nitrogen contributes one electron in a
p
orbital
to the
11.3 Aromaticity and
heteroaromaticity
electron system, and its lone pair is located in
an
sp
2
orbital that is in the plane of the ring and per-
pendicular to the
π
Pyridine
is structurally related to benzene: one
CH unit has been replaced by N. If we consider
the constitutions of the two compounds in more
electron system. It also conforms
to H uckel's rule, in that we still have an aromatic
sextet of
π
π
electrons.
N
NH
lone pair in
sp
2
orbital
H
H
H
H
H
H
H
H
H
N
N
H
H
H
H
H
H
H
benzene
pyridine
pyrrole
One of the structural features of benzene that
derives from aromaticity is the equal length of the
C-C bonds (1.40 A), which lies between that for
normal single (1.54 A) and double (1.34 A) bonds.
Nevertheless, we continue to draw benzene with
single and double bonds because this allows us to
represent reaction mechanisms in terms of electron
movements (see Section 5.1). Pyridine does not have
a perfect hexagon shape; the symmetry is distorted
because the C-N bonds are slightly shorter (1.34 A)
than the C-C bonds (1.39 - 1.40 A).
