 3.4. Hyperconjugation Hyperconjugation is defined as the (usually stabilizing) interaction of an occupied s-bonding orbital with a vacant orbital (usually a p- or p-orbital). Negative hyperconjugation, which is far less common and whose existence was controversial for many years, is the interaction of a high lying occupied orbital (usually of an anionic centre) with a vacant s*-antibonding orbital. Thus, hyperconjugation is a major stabilising factor for carbocations and negative hyperconjugaion occurs for carbanions. Negative hyperconjugation is closely related to the anomeric effect, which will also be discussed below. The ethyl
cation is the prototype system for demonstrating the
effect of hyperconjugation. Consider classical CH3CH2+
as a combination of a methyl group and a -CH2+
centre. The group orbitals of the methyl group
(equivalent to the MOs of NH3) include the pCH3-orbital
shown schematically in Figure 3.18a. This orbital has the
correct nodal characteristics to interact strongly with
the unoccupied p-orbital of the -CH2+
group (shown in Figure 3.18b):
The
"correct nodal characteristics" in this case
means that both orbitals have a nodal plane containing
the connecting bond and perpendicular to the page in
Figure 3.18. This means that they can overlap strongly
with each other to give the interaction diagram shown in
Figure 3.19.
Note that the
two orbitals have the same symmetry, which is a neccesary
condition for non-zero overlap, but that this is not
sufficient for strong overlap. In this case, the only
symmetry element is a mirror plane corresponding to the
plane of the page, but the fact that both orbitals have a
nodal plane as described ensures that they can overlap
strongly. This sort of consideration, which involves what
might be called "approximate symmetry" is
common in MO-arguments and will be considered again for
the Woodward-Hoffman rules. The two orbitals form bonding
and antibonding combinations with contributions that
depend on the relative stabilities of the two component
group orbitals, exactly analogously to the interaction
diagram shown for LiH in Figure 2.5. Because only the bonding combination is
occupied, this interaction results in a stabilisation.
This is usually expressed as a stabilisation of the
cationic centre by the occupied orbital but, because the
orbital involved on the cationic centre was originally
vacant, it is more accurate to say that the s-system is stabilised by interaction with
the cation centre. This interaction can be seen in the HOMO-1
and the LUMO of the staggered ethyl cation, which correspond to the bonding
and antibonding combinations of group orbitals shown in
Figure 3.19. One
consequence of the hyperconjugative overlap shown above
is that electron density is removed from the CH-bond of
the methyl group (and especially the one in the plane of
the page) and that bonding overlap occurs between the
methyl hydrogens and the cationic carbon. In most cases,
this results in a shortening of the connecting bond and a
lengthening of the hyperconjugating bonds. In the ethyl
cation, however, the interaction is so strong that it
leads to a symmetrically bridged structure in which the
unique hydrogen of the methyl group is shared between the
two carbons. The two orbitals shown above then become the
HOMO-1 and the LUMO
of the bridged ethyl cation, which is the global minimum structure for this cation (Figure
3.20).
Hyperconjugation
is the reason that highly substituted cations such as tbutyl
are very stable. Note also that carbon-carbon bonds
generally hyperconjugate better than carbon-hydrogen
bonds. 3.4.2. The Cyclopropylcarbinyl Cation It can be
seen from Figure 3.19 that the smaller the energy gap
between the occupied s-MO and the unoccupied orbital on
the cationic centre, the larger will be the
stabilisation. One way to obtain high energy s-MOs is to consider strained molecules such
as cyclopropane. One of the two degenerate Walsh orbitals
of cyclopropane shown in Figure 3.11 has the correct
nodal characteristics to interact with a cationic centre
in the correct orientation. As this orbital lies
particularly high in energy, it should be an especially
efficient hyperconjugator. The two interacting orbitals
are shown in Figure 3.21:
This
interaction can be seen particularly well in the HOMO-1
of the bisected cyclopropylcarbinyl cation, but also to some extent in the LUMO.
Once again, the hyperconjugation is so strong that the
cation distorts to a non-classical structure that is not
reproduced by the AM1 calculations used in this book.
Another consequence of the hyperconjugation is the large
calculated energy difference (14 kcal mol-1 at
AM1) between the bisected and perpendicular conformations of the cation, even
though the perpendicular conformation also enjoys some
hyperconjugation, as shown by the HOMO-5
and the LUMO (which cyclopropane MO is involved?). 3.4.3. Negative Hyperconjugation Just as the
acceptor orbitals at cationic centres can interact with
high lying s-MOs, the high energy donor
orbitals at anionic centres can interact with low lying s*-orbitals. This effect is known as negative
hyperconjugation. It occurs when an electronegative atom,
X, such as oxygen or fluorine is bound to a carbon atom
attached to an anionic centre. The CX-bond is strongly
polarised towards the electronegative element, so that
the s*CX-MO is therefore
concentrated on carbon, analogously to the LiH MOs shown
in Figure 2.5.
This polarisation towards carbon allows a strong overlap
with the non-bonding electron pair at the anionic centre,
as shown in Figure 3.22:
This
interaction leads to a stabilisation of the anionic
centre, but also to a weakening of the C-X-bond and
partial double bond character in the CC-bond. If these
changes are extrapolated further (Figure 3.23), they lead
to elimination of X- to give an olefin:
The HOMO
and the LUMO+2 of the b-fluoroethyl anion demonstrate negative hyperconjugation. A further
impressive example of negative hyperconjugation, this
time in a neutral compound, is NF3O, whose highest occupied and lowest
unoccupied MOs show strong mixing between lone pairs and s*-orbitals. This leads to unusual stability
and bond lengths. The anomeric
effect is simply negative hyperconjugation under another
name. It is best known as the effect that stabilises the
normally less stable axial position of oxygen
substituents on sugars and can be demonstrated using the
axial and equatorial hydroxypyranes in Figure 3.24 below:
In the axial
conformation, negative hyperconjugation between the
higher lying of the two lone pairs on the ring oxygen and
the exocyclic s*CO-orbital leads to a
stabilisation that is not possible in the equatorial
conformation, as shown on Figure 3.25.
In the
alternative equatorial conformation, only a much weaker
interaction between the s*CO-orbital
and the lower of the two ring oxygen lone pairs is
possible. This leads to the unusual preference of the
hydroxyl substituent for the axial position. The AM1
calculations predict that the axial conformation is 0.7
kcal mol-1 more stable than the equatorial
that is also found in sugars. Thus, the anomeric effect
is simply another manifestation of negative
hyperconjugation, using an oxygen lone pair instead of an
anionic centre. These effects can be seen in the HOMO
and LUMO+1 of the axial
2-hydroxypyrane.
It is a useful exercise to identify the orbitals involved
in the alternative interaction in the equatorial
2-hydroxypyrane
from the HOMO and LUMO+1.
The anomeric effect is also responsible for the fact that
polyhalogenated methanes have increasing CX-bond
strengths with increasing degree of halogen substitution
because the lone pairs of the halogens interact with
adjacent s*CX-orbitals
(analogously to NF3O). |
