Welcome
Content
Introduction
Linear Combination of Atomic Orbitals
Larger Molecules
Ethylene
Cyclopropane
p-Systems
Hyperconjugation
The Ethyl Cation
The Cyclopropylcarbinyl Cation
Negative Hyperconjugation
The Anomeric Effect
Reactions
Elementary Symmetry
Glossary
Molecules in VRML
General Information - Installation - Use
| 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. 3.4.1. The Ethyl Cation 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 p CH3-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): Figure 3.18 The p CH3 orbital of the methyl group and the unoccupied p-orbital of the cationic centre involved in hyperconjugtion in the ethyl cation. 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. Figure 3.19 Interaction diagram illustrating the hyperconjugation interaction in the classical ethyl cation. 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). Figure 3.20 Structure of the bridged "non-classical" ethyl cation. 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: Figure 3.21 The high lying Walsh orbital of cyclopropane (a) and the accepting orbital of the cationic centre (b) involved in the strong hyperconjugative stabilisation of the cyclopropylcarbinyl cation (c). 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: Figure 3.22 Negative hyperconjugation between a non-bonding electron pair at an anionic centre and a low lying s CX-orbital, where X is an electronegative element. 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: Figure 3.23 VB-approach to negative hyperconjugation. 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. 3.4.4. The Anomeric Effect 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: Figure 3.24 Axial (left) and equatorial (right) conformers of 2-hydroxypyrane. 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. Figure 3.25 The highest lying ring oxygen lone pair (a) and the exocyclic s*CO-orbital (b) responsible for the anomeric effect in the axial conformation. 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). Ethylene Cyclopropane p-Systems Hyperconjugation