Hi Nicole, good to hear from you, hope your classes are treating you all right! Let's see, descriptions of the bonding in benzene... several ways as you've noted. Here's a few comments about each:
1. Resonance Description. These are the simplest descriptions. Resonance structures are simply different ways of drawing Lewis dot structures of the same molecule. The difference is the way the electrons are positioned. These depictions predate hybridized orbitals and molecular orbital theory. Nevertheless, they tie in with orbital descriptions well.
2. The "Orbital Picture". This is a description of the bonding using hybridized atomic orbitals. Each carbon is sp2 hybridized, giving the trigonal planar bonding geometry to the hydrogen and two other carbons, leaving the unhybridized 2p orbital extending above and below the plane of the molecule. These are then able to overlap in a side-to-side fashion with the 2p orbitals on adjacent carbons, forming the doughnut-shaped "pi clouds" above and below the plane of the molecule.
3. Molecular Orbital Theory. Very similar to the orbital approach above, but MO theory gives new molecular wavefunctions with associated energy levels, rather than stopping at a simpler orbital overlap picture. You've probably seen the MO diagram of the pi-bonding in benzene, three bonding and three antibonding MOs holding the six pi-electrons:
____p6*
____p4* ____p5*
!¡ p2 !¡ p3
!¡ p1
This shows how "All pie bonding electrons are accommodated in bonding molecular orbitals (MO) in the ground state."
The conjugation phenomenon can likewise be more simply described as an extended side-to-side overlap of more than two p-orbitals. This occurs when you have, for example, alternating single and double bonds and the atoms are in the same plane. The p-orbitals are parallel to each other and as such can overlap in this manner. MO theory takes this further, giving the pi bonding and antibonding molecular orbitals similar to the benzene system. On paper, you can draw resonance structures of molecules like cyclooctatetraene, a ring of eight carbons with four double bonds alternating with four single bonds, just as you can with benzene. However, it turns out that this does not occur in cyclooctatetraene. The molecule is bent out of a planar shape so that the p-orbitals cannot all overlap with each other. Thus this molecule is not conjugated throughout the entire ring. In fact, if it was, it would be antiaromatic, an even less favorable situation. There are other examples of molecules that would be aromatic like benzene if the ring could be planar, but is hindered from being so because of steric effects, such as hydrogen atoms getting in each other's way. So the resonance picture alone (using dot structures) cannot tell us with certainty that the system is actually conjugated, unless we know from other evidence that it is planar or demonstrates extra stability and/or reactivity characteristic of conjugated systems.
Steve
Free Forum Hosting
Important Announcement
Prev Message
