![]() The highest energy level (the “penthouse” of our building) has (n–1) nodes. Here’s what the “ground floor” looks like for butadiene: This provides the greatest possible delocalization of the electrons, and hence is the lowest in energy. The lowest energy level (the “ground floor”, if you will) has all the p-orbitals aligned the same way, and therefore has zero nodes between the p orbitals ( not counting the node inherent to the p-orbitals that lies in the plane of the molecule). where they can’t constructively overlap). A “node” is where there is a change in phase between adjacent p-orbitals (i.e. The number of nodes increases with each successive energy level.Both hexatriene and benzene have six contributing p-orbitals (n = 6), so we should expect six pi orbitals for each. For butadiene (n=4) we saw that the energy levels of the pi system stacked like a four-story apartment building. The number of pi molecular orbitals in the pi-system equals the number of contributing atomic p orbitals.Think of drawing out the pi orbitals as a bit like constructing an apartment building, albeit with some strange municipal building codes and quirky tenant behaviour. We learned some key lessons for drawing out the molecular orbitals of (linear) pi systems that I will quickly rehash here. Previously we’ve looked at the molecular orbitals of the allyl system, and of butadiene. Quickie Review: How To Draw Pi Molecular Orbitals For A Given Pi System (Advanced) References and Further Readingġ.Why Is Benzene More Stable Than Hexatriene?.Filling Out The Rest of The Picture For Benzene.The Benzene Molecular Orbital Diagram: Putting It All Together.Where Do We Place The Nodes In The Intermediate Energy Levels Of Benzene?.The Maximum Energy Level Has 3 Nodal Planes Diagram (Highest Energy Level) Has The Maximum Number Of Nodes The Lowest-Energy Molecular Orbitals Of Hexatriene And Benzene Have Zero Nodes. ![]() Building The Pi Molecular Orbital Diagram For Benzene: Hexatriene and Benzene Each Have Six Pi Molecular Orbitals.Quickie Review: How To Draw Pi Molecular Orbitals For A Given Pi System.The rest of this post will describe how we came up with this drawing and what it means. Here’s of what the molecular orbitals of benzene look like. The big takeaway is that from contrasting the molecular orbitals of these two 6-electron pi systems, we will unlock the deep, mysterious riddle of why benzene is so unusually stable. We’ll compare them with the molecular orbitals for (linear) hexatriene. Today, let’s go through how to draw out the molecular orbitals of benzene. ![]()
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