UNIT I-PART-3.pptx

POC-II UNIT 1

1. 1 UNIT I: Benzene and its derivatives Orbital picture

2. 2 Each carbon in the benzene ring is sp 2   hybrized with a p orbital perpendicular to the ring plane Being planar and cyclic allows benzene's p orbitals to undergo cyclic overlap 

3. 3 The orbital structure of benzene: All the carbon atoms in benzene are sp  hybridised . The three sp  hybridorbitals are lying in one plane and oriented at an angle of 120°. There is six sigma C-C bonds and six sigma C-H bonds. There is still one un hybridized 2p orbital on each carbon atom. This orbital consists of two lobes, one lying above and the other below the plane of the ring.

4. The unhybridized 2p orbital on each carbon atom can overlap sidewise with the 2p orbital of the two adjacent carbon atoms in two different ways as shown below giving rise to two sets of -bonds. Since 2p orbital on any carbon atom can overlap sideways with the 2p orbital on adjacent carbon atom on either side equally well, a continuous -molecular 3 orbitals will result which embraces all the six p-electrons as shown:

5. 5 The net result is that there are two continuous rings-like electron clouds, one above and the other below the plane of atoms as shown. This delocalisation of π-electrons imparts unique stability to the benzene molecule.

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8. 8 Pi Molecular Orbital Diagram For Benzene

9. 9 The number of pi molecular orbitals in the pi-system equals the number of contributing atomic p orbitals .   For butadiene (n=4) we saw that the energy levels of the pi system stacked like a four-story apartment building.  Both hexatriene and benzene have six contributing p- orbitals  (n = 6), so we should expect six pi orbitals for each. Building The Pi Molecular Orbital Diagram For Benzene Six p orbital's in the pi systems of benzene will produce six pi molecular orbitals . The Lowest-Energy Molecular Orbital's Of Benzene Have Zero Nodes  phases of all p- orbitals aligned the same way. It doesn’t matter if you draw the “shaded” or “white” lobes up or down, so long as they are all drawn the same way

10. 10 What about benzene?  This is where things get interesting. Benzene Has Nodal  Planes . The Maximum Energy Level Has 3 Nodal Planes In the case of cyclic systems, the (n–1) rule fails The highest energy level has (n–1) nodes The Highest Energy molecular orbitals  have p orbitals with completely alternating phases.

11. 11 This orbital has zero overlap between adjacent p orbitals and therefore electrons in this orbital have the minimum possible delocalization. They are therefore the highest energy

12. 12 Where Do We Place The Nodes In The Intermediate Energy Levels Of Benzene? As we said above, the tricky thing in building pi molecular orbitals is knowing where to put the nodes in the intermediate levels. For hexatriene , the second floor (one node) is fairly straightfoward : we put the node in the centre, like this: It’s impossible to draw a cyclic pi system with one node, but we can draw a system with one nodal  plane . Here, for instance, we’ve drawn a nodal plane that cuts through two of the single bonds: a second way to do it. We can also draw a nodal plane through the  atoms These two molecular orbitals (π 2  and π 3  ) have the same number of nodal planes, and therefore have the same energy

13. 13 This is really the key difference in the molecular orbital picture of a cyclic system versus an acyclic system: two units can co-exist on the same floor. For benzene,  that results in a lowering of energy.

14. 14 Here’s the third, fourth, and fifth “floors” for the hexatriene pi system, which have two, three, and four nodes, respectively. For benzene, the next level up has two nodal planes. Again, there’s two ways to do it: cut through the bonds, or cut through the atoms. Again, these are of the same energy.

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