Chapter 16 Notes - CHAPTER 16 Conjugated Systems:...

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CHAPTER 16 Conjugated Systems: Conjugation, Resonance, and Dienes 16.1 What are Dienes? Compounds containing two carbon-carbon double bonds. There are three types of dienes: unconjugated dienes (double bonds separated by 2 or more single bonds). Conjugated dienes (double bonds separated by one single bond). Cumulated dienes (two double bonds share a carbon). Examples: 16.2 Conjugated Dienes Also called 1,3-dienes. The simplest example is 1,3-butadiene. Three stereoisomers (discrete molecules) are possible. Two possible conformations result from rotation around the carbon-carbon single bond that joins the 2 double bonds. These conformations interconvert. Done by Dr. Felix N. Ngassa for Chemistry 242: Organic Chemistry for Life Sciences 2, GVSU, Spring/Summer 2011. 1
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Conjugation gives extra stability to a molecule. Compounds containing conjugated double bonds are more stable than isomeric compounds containing unconjugated double bonds. A more stable diene has a lower heat of hydrogenation . In conjugated dienes, the extra stability results from the delocalization of electron density between many atoms (4 atoms). A. Molecular Orbital (MO) Consideration of Conjugated Dienes: According to the molecular orbital theory, a conjugated diene is represented as a set of four pi molecular orbitals arising from the combination of four 2p atomic orbitals . The combination of four parallel 2p atomic orbitals gives 2 pi-bonding MOs and 2 pi- antibonding MOs . In the ground state, each pi-bonding MO is filled with 2 π-electrons while the pi- antibonding MOs are unoccupied . Two important MOs are the HOMO (highest occupied MO), and the LUMO (lowest unoccupied MO). Diagram: Done by Dr. Felix N. Ngassa for Chemistry 242: Organic Chemistry for Life Sciences 2, GVSU, Spring/Summer 2011. 2
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Chapter 16 Notes - CHAPTER 16 Conjugated Systems:...

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