Chapter 20 and 24.6

Chapter 20 and 24.6 - CHEM 262 Wei You Chapter 20....

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11 CHEM 262 Wei You Chapter 20. Conjugated Systems
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22 Conjugated Dienes H 0 -237 (-56.5) 1,3-Butadiene -126 (-30.1) -127 (-30.3) kJ (kcal)/mol Name 1-Pentene 1-Butene trans- 1,3-Pentadiene 1,4-Pentadiene trans- 2-Butene -115 (-27.6) cis- 2-Butene -120 (-28.6) -226 (-54.1) -254 (-60.8) Structural Formula from heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes
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33 Conjugated Dienes conjugation of the double bonds in 1,3-butadiene gives an extra stability of approximately 17 kJ (4.1 kcal)/mol 2H 2 + catalyst H 0 = 2(-127 kJ/mol)           = -254 kJ/mol) 2 2 2H 2 + H 0  = -237 kJ/mol catalyst
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44 Conjugated Dienes the pi system of butadiene is derived from the combination of four 2p atomic orbitals; there are two bonding MOs and two antibonding MOs  
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55 Conjugated Systems systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds 3-Cyclohexenone (less stable) 2-Cyclohexenone (more stable) O O
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66 1,2- and 1,4- addition Addition of 1 mol of HBr to butadiene at -78°C gives a mixture of two
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Chapter 20 and 24.6 - CHEM 262 Wei You Chapter 20....

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