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lectures 19-20 preview - Lecture 19 Chemistry 310N Organic...

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6/15/09 Lecture 19 Chemistry 310N    Organic Chemistry II for Prehealth Professionals Unique number: 53185 Tu,Th 12:30-2:00 pm, Welch 2.224   Professor:  Jonathan Sessler, Ph.D. [email protected]
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6/15/09 Chapter 20 Conjugated Systems
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6/15/09 Conjugated Dienes From heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated 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
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6/15/09 Conjugated Dienes Butadiene 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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6/15/09 Conjugated Dienes Butadiene Conjugation of double bonds in butadiene gives the molecule an additional stability of approximately 17 kJ/mol.
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6/15/09 Structure of Butadiene MOs Combination of four parallel 2p atomic orbitals gives two π -bonding MOs (this screen) and two π -antibonding MOs (the next screen).
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6/15/09 Structure of Butadiene MOs the two π -antibonding MOs of butadiene.
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6/15/09 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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6/15/09 1,2- and 1,4-Addition Addition of one mol of HBr to butadiene at -78°C gives a mixture of two constitutional isomers. We account for these products by the following 1-Bromo-2-butene 10% (1,4-addition) -78 ° C + + 3-Bromo-1-butene 90% (1,2-addition) CH 2 = CH- CH= CH 2 HBr CH 2 = CH- CH- CH 2 CH 2 - CH= CH- CH 2 1,3-Butadiene H H Br Br
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6/15/09 1,2- and 1,4-Addition The key intermediate is a resonance-stabilized allylic carbocation. CH 2 = CH- CH= CH 2 H- Br CH 2 = CH- CH-CH 2 CH 2 -CH= CH-CH 2 + + + CH 2 = CH- CH-CH 2 Br H Br CH 2 -CH= CH-CH 2 Br Br (1,4-Addition) (1,2-Addition) _ _ H H H
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6/15/09 1,2- and 1,4-Addition Addition of one mole of Br2 to butadiene at -15°C also gives a mixture of two constitutional isomers. We account for the formation of these 1,2- and -15 ° C 3,4-Dibromo-1-butene (54%) (1,2-addition) 1,4-Dibromo-2-butene (46%) (1,4-addition) + + 1,3-Butadiene CH 2 = CH- CH= CH 2 Br 2 CH 2 - CH= CH- CH 2 CH 2 - CH- CH= CH 2 Br Br Br Br
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6/15/09 Experimental Information For addition of HBr at -78°C and Br2 at - 15°C, the 1,2-addition products predominate; at higher temperatures (40° to 60°C), the 1,4-addition products predominate. If the products of the low temperature addition are warmed to the higher temperature, the product composition becomes identical to the higher temperature distribution. The same result can be accomplished using a
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6/15/09 1,2- and 1,4-Addition We interpret these results using the concepts of kinetic and thermodynamic control of reactions.
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