lecture 20.1

lecture 20.1 - 20-20-1Organic Organic H BrownWilliam H...

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Unformatted text preview: 20-20-1Organic Organic ChemistryChemistryWilliam H. BrownWilliam H. BrownChristopher S. FooteChristopher S. FooteBrent L. IversonBrent L. Iverson20-20-2Conjugated Conjugated SystemsSystemsChapter 20Chapter 2020-20-3Conjugated DienesConjugated Dienes•from heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes∆H-237 (-56.5)1,3-Butadiene-126 (-30.1)-127 (-30.3)kJ (kcal)/molName1-Pentene1-Butenetrans-1,3-Pentadiene1,4-Pentadienetrans-2-Butene-115 (-27.6)cis-2-Butene-120 (-28.6)-226 (-54.1)-254 (-60.8)StructuralFormula20-20-4Conjugated DienesConjugated Dienes•conjugation of the double bonds in 1,3-butadiene gives an extra stability of approximately 17 kJ (4.1 kcal)/mol 2H2+catalyst∆Η= 2(-127 κϑ/μ ολ29= -254 κϑ/μ ολ29222 Η2+∆Η= -237 κϑ/μ ολχαταλψστ20-20-5Conjugated DienesConjugated 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 20-20-6Conjugated SystemsConjugated Systems•systems containing conjugated double bonds, not just those of dienes, are more stable than those containing unconjugated double bonds3-Cyclohexenone(less stable)2-Cyclohexenone(more stable)OO20-20-71,2- and 1,4-Addition1,2- and 1,4-Addition◆Addition of 1 mol of HBr to butadiene at -78°C gives a mixture of two constitutional isomers•we account for these products by the following two-step mechanism1-Bromo-2-butene10%(1,4-addition)-78� C++3-Bromo-1-butene90%(1,2-addition)CH2= CH- CH= CH2HBrCH2= CH- CH- CH2CH2- CH= CH- CH21,3-ButadieneHHBrBr20-20-81,2- and 1,4-Addition1,2- and 1,4-Addition•the key intermediate is a resonance-stabilized allylic carbocationCH2=CH-CH=CH2H-BrCH2=CH-CH-CH2HCH2=CH-CH-CH2BrHBrBrCH2-CH=CH-CH2HBrCH2-CH=CH-CH2H(1,4-Addition)(1,2-Addition)__+++20-20-91,2- and 1,4-Addition1,2- and 1,4-Addition◆Addition of 1 mole of Br2to butadiene at -15°C...
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lecture 20.1 - 20-20-1Organic Organic H BrownWilliam H...

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