厦门大学化学化工学院：《有机化学》课程教学资源（课件讲稿，双语版）Chapter 13 Conjugated Unsaturated Systems（13.6-13.8）Alkadienes and Polyunsaturated Hydrocarbons

ORGANIC CHEMISTRY Chapter 13 13.6 Alkadienes and polyunsaturated hydrocarbons Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.6 Alkadienes and Polyunsaturated Hydrocarbons

ORGANIC CHEMISTRY Chapter 13 o a hydrocarbon whose molecules contain two double bonds is called an alkadiene( alkatriene for three double bonds) g a hydrocarbon with two triple bonds is called an alkadiyne o a hydrocarbon with a double and triple bond is called an alkenyne An alkadiene An alkatriene HC≡C-CH2-C≡CH HC≡C-CH2-CH=CH An alkadiyne An alkenyne Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 A hydrocarbon whose molecules contain two double bonds is called an alkadiene(alkatriene for three double bonds); A hydrocarbon with two triple bonds is called an alkadiyne; A hydrocarbon with a double and triple bond is called an alkenyne. HC C CH2 C CH HC C CH2 CH CH2 An alkadiene An alkatriene An alkadiyne An alkenyne

ORGANIC CHEMISTRY Chapter 13 ● Cumulated dienes(累积二烯)— The molecules have cumulated double bonds g Isolated dienes(孤立二烯)— one or more saturated carbon atoms intervene between the double bonds of an alkadiene o Conjugated dienes(共轭二烯)— the double and single bonas alternate along the chain C=C C=C=C (CH a cumulated diene A conjugated diene An isolated diene(n+0) Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 Cumulated dienes（累积二烯）— The molecules have cumulated double bonds Isolated dienes（孤立二烯）— one or more saturated carbon atoms intervene between the double bonds of an alkadiene. Conjugated dienes（共轭二烯）— the double and single bonds alternate along the chain C C C C C (CH2)nC C C C C C A cumulated diene A conjugated diene An isolated diene (n = 0)

ORGANIC CHEMISTRY Chapter 13 13.71.3-Butadiene. electron Delocalization Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.7 1,3-Butadiene: Electron Delocalization

ORGANIC CHEMISTRY Chapter 13 13.7A Bond Lengths of 1, 3-Butadiene o All of the 1, 3-butadiene are sp hybridized and as a result the central bond of butadiene results from overlapping sp? orbitals Bond lengths 1337pm C-C C=C CH=CH一CH=CH 154pm 134pm 146pm 足 Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.7A Bond Lengths of 1,3-Butadiene All of the 1,3-butadiene are sp2 hybridized and as a result, the central bond of butadiene results from overlapping sp2 orbitals. C C C C 154 pm 134 pm Bond lengths CH2 CH CH CH2 133.7 pm 146 pm C C C C

ORGANIC CHEMISTRY Chapter 13 13.7B Conformations of 1. 3-Butadiene S-cis conformation 9 S-trans conformation 9 s-cis and s-trans conformations can be interconverted through rotation about the single bond o The s-trans conformation is the predominant one at room temperature rotate about C2--C3 S-cis conformation S-trans conformation S-cis and s-trans conformations of 1. 3-butadiene Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.7B Conformations of 1,3-Butadiene s-cis conformation s-trans conformation s-cis and s-trans conformations can be interconverted through rotation about the single bond. The s-trans conformation is the predominant one at room temperature. s-cis conformation s-trans conformation rotate about C2—C3 s-cis and s-trans conformations of 1,3-butadiene

ORGANIC CHEMISTRY Chapter 13 13. 7c Molecular Orbitals of 1,3-Butadiene Y4 00-V3 LUMO 4444 y2 HOMO °HoMo—( hihgest occupied molecular orbital) o LUMO-( lowest unoccupied molecular orbital Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.7C Molecular Orbitals of 1,3-Butadiene ψ1 ψ2 ψ3 ψ4 HOMO LUMO HOMO——(hihgest occupied molecular orbital) LUMO——( lowest unoccupied molecular orbital)

ORGANIC CHEMISTRY Chapter 13 13.8 The stability of conjugated Dienes Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 13.8 The Stability of Conjugated Dienes

ORGANIC CHEMISTRY Chapter 13 o Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes Heats of hydrogenation CH3 CHCH=CH2CH2=CH--CH=CH2 ∧H°:127kJ/mol 239kJ/ mol △HC=C:127kJ/mol 119.5kJ/mol 2 +2H2 △H° 15 kJ mol +2H 254kJ mol-1 239kJ mol-l Department of Chemistry, Xiamen University

ORGANIC CHEMISTRY Department of Chemistry, Xiamen University Chapter 13 Conjugated alkadienes are thermodynamically more stable than isomeric isolated alkadienes. CH3CH2CH CH2 CH2 CH CH CH2 ∆H°：127 kJ / mol 239 kJ / mol ∆H°/C=C: 127 kJ / mol 119.5 kJ / mol Heats of hydrogenation 2 + 2H2 + 2H2 −254 kJ mol−1 −239 kJ mol−1 15 kJ mol−1 ∆H°