lecture 20 - Conjugated Systems Chapter 20 Unconjugated...

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Conjugated Conjugated Systems Systems Chapter 20 Unconjugated diene: double bonds are sparated by two or more single bonds. Conjugated diene: double bonds are sparated by one single bond. Cumulated diene: two double bonds share an sp-hybridized carbon. These double bonds do not overlap and are not conjugated. Which of these molecules contain conjugated double bonds? Which of these terpenes contain conjugated double bonds? Conjugated Conjugated Dienes Dienes from heats of hydrogenation, we can compare relative stabilities of conjugated and unconjugated dienes Δ 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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Conjugated Conjugated Dienes 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 Δ H 0 = 2(-127 kJ/mol) = -254 kJ/mol) 22 2 + Δ H 0 = -237 kJ/mol catalyst Conjugated Systems 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 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 Conjugated Systems Conjugated Systems Systems containing conjugated double bonds, are more stable than those containing unconjugated double bonds because the electron density is more delocalized Delocalization increases stability Structure of Butadiene MOs ± Combination of four parallel 2p atomic orbitals gives two π -b onding MOs (this screen) and two π -antibonding MOs (the next screen ). Structure of Butadiene MOs ± the two π -antibonding MOs of butadiene.
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1,2 1,2 - and 1,4 and 1,4 -Addition Addition ± Addition of 1 mol of HBr to butadiene at -7 8°C gives a mixture of two constitutional isomers we account for these products by the following two- step mechanism 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 1,2 1,2 - and 1,4 and 1,4 -Addition Addition ± Addition of 1 mole of Br 2 to butadiene at -1 5°C also gives a mixture of two constitutional isomers we account for the formation of these 1,2- and 1,4- addition products by a similar mechanism -15°C
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This note was uploaded on 09/06/2009 for the course CHEM chem 12 AB taught by Professor Valentin during the Spring '09 term at Evergreen Valley.

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lecture 20 - Conjugated Systems Chapter 20 Unconjugated...

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