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Ch_322b_13.05 - Chapter 13.5 lecture note

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Alkadienes and Polyunsaturated Hydrocarbons Hydrocarbons may contain more than one carbon-carbon double or triple bond. They also may have combinations of double and triple bonds. Examples CH 2 =C=CH 2 1 2 3 1,2-propadiene CH 2 =CH-CH=CH 2 1 2 3 4 1,3-butadiene C=C CH=CH 2 CH 3 H H 1 2 3 4 5 (3 Z)-1,3-pentadiene ( cis -1,3-pentadiene) CH 3 C=C C=C C=C CH 3 H H H H H H 1 2 3 4 5 6 7 8 (2E, 4E, 6E)-2,4,6-octatriene ( trans, trans, trans -2,4,6-octatriene) HC C-CH 2 CH=CH 2 1 2 3 4 5 1-pentene-4-yne 1,4-cyclohexadiene 1 2 3 4 5
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Designation of Positions of Multiple Bonds Cumulenes have double bonds in adjacent bonding positions. The two double bonds have a common central carbon that is sp hybridized . C=C=C a "cumulated" double bond C C C C=C=C H H H H allene (1,2-propadiene) Conjugated dienes have double bonds connected by a single bond: C=C C=C a "conjugated" diene CH 2 =CH-CH=CH 2 1,3-butadiene Isolated dienes have double bonds electronically isolated from each other. CH 2 =CH-CH 2 -CH 2 -CH=CH 2 1,5-hexadiene
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1,3-Butadiene: Properties of a Conjugated Diene The C-C bond lengths in 1,3-butadiene have been measured: CH 2 =CH CH=CH 2 1 2 3 4 1.34 A o 1.34 A o 1.47 A o for comparison CH 2 =CH 2 1.34 A o CH 3 CH 3 1.54 A o
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