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CHEM152L(7_01_09)

CHEM152L(7_01_09) - CHEM 152L Summer 2009 Dienes are...

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CHEM 152L Summer 2009 7/01/09 Dienes are compounds with two carbon – carbon double bonds. Conjugated double bonds, alternating double single double bond systems. Cumulenes/allenes – carbon participates in two carbon-carbon double bonds. Cumulated double bonds Dienes that are separated by two or more single bonds basically have the same properties as those of isolated (or simple alkenes). We will call these types of dienes “ordinary”. Structure and stability of dienes – if we look at conjugated dienes and isomers that have the same number of double bonds, but are “non conjugated” we see that the conjugated system is more stable. Two reasons for this additional stability – 1) the single bond in a conjugated system is made up of sp 2 – sp 2 overlap, so ultimately this a stronger bond than the sp 2 – sp 3 bond that we see in an ordinary alkene. The stronger bond gives more stability. 2) delocalization – the pi system is just not shared in the alkene unit, but it is shared over the entire pi system. This is the idea of delocalization. Bond length of the single bond is shorter than what would be expected in a normal alkane.
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CHEM 152L Summer 2009 Conjugated dienes exist in two stable forms – transoid or s-trans conformation Cisoid or s-cis conformation.
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