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Ch15Su08 - Conjugated Systems Orbital Symmetry and UV...

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Conjugated Systems, Orbital Symmetry and UV Spectroscopy Introduction There are several possible arrangements for a molecule which contains two double bonds (diene): Isolated : (two or more single bonds between them) Conjugated : (one single bond between them) Cumulated : (zero single bonds between them: allenes) Conjugated double bonds are found to be the most stable. Stabilities Recall that heat of hydrogenation data showed us that di-substituted double bonds are more stable than mono-substituted double bonds. When a molecule has two isolated double bonds, the heat of hydrogenation is essentially equal to the sum of the values for the individual double bonds. 8ed5634cec08ebe95482105b809817e0802bc386.doc Page 1 C C C H 2 , Pt H o = -30.0kcal H 2 , Pt H o = -27.4kcal H 2 , Pt H o = -60.2kcal
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For conjugated dienes, the heat of hydrogenation is less than the sum of the individual double bonds. The conjugated diene is more stable by about 3.7kcal/mol. (Predicted -30 + (-24.7) = –57.4kcal , observed –53.7kcal) Allenes, which have cumulated double bonds are less stable than isolated double bonds. Increasing Stability Order (least to most stable) Cumulated diene -69.8kcal Terminal alkyne -69.5kcal Internal alkyne -65.8kcal Isolated diene -57.4kcal Conjugated Diene -53.7kcal 8ed5634cec08ebe95482105b809817e0802bc386.doc Page 2 H 2 , Pt H o = -53.7kcal C C C H CH 2 CH 3 H H H 2 , Pt H o = -69.8kcal
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Molecular Orbital (M.O) Picture The extra stability of conjugated double bonds versus the analogous isolated double bond compound is termed the resonance energy . Consider 1,3-butadiene: (2 x –30.1 = -60.2kcal). Resonance energy of 1,3-butadiene is 3.6kcal. The C2-C3 bond is much shorter than a normal alkane single bond (1.48Å vs. 1.54Å). This is mainly due to the π bonding overlap (resulting in some double bond character). The planar arrangement, and the alignment of the p orbitals allows overlap between the two double bonds. The electrons are delocalized over the full length of the molecule. This delocalization of electrons creates partial double bond character between C2 and C3. 8ed5634cec08ebe95482105b809817e0802bc386.doc Page 3 H 2 , Pt H o = -30.1kcal H 2 , Pt H o = -56.6kcal
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Lewis structures cannot accurately depict delocalized structures, and we turn to molecular orbital theory. M.O.’s of 1,3-butadiene All four carbons are sp 2 hybridized, and in the planar conformation, all the p orbitals overlap. But first, let us recap simple MO theory using ethene: “An orbital is a mathematical function that describes the wavelike character of an electron.” θ θ +1.0 +0. -0.45 2p y = Rcos θ Each p orbital has two lobes, with differing wavefunction sign (+/-, black/white, shaded/unshaded – not electrical charges). A π bonding orbital is formed by overlap of p lobes with the same wavefunction sign. (Constructive overlap). A π anti-bonding orbital is formed by overlap of p lobes with opposite wavefunction sign. (Destructive overlap).
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