chapter 15 - Start of Chapter 15 Organic text by Wade When...

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Start of Chapter 15 Organic text by Wade
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When a molecule has more than 1 double bond the position of those Pi bonds in relation to one another is important. Isolated double bonds Allenes or 1,2-dienes Conjugated dienes (1,3-dienes) C Use Heats of hydrogenation ( Hº) to measure the stability of an alkene -a less negative Hº value indicates a MORE stable alkene Shown here is a 2,4-diene
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Conjugated systems Double bonds alternate with single bonds Dienes Polyenes Enones (alkene + ketone) O
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Circle any of the following molecules that have a conjugated system and Indicate what the conjugated portion of the molecule is? H C C N H 2 C OCH 3 O H 2 C No conjugated double bonds here
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Ways to explain this increased stability: Conjugated systems have a lower heat of hydrogenation than what’s expected – so they are MORE STABLE (or highly stable) - resonance energy - the Pi electrons in a conjugated system are spread over 4 atoms - using the molecular orbital theory - the lowest energy molecular orbital with electrons in it has Pi orbital overlap between all four C atoms in a conjugated diene - the sigma bond between the double bonds is formed by 2 sp 2 hybridized C atoms - it’s bond length is 1.48 A (between the normal length for a C=C and a C-C bond)
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How can we use UV spectroscopy to give us information about conjugated systems? UV spectroscopy – is the light in the range of 200-400 nm wavelengths - in this energy range Pi electrons in a conjugated arrangement will absorb energy In comparison to the range of wavelengths that make up visible light UV range has a shorter wavelength and higher energy Measuring the absorbance is useful in cases where you have a
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This note was uploaded on 02/13/2011 for the course CHEM 2344 taught by Professor Slack,wu during the Spring '11 term at University of Houston.

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chapter 15 - Start of Chapter 15 Organic text by Wade When...

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