14' Conjugated Dienes and Ultraviolet Spectroscopy

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14. Conjugated Dienes and Ultraviolet Spectroscopy
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Topics to discuss

• Conjugated Dienes
• What are they?
• Electrophilic addition reactions
• Allylic carbocations and their stability
• Diels-Alder Cycloadditions
• Dienes and dienophiles
• Required physical properties
• UV Spectroscopy
• What is it and why is it useful?
• Effect of conjugation

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Conjugated and Non-conjugated Dienes

• Q What is a conjugated diene?

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Polyenes

• Q What is a polyene?
• Q Why is extended conjugation important?

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Preparation Stability of Conjugated Dienes

• Q How are conjugated dienes prepared?

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Measuring Stability

• Q How do we measure the stability of conjugated
  dienes?

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Molecular Orbital Description of 1,3-Butadiene

• Q Why are conjugated dienes more stable than
  non-conjugated dienes?

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Electrophilic Additions to Conjugated Dienes

• Review addition of electrophile to CC

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Allylic Carbocations from Conjugated Dienes

• Addition of H leads to delocalized secondary
  allylic carbocation

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Products of Addition to Delocalized Carbocation
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The Diels-Alder Cycloaddition Reaction
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Generalized View of the Diels-Alder Reaction
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Characteristics of the Diels-Alder Reaction
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Stereospecificity of the Diels-Alder Reaction
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Regiochemistry of the Diels-Alder Reaction

• Reactants align to produce endo (rather than exo)
  product

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Conformations of Dienes in the Diels-Alder
Reaction
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UV Spectroscopy

• What is UV spectroscopy?
• What is a UV spectrum?

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Ultraviolet Spectrum of 1,3-Butadiene
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Quantitative Use of UV Spectra

• Absorbance for a particular compound in a
  specific solvent at a specified wavelength is
  directly proportional to its _____________.
• You can follow changes in
• Beers law absorbance ecl

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Effect of Conjugation

• ?max
• Energy difference between HOMO and LUMO
• ?max increases as
• Substituents on ? system increase

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Conjugation and Color

• Visible region is
• Extended systems of conjugation absorb in visible
  region
• b-Carotene, 11 double bonds in conjugation, ?max
  455 nm
• Visual pigments are responsible for absorbing
  light in eye and triggering nerves to send signal
  to brain

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Summary

• Conjugated Dienes
• What are they?
• Electrophilic addition reactions
• Allylic carbocations and their stability
• Diels-Alder Cycloadditions
• Dienes and dienophiles
• Required physical properties
• UV Spectroscopy
• What is it and why is it useful?
• Effect of conjugation