Electronic Absorption Spectroscopy of Organic Compounds Day 2 Dienes

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Electronic Absorption Spectroscopy of Organic
CompoundsDay 2Dienes

• W. R. Murphy, Jr.
• Department of Chemistry and Biochemistry
• Seton Hall University

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Conjugated dienes

• Strong UV absorber
• ?max affected by geometry and substitution
  pattern
• S-trans ? 217 nm
• S-cis ? 253 nm
• Replacement of hydrogen with alkyl or polar
  groups red shift these base values
• Extending conjugation also red shifts ?max

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Simple diene
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Dienes with additional double bonds

• Through conjugation 30 nm
• Cross conjugation (poor correlation)
• Exocyclic at least one of the carbons of the
  double bond is in a ring

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Exocyclic double bonds

• Both double bonds in this example count as
  exocyclic, since at least of one two carbons in
  each double bond is in a ring

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Conjugated Polyenes
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Extending conjugation

• Note that there is a convergence as of
  conjugated double bonds increases
• Change from cis to trans at one or more double
  bonds lowers the wavelength and intensity of the
  max

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Increasing polyene conjugation
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Conjugated Polyenes
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Polyeneynes and poly-ynes

• Important due to presence in natural products
• Spectra tend to have spiky appearance
• When two or more triple bonds are conjugated,
  spectrum shows a series of low intensity bands
  (200) at intervals of 2300 cm-1 and high
  intensity bands (105) at intervals of 2600 cm-1

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Polyenenyes
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Maxima for poly-ynes
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Substituted acyclic dienes

• ?max shifts
• Presence of substituents
• Length of conjugation

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Effects of Ring Strain
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Structural distortions can lead to both red and
blue shifts

• Bicyclic has an observed max of 245.5 nm calc.
  max is 229 nm
• Cyclohexane on right has an observed max of 220
  nm (5500) calc. max is 273 nm

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Cyclo distortions

• cyclopentadiene 238.5 (3400)
• cycloheptadiene 248 (7500)
• cyclohexadiene 256 (8000)
• Generally, steric hindrance to coplanarity about
  a double bond raises the ground state energy
  level but leaves the excited state level unchanged

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Single bond distortions

• Mild steric hindrance to coplanarity about a
  single bond gives rise to a marked decrease in
  intensity but may also lead to a max shift
• 2,4,6-trimethylacetophenone max 242 (3200) max
  calc. 262
• p-methylacetophenone max is 252 (15000)

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Coplanarity

• Extreme steric hindrance about a single bond
  leads to a loss of overlap between the separated
  chromophores
• No visible max in parent, but hydrolysis breaking
  up the lactone ring yields a max at 227 (5500)

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Comments

• Examples 4 and 5 are straightforward
• Example 6 is a case of through conjugated triene
• Could be either a homo- or heteroannular system
• Generally, use base value with the longer
  wavelength absorption
• In 6, use homoannular base
• Example 7 is a cross conjugated system. Neither
  value for the base works particularly well.
  Choose longest wavelength analysis.

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Additional comments

• Other ring sizes
• Cyclopentadiene try 228 nm
• Cycloheptadiene try 241 nm (this base includes
  the two ring substituent carbons)
• Steric interactions distorting the ?-system will
  invalidate the prediction
• 1,2-dimethylenecyclohexene absorbs at 220 nm
  rather than the predicted 234 nm

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Diene Example 1
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Diene Examples 2
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Hetero- vs Homoannular
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Diene Examples 2Homo- vs Heteroannular
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Distinguishing between polyenes