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uv spectroscopy
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UV-Visible Spectroscopy
LECTURE 4
ORGANIC CHEMISTRY 1
FS AY 2010-2011
West Kristian D Paraiso, RPh
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Electromagnetic radiation
Spectroscopy• Study of the interaction between matter and
electromagnetic radiation
Electromagnetic radiation• Radiant energy having the properties of both a
particle and a wave
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Electromagnetic radiation
UV 180-400 nmVisible 400-720 nm
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Electromagnetic radiation
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Electronic transitions
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Spectrophotometer
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UV Spectrum of acetone
λmax – wavelength of maximum absorbance
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Chromophores and auxochromes
Chromophore – part of the molecule which absorbs UV/visible light
Auxochrome – substituent which alters λmax and absorption intensity when attached to a chromophore
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Red shift and blue shift
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Red shift and blue shift
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Beer’s Law
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Beer’s Law
• A solution of 4-methyl-3-penten-2-one in ethanol shows an absorbance of 0.52 at 236 nm in a cell with a 1-cm light path. Its molar absorptivity in ethanol at that wavelength is 12,600. What is the concentration of the compound?
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Effect of conjugation on λmax
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Effect of conjugation on λmax
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Exercises
• How could one use UV spectroscopy to distinguish between the compounds in each of the following pairs?
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Calculations of λmax
• To the base number,1. + 30 for each extra conjugated double bond2. + 5 if a conjugated double bond is an exocyclic double
bond3. + 36 for conjugated double bonds that are frozen in
the s-cis conformation4. + 5 for each alkyl group or halogen bonded to the
conjugated system of a diene5. + 10 for an α-substituent of a conjugated aldehyde /
ketone6. + 12 for a β-substituent of a conjugated aldehyde /
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Calculations of λmax
• Compute for the λmax of the following:
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Visible spectrum and color
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Visible spectrum and color
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Visible spectrum and color
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