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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