UV-Vis Spectroscopy
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Transcript of UV-Vis Spectroscopy
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UV-Vis Spectroscopy• Conjugated compound’s π electron systems can be identified
and analyzed UV-Vis Spectroscopy
• Ultraviolet light: 10 nm – 400 nm• Visible light: 400 – 750 nm
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• Absorption spectrum for 2-methyl-1,3-butadiene produced by UV-Vis spectroscopy
215.2 Ultraviolet-Visible Spectroscopy
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Conjugated vs. Non-congugated systems
315.2 Ultraviolet-Visible Spectroscopy
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• max is the most important part of the UV-vis spectrum
• The longer the conjugated -electron system, the higher the wavelength of absorption
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• Longer wavelength (λ) means a smaller ΔE
915.2 Ultraviolet-Visible Spectroscopy
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-carotene, max = 455 nm
max is at 455 – in the far blue region of the spectrum – this is absorbed
The remaining light has the complementary color of orange
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• Chromophore: The structural feature of a molecule responsible for its UV-vis absorption
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The colors of M&M’sBright Blue
Common Food Uses Beverages, dairy products, powders, jellies, confections, condiments, icing.
Royal Blue
Common Food Uses Baked goods, cereals, snack foods, ice-cream, confections, cherries.
Orange-red Common Food Uses Gelatins, puddings, dairy products, confections, beverages, condiments.
Lemon-yellow Common Food Uses Custards, beverages, ice-cream, confections, preserves, cereals.
Orange Common Food Uses Cereals, baked goods, snack foods, ice-cream, beverages, dessert powders, confections
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Problems
• Which of the following compounds would you expect to show ultraviolet absorptions?
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Estimating max
• Besides the number of multiple bonds in a conjugated molecule, there are other factors that influence max absorption– Conformational effects:• S-trans vs. S-Cis
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• Substituent effects:
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1,2- and 1,4-Additions
• Conjugated dienes react with H-X like normal alkenes, but two types of addition can occur
• 1,2-addition: Occurs at adjacent carbons• 1,4-addition: Occurs to carbons that have a
1,4-relationship1815.4 Addition of Hydrogen Halides to Conjugated Dienes
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1,2- and 1,4-Additions
1915.4 Addition of Hydrogen Halides to Conjugated Dienes
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1,2- and 1,4-Additions
2015.4 Addition of Hydrogen Halides to Conjugated Dienes
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Allylic Carbocations
2115.4 Addition of Hydrogen Halides to Conjugated Dienes
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MO Diagram of the Allyl Cation
2215.4 Addition of Hydrogen Halides to Conjugated Dienes
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Delocalization in the Allyl Carbocation
• The 1 MO shows that electrons are spread across all three carbons
• Lewis notation:
• The 2 MO shows that electron deficiency is split between C1 and C3
2315.4 Addition of Hydrogen Halides to Conjugated Dienes
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Problem
• Give the structure of both the 1,2 and 1,4 products resulting from a reaction of 1 equivalent of HBr with the following substance:
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