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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 1

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. Absorption spectrum for 2-methyl-1,3-butadiene produced by UV-Vis spectroscopy. - PowerPoint PPT Presentation

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Page 1: UV-Vis Spectroscopy

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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Page 2: UV-Vis Spectroscopy

• 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

Page 22: UV-Vis Spectroscopy

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

Page 24: UV-Vis Spectroscopy

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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