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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• Absorption spectrum for 2-methyl-1,3-butadiene produced by UV-Vis spectroscopy

215.2 Ultraviolet-Visible Spectroscopy

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

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

1,2- and 1,4-Additions

1915.4 Addition of Hydrogen Halides to Conjugated Dienes

1,2- and 1,4-Additions

2015.4 Addition of Hydrogen Halides to Conjugated Dienes

Allylic Carbocations

2115.4 Addition of Hydrogen Halides to Conjugated Dienes

MO Diagram of the Allyl Cation

2215.4 Addition of Hydrogen Halides to Conjugated Dienes

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

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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