1. 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

2. Absorption spectrum for 2-methyl-1,3-butadiene produced by UV-Vis spectroscopy 2 15.2 Ultraviolet-Visible Spectroscopy

3. Conjugated vs. Non-congugated systems 3 15.2 Ultraviolet-Visible Spectroscopy

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8. max is the most important part of the UV-vis spectrum The longer the conjugated  -electron system, the higher the wavelength of absorption 8

9. Longer wavelength (λ) means a smaller ΔE 9 15.2 Ultraviolet-Visible Spectroscopy

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11. 11 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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13. Chromophore: The structural feature of a molecule responsible for its UV-vis absorption

14. 14 The colors of M&M ’ s Bright 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

15. Problems Which of the following compounds would you expect to show ultraviolet absorptions? 15

16. 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 16

17. Substituent effects: 17

18. 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-relationship 18 15.4 Addition of Hydrogen Halides to Conjugated Dienes

19. 1,2- and 1,4-Additions 19 15.4 Addition of Hydrogen Halides to Conjugated Dienes

20. 1,2- and 1,4-Additions 20 15.4 Addition of Hydrogen Halides to Conjugated Dienes

21. Allylic Carbocations 21 15.4 Addition of Hydrogen Halides to Conjugated Dienes

22. MO Diagram of the Allyl Cation 22 15.4 Addition of Hydrogen Halides to Conjugated Dienes

23. 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 C 1 and C 3 23 15.4 Addition of Hydrogen Halides to Conjugated Dienes

24. 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: 24