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

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UV-Vis Spectroscopy

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  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

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

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

  4. lmax is the most important part of the UV-vis spectrum • The longer the conjugated p-electron system, the higher the wavelength of absorption

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

  6. lmax is at 455 – in the far blue region of the spectrum – this is absorbed The remaining light has the complementary color of orange

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

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

  9. Estimating lmax • 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

  10. Substituent effects:

  11. 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 15.4 Addition of Hydrogen Halides to Conjugated Dienes

  12. 1,2- and 1,4-Additions 15.4 Addition of Hydrogen Halides to Conjugated Dienes

  13. 1,2- and 1,4-Additions 15.4 Addition of Hydrogen Halides to Conjugated Dienes

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

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

  16. Delocalization in the Allyl Carbocation • The p1 MO shows that p electrons are spread across all three carbons • Lewis notation: • The p2 MO shows that electron deficiency is split between C1 and C3 15.4 Addition of Hydrogen Halides to Conjugated Dienes

  17. 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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