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
• 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
5
6
7
• max is the most important part of the UV-vis spectrum
• The longer the conjugated -electron system, the higher the wavelength of absorption
8
• Longer wavelength (λ) means a smaller ΔE
915.2 Ultraviolet-Visible Spectroscopy
10
11
-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
12
• Chromophore: The structural feature of a molecule responsible for its UV-vis absorption
14
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?
15
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
• Substituent effects:
17
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:
24