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Chemistry 3B Lecture 6

Chemistry 3B Lecture 6 - Chemistry 3B Lecture 6 Tuesday...

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Chemistry 3B Lecture 6 Tuesday February 10 th , 2009 UV light is from 200-400 nanometers, and the energy is about 70-140kcal/mol and those are the energies required to move an electron from a pi bonding orbital to a pi antibonding orbital. To absorb UV light, the molecule has to be conjugated because the orbitals need to be close enough so that we have the energies correct for popping an electron from one orbital to the other. UV light can hurt us. Visible light is 400-700 nanometers, and the energy range is 40- 70kcal/mol. In order for a molecule to take up colors, their HOMO LUMO gap needs to be smaller than it is in a typical conjugated molecule. This is a conjugated molecule known as Beta-Carotene and supplies the color you find in oranges. It has 11 pi bonds, or 22 p orbitals in a row. All 22 p orbitals are parallel to each other so we have 22 pi energy levels. If you take an energy region, you have to squeeze 22 orbitals in them. The more orbitals you squish into a defined region, the gaps between them becomes smaller. That gap is small enough that the energy range gets within the visible range. The Lambda max, or the top of the UV spectrum peaks for Beta Carotene is 497 nanometers which is within the visible range. Not all colored compounds need to have lots of pi bonds in a row, there are other ways for it to happen. A highly conjugated molecule might be visible. Aromaticity: not all compounds that smell have aromaticity. The prototypical aromatic compound is benzene. Nomenclature: you can use it as a
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substituent. It is some type of ethanol, so it will be 2-something-1-ethanol where the benzene is called the phenyl group: 2-phenyl-1-ethanol. These are all mono substituted benzenes: Bromobenzene, ethyl benzene, nitro benzene, toluene, phenol. There is aniline, benzaldehyde, acetophenone is a ketone, and benzoic acid. What if you have more than one substituent? Two substituents on a benzene ring gives 3 possible ways to orient them. For disubstituted molecules, ortho, meta, and para are more commonly used.
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