chemistry 242 exam 1 2008

Chemistry 242 exam - CHEMISTRY 242 FIRST EXAMINATION WEDNESDAY ANSWER KEY 1(14 Points a Provide a molecular orbital diagram for the pi system of

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CHEMISTRY 242 FIRST EXAMINATION WEDNESDAY, September 24, 2008 ANSWER KEY 1. (14 Points) a. Provide a molecular orbital diagram for the pi system of the allyl anion (C 3 H 5 ) using the particle in a box method. Include both the relative energies and phasing of all the orbitals and place the required number of electrons into the correct orbital(s). E ! 1 ! 2 ! 3 b. Using your diagram, illustrate what happens when the allyl anion absorbs a photon in the ultraviolet region of the spectrum. Will the longest wavelength absorption for the allyl radical occur at longer, shorter or the same wavelength as the allyl anion? Explain your reasoning.
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E h ! = ! E The lowest energy (longest wavelength) electronic transition will occur when an electron is promoted from the psi 2 (HOMO) to the psi 3 (LUMO) level of the allyl anion. In the allyl radical the energy levels will be the same, so the radical will have the SAME absorption energy. 2. (14 Points) Consider the reaction between the allyl anion and ethylene to form cyclopentyl anion. Using orbital symmetry arguments, predict whether this is a symmetry allowed or symmetry forbidden reaction. Show all of your work. H H H H H H H H H For a cycloaddition reaction to be symmetry allowed, the symmetry of the HOMO of one fragment must be the same as the LUMO of the other fragment. In the case of the allyl anion with ethylene, the two HOMO-LUMO interactions each have the correct symmetry to allow an interaction. Thus this is a symmetry allowed reaction.
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! 1 ! 2 ! 3 3. (8 Points) For each of the following molecules, label them as aromatic, anti-aromatic, or non-aromatic. N
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This note was uploaded on 03/07/2012 for the course BIOL 212 taught by Professor Kina during the Fall '12 term at UPenn.

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Chemistry 242 exam - CHEMISTRY 242 FIRST EXAMINATION WEDNESDAY ANSWER KEY 1(14 Points a Provide a molecular orbital diagram for the pi system of

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