CHEM 216 Prelim 2 (Spring 2008) Answer Key

CHEM 216 Prelim 2 - Chem 216 Please do not open this exam until told to do so by a TA Please check there are 4 questions to your exam Three blank

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Unformatted text preview: Chem 216: Please do not open this exam until told to do so by a TA. Please check there are 4 questions to your exam, Three blank pages have been included for you to do your work on. The TA’s have been told not to answer questions during the exam. If you finish at least 15 minutes early you may turn your exam in. Otherwise please wait at your desk for a TA to pick up your exam. No calculators may be used on this test. Two 5x8 index cards of notes are allowed. name: K93, day of the week your lab meets: Professor Kenichi Fukui (shown on left) together with Roald Hoffmann are the two theorists most responsible for bringing MO theory to our under— standing of organic chemical reactions. question 1 [4? question 2 3 0 question 3 "l 0 question 4 1 0 . Total I 0 E2 1. (16 pts) The unstable ion H42+ would have the tetrahedral geometry shown on the right. Every H atom is equidistant from every other hydrogen atom. Write a 4x4 Hamiltonian for the electrons in this ion. Use only the H 1s orbitals for this Hamiltonian. The H ts orbital is at —13.6 eV. Just as in class, call the interaction between atomic orbitals B. One of the MOs of this molecule is shown below on the right. Find its eigenvalue. Is this orbital bonding, antibonding, or non—bonding? The 4x4 Hamiltonian t/(ri vii: ' W1 [ (5 HM] W2. 7 0 —~ ’ O The two H is orbitals in this MO are e ual—sized. lAI}. q ['5 M "A: p ‘— . 0 f’ o eigenvalue of _ Is this MO bonding, anti— displayed orbital bonding, or non—bonding? 2(30 pt) As we saw in a problem set, the approximate MO's of pyridine are as shown below. in this problem we place an —OCH3 group ortho to the N atom of the pyridine (see molecule below). Consider how this ——OCH3 group changes the ability of pyridine to ' undergo electrophilic aromatic substitution (EAS). Recall that in electrophilic aromatic substitution, the aromatic system reacts with electrophiles such as N02+. Derive the shape and energy change of the pertinent MO's. Determine on which carbon (or carbons) electrophilic substitution is/are most likely to occur. State very briefly your reasoning. Recall that O is a very electronegative atom. / __ _ 2—methoxy—pyridine i . \ N ocn3 OCH3 N _ at. at; tel wJJ M N from?” OCH3__ HMM-Aeawl Cristrl‘tJaN '5- cumin/10.9.:- uLk/LUVEPA w.» aw.“ ULLL “A mot/“Tut; w QCH3 ‘ or: N02. . gLLL fi Does the ~—OCH3 group activate or deactivate the EAS reaction? 3. (40 pts) Derive the 1c~MO diagram for the planar molecule, shown on the right. Assume this molecule has three perpendicular mirror planes. Place your final rtv—MO diagram in the box on the right. Place the M05 in the order of their energy.(The 1 energies of some MOs may be near or equal to the energies of other MOs. You need not decide the correct order of close—in—energy M03.) in you final MO diagram mark 2 clearly which orbitals are filled. [I II The HOMO and the LUMO may prove to be the same orbital(s). Based on your MO diagram, if this molecule reacted with H+ woutd the site of attack be 1 or 2? if the molecule reacted with H“, would the site of attack be 1 or 2? Based on your MO diagram, is this molecule more or less reactive than benzene? 'Do ‘Hnis FTD'olUH bra, fiat-smoky . 69 m an. M0) ~2/t Cat'ka T Reaction with H+ at 1 or 2? Reaction with H- ‘ at 1 or 2? ‘3 A! More or less reactive than benzene? we. reaabbt 4. (20 pts) only MOs with length .1. As we know from energy is subsequently measured, what .2: Cir All lobes in this MO are exactly the same size. Viz Va; Viz If“: WE M09 in MO diagrams are always orthogonal to one anot this course, benzene hast possible energies will be obtaine or.+2[3 her. It is also wise in problems such as the one given below to consider 0i+B he rc—MO diagram shown b d? What are the respec elow. tive probabilities of these various measurements? If an electron is in the state ‘P, and its possible measured energies ".7 respective probabilities of measured energies ? vs, 2/5 ...
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This note was uploaded on 10/26/2008 for the course CHEM 2160 taught by Professor Lee, s during the Spring '08 term at Cornell University (Engineering School).

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CHEM 216 Prelim 2 - Chem 216 Please do not open this exam until told to do so by a TA Please check there are 4 questions to your exam Three blank

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