8 - 2/8/2012 Is cyclobutadiene predicted by the following...

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Unformatted text preview: 2/8/2012 Is cyclobutadiene predicted by the following MO diagram to be stabilized by electron delocalization? Show your calculation. -bond Benzene -system Cyclobutadiene -system What is the stabilization due to electron delocalization? Cyclobutadiene -system -bond -2 - 0.0 energy 4 -2 - 0.0 energy 4 2 3 2 3 2 2 E(cyclobutadiene) = 2 x 2 = 4 E(2 Sketch all the MOs for cyclobutadiene (2 and 3 both have 2 nodes). 1 ) = x 2 x 2 = 4 1 delocalization energy = 0! What is the energy for cyclobutadiene? Cyclobutadiene -system Do MO and resonance theory lead to different conclusions with respect to benzene and cyclobutadiene? Cyclobutadiene -system -bond -2 - 0.0 energy 4 resonance theory molecular orbital theory due to electron delocalization stabilization due to electron delocalization 4 NO stabilization 2 3 stabilization due to electron delocalization 2 3 2 1 1 What is the energy for cyclobutadiene? Cyclobutadiene -system Sketch all the MOs for cyclobutadiene (2 and 3 both have 2 nodes). Which is the least stable? Nodes Cyclobutadiene -system -bond -2 - 0.0 energy 4 4 4 2 2 3 energy A B C 0 D 2 3 2 E(cyclobutadiene) = 2 x 2 = 4 1 1 1 2/8/2012 Is cyclobutadiene less stabilized by electron delocalization than butadiene? A. Yes B. No -bonds butadiene -system Cyclobutadiene -system Which of the following terms best describes cycloheptadiene? butadiene -system cycloheptatriene -system -2 - 0.0 2 4 A. non aromatic 2 3 B. aromatic C. anti aromatic E 1 butadiene stabilization energy = 8 kJ/mol (experimental) E Which of the following terms best describes cyclobutadiene? butadiene -system benzene cyclobutadiene -system Is there a rule that uses MO theory to predict aromaticity? A Yes B No 4 A. non aromatic B. aromatic C. anti aromatic 2 3 does not obey Hckel's rule (n = no integer) obeys Hckel's rule (n = 1) Hckel's Rule: If a cyclic, conjugated compound system contains 4n + 2 -electrons, where n any integer, then it will be stabilized by electron delocalization. E 1 4 -electrons = 4(0.5) + 2 6 -electrons = 4(1) + 2 Which of the following terms best describes benzene? benzene -system butadiene -system Which of the following compounds obey Hckel's rule? 1 2 3 4 A. non aromatic B. aromatic C. anti aromatic E A=1 B=2 C=3 D=4 E=1+3 F=3+4 2 2/8/2012 Which of the following compounds obey Hckel's rule? A B According to Hckel's rule which of the following compounds is (are) aromatic? B. 2 C. 1 + 2 A. 1 1 6 -electrons pKa = A=1 B=2 C=3 16 D=4 ~40 E=1+3 F=3+4 2 6 -electrons 4n + 2 4(1) + 2 = 6 Which shape would be expected for the methyl anion? Why? Which of the following compounds is more basic? A B sp3 p A B The greater the Ka the more basic the conjugate base. (A) true (B) false Which shape would be expected for cyclopentadienide anion? Why? A B Which of the following compounds is more basic? A B pKa = -5 pKa = 5.25 3 2/8/2012 Which nitrogen atom is the most basic? A. 1 B. 2 C. 1 = 2 What is a good mechanism for the following reaction that accounts for the catalytic role of FeBr3? 1 2 Why is the brominum ion not involved? 22 Which hydrogen atom is the most acidic? A. 1 B. 2 C. 1 = 2 What is a good mechanism for the following reaction that accounts for the catalytic role of FeBr3? 1 2 Would Br be predicted to be(A) more or (B) less reactive than I? 23 Which of the following compounds would react faster with Br2? What is the slow step in the following reaction? A B no reaction A Eact Eact B How can Br2 be made more reactive as an electrophile (electron acceptor)? 21 energy reaction 24 4 ...
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This note was uploaded on 03/27/2012 for the course CHE 322 taught by Professor Staff during the Spring '08 term at SUNY Stony Brook.

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