Feb_08 - Arrange the following with respect to increasing stability due to πelectron delocalization aromatic Aromatic = stabilization due to

Info iconThis preview shows page 1. Sign up to view the full content.

View Full Document Right Arrow Icon
This is the end of the preview. Sign up to access the rest of the document.

Unformatted text preview: 2/9/2010 Arrange the following with respect to increasing stability due to πelectron delocalization? aromatic Aromatic = stabilization due to cyclic deolcalization of electrons. anti aromatic aromatic non aromatic anti aromatic lots of delocalization energy no delocalization energy some delocalization energy molecular orbital theory: a review. 1 molecular orbital theory: a review. 2 3 4 An important failure of simple resonance theory is the inability to predict the instability of cyclobutadiene. only overlap between p orbitals on positions 2 and 3. LUMO LUMO lots of delocalization energy no delocalization energy HOMO HOMO An important success of simple molecular orbital theory is the ability to predict the instability of cyclobutadiene. energy 2 1 3 4 Does the interaction of two filled molecular orbitals (HOMO-HOMO interaction) result in stability for butadiene? A. yes B. no Using this model, what is the origin of stability of butadiene? LUMO LUMO LUMO LUMO HOMO HOMO HOMO HOMO energy Are HOMO-LUMO interactions allowed? energy HOMO-LUMO interactions are allowed and predict stabilization of benzene by electron delocalization. 1 2/9/2010 2 1 4 3 Allow overlap between positions 2 and 3 as well as 1 and 4. LUMO The molecular orbitals of benzene LUMO HOMO HOMO energy energy 2 1 4 3 HOMO-LUMO interactions are forbidden because of the symmetry of the molecular orbitals!!! LUMO LUMO HOMO HOMO energy energy Can I do these molecular calculations? Hückel http://www.ic.sunysb.edu/Class/orgchem/ energy 2 2/9/2010 Hückel Hückel Hückel’s Rule: Cyclic compounds with 4n + 2 π-electrons will be stabilized by cyclic delocalization. Hückel’s Rule: Cyclic compounds with 4n + 2 π-electrons will be stabilized by cyclic delocalization. Chose those compounds predicted by Hückel’s Rule to be stabilized by the cyclic delocalization of electrons. Since Since benzene has 6 π-electrons Hückel’s rule (n = 1) predicts benzene to be stabilized by the cyclic deolcalization of electrons. Does Does cyclobutadiene obey Hückel’s rule? i ii iii A. i + ii + iii D. i B. ii + iii E. ii C. i + iii F. iii Hückel’s Rule: Cyclic compounds with 4n + 2 π-electrons will be stabilized by cyclic delocalization. Chose those compounds predicted by Hückel’s Rule: to be stabilized by the cyclic delocalization of electrons. H H H H H H H Which order has the following compounds correctly arranged with respect to increasing acidity. i ii iii H H H H + H + H H H H H H H + H H H i ii H iii H H H H A i < ii < iii D ii < iii < ii B i < iii < ii E iii < ii < i C ii < i < iii F iii < i < ii Which order has the following compounds correctly arranged with respect to increasing acidity. H H pKa + H 16 Which of the following molecules would Hückel’s rule predict to be stabilized by cyclic delocalization of the π-electrons? i 6π electrons. An aromatic anion! N N ii H H 1 + H ~40 2 3 iii A i < ii < iii D ii < iii < ii H H + H ~50 B i < iii < ii E iii < ii < i C ii < i < iii F iii < i < ii (A) 1 + 2 + 3 (B) 1 + 3 (C) 2 + 3 (D) 1 (E) 2 (F) 3 3 2/9/2010 Which of the following molecules would Hückel’s rule predict to be stabilized by cyclic delocalization of the π-electrons? Which of the following molecules would Hückel’s rule predict to be stabilized by cyclic delocalization of the π-electrons? N H N H 1 2 3 1 2 3 (A) 1 + 2 + 3 (B) 1 + 3 (C) 2 + 3 (D) 1 (E) 2 (F) 3 (A) 1 + 2 + 3 (B) 1 + 3 (C) 2 + 3 (D) 1 (E) 2 (F) 3 Which of the following compounds would you predict to be more basic? Which of the following nitrogen atoms in imidazole would you predict to be more basic? A + H N H A pKa = 5.21 B 1. Draw the two cations that are formed by protonating each nitrogen atom. B + H N H H pKa = 11.2 2. Circle the more acidic cation. 3. Circle the more basic nitrogen atom. ∆pKa (5.7 kJ/mol) = ∆pKa = (11.2 – 5.21)(5.7 kJ/mol) = 18.5 kJ/mol) Students who have their answers on their personal workshop page and whose University IDs end in the following number should turn in their sheets for extra credit!!! Which has the following compounds correctly classified as aromatic, anti aromatic and non aromatic. H H A B 6 π-electrons 8 π-electrons 6 π-electrons A B C D aromatic non aromatic non aromatic anti aromatic anti aromatic anti aromatic aromatic non aromatic non aromatic aromatic anti aromatic aromatic 4 2/9/2010 What is the correct classification of cyclopentadiene? A anti aromatic B. non aromatic C. aromatic 4 π electrons cyclopentadiene H H 6 π electrons H H Hückel’s Rule: If a cyclic π-system contains 4n + 2 electrons it will be stabilized by cyclic delocalization. 5 ...
View Full Document

This note was uploaded on 04/06/2010 for the course CHE 326 taught by Professor Fowler during the Spring '08 term at SUNY Stony Brook.

Ask a homework question - tutors are online