Understanding Resonance Structures and Electron Delocalization in Organic Chemistry, Assignments of Organic Chemistry

Download Understanding Resonance Structures and Electron Delocalization in Organic Chemistry and more Assignments Organic Chemistry in PDF only on Docsity! Announcements: Homework problems for Chapter 1: 1.17; 1.18; 1.21; 1.24; 1.26; 1.30; 1.33 The order in which the atoms of a molecule are connected is called its constitution or connectivity. The constitution of a molecule must be determined in order to write a Lewis structure. Constitution or Structural Isomers Bond-line formulas H Cl C C H2C H2C CH2 CH2 HH is shown as Cl Omit atom symbols. Represent structure by showing bonds between carbons and atoms other than hydrogen. Atoms other than carbon and hydrogen are called heteroatoms. Resonance Step 1: The connectivity must be the same in all resonance structures Example: The Lewis formulas below are not resonance forms of the same compound. Introduction to the Rules of Resonance N H H C O H N H C OH H Step 4: Calculate formal charges on the second and third structures. Example: These structures have formal charges; these are less stable Lewis structures. Introduction to the Rules of Resonance N H H C O H N H H C O H same atomic positions differ in electron positions more stable Lewis structure less stable Lewis structure .. .. C O N OH H H .. :.. + – .. .. C O N OH H H .. :.. Resonance Structures of Methyl Nitrite same atomic positions differ in electron positions more stable Lewis structure less stable Lewis structure .. .. C O N OH H H .. :.. + – .. .. C O N OH H H .. :.. Resonance Structures of Methyl Nitrite Ozone (O3) Lewis structure of ozone shows one double bond and one single bond Resonance: Example O O ••O•• •••• •••• –+ O O ••O•• •••• •••• –+ O OO •• •••• •••• – + •• Electrostatic potential map shows both end carbons are equivalent with respect to negative charge. Middle carbon is positive. Example O O ••O•• •••• •••• –+ O OO •• •••• •••• – + •• The Shapes of Some Simple Molecules tetrahedral geometry each H—C—H angle = 109.5° Methane bent shape H—O—H angle = 105° but notice the tetrahedral arrangement of electron pairs O H .. H : Water trigonal pyramidal shape H—N—H angle = 107° but notice the tetrahedral arrangement of electron pairs N H H H : Ammonia The greater the difference in electronegativity between two bonded atoms; the more polar the bond. Generalization polar bonds connect atoms of different electronegativity :O C δ+δ− F: .. ..H δ+ δ− O .. ..H δ+ δ− H δ+ O:.. .. δ− Electrostatic potential maps show the charge distribution within a molecule. Electrostatic Potential Maps Red is negative charge; blue is positive. F: .. ..H δ+ δ− Solid surface Electrostatic potential maps show the charge distribution within a molecule. Electrostatic Potential Maps Red is negative charge; blue is positive. F: .. ..H δ+ δ− Transparent surface —+ polar A substance possesses a dipole moment if its centers of positive and negative charge do not coincide. μ = e x d (expressed in Debye units) Dipole Moment molecule must have polar bonds necessary, but not sufficient need to know molecular shape because individual bond dipoles can cancel O C O δ+δ- δ- Molecular Dipole Moments O C O Carbon dioxide has no dipole moment; μ = 0 D Molecular Dipole Moments Resultant of these two bond dipoles is μ = 1.62 D Resultant of these two bond dipoles is The individual bond dipoles do not cancel in dichloromethane; it has a dipole moment. Figure 1.7 IN-CLASS EXERCISE