Chapter 1 1.2-1.3 Bonding Atoms trying to attain the stable configuration of a noble (inert) gas - often referred to as the octet rule 1.2 Ionic Bonding - Electrons Transferred 1.3 Covalent Bonding - Electrons Shared type of bond that is formed is dictated by the relative electronegativities of the elements involved Electronegativity the attraction of an atom for electrons 1.2 Ionic bonding Electrons Transferred Big differences in E.N. values Metals reacting with non-metals 1
Important Electronegativity Values 2.1 Li Be B C N O F 1.0 2.0 2.5 3.0 3.5 4.0 Cl 3.0 Br 2.8 I 2.5 1.3 Covalent Bonding - Similar electronegativities Lewis dot representations of molecules. +. : ydrogen atoms B.D.E ydrogen molecule +104 kcal/mol C + 4 C B.D.E +104 kcal/mol B.D.E. = bond dissociation energy 1.3 Lewis Dot Structures of Molecules 2
1.4 Double bonds and triple bonds C : : C Double bonds - alkenes C C Triple bonds - alkynes : C : : : C : C C 1.5 Polar covalent bonds and electronegativity 2 F 2 O C 4 C 3 Cl Based on electronegativity δ+ δ F: δ δ+ Li 1.6 Structural Formula - Shorthand in Organic Chemistry 3
1.6 Constitutional Isomers Same molecular formula, completely different chemical and physical properties 1.7 Formal Charge O O N O Formal charge = group number - number of bonds O O O - number of unshared electrons 1.8 Resonance Structures - Electron Delocalization Table 1.6 formal rules for resonance 4
1.9 Shapes of Molecules Shapes of molecules are predicted using VSEPR theory 1.9 Shape of a molecule in terms of its atoms Figure 1.9 Table 1.7 VSEPR and molecular geometry 5
Trigonal planar geometry of bonds to carbon in 2 C=O Linear geometry of carbon dioxide 1.10 Molecular dipole moments Figure 1.7 1.11 Curved Arrows Extremely Important Curved arrows are used to track the flow of electrons in chemical reactions. Consider the reaction shown below which shows the dissociation of AB: A B A + + B - 6
Curved Arrows to Describe a Reaction Many reactions involve both bond breaking and bond formation. More than one arrow may be required. O + C Br O C + Br - 1.12 Acids and Bases - Definitions Arrhenius An acid ionizes in water to give protons. A base ionizes in water to give hydroxide ions. Brønsted-Lowry An acid is a proton donor. A base is a proton acceptor. Lewis An acid is an electron pair acceptor. A base is an electron pair donor. 1.13 A Brønsted-Lowry Acid-Base Reaction A proton is transferred from the acid to the base. + B. + A B +. A base acid conjugate acid conjugate base 7
Proton Transfer from Br to Water hydronium ion ( 3 O + ) O. + + Br O Br. + base acid conjugate acid conjugate base Equilibrium Constant for Proton Transfer. O. + + Br. O. Br.. + [ 3 O + ][Br ] K a = [Br] pk a = log 10 K a Acids and Bases: Arrow Pushing O + Br O + Br? O + Br O + Br O + Br O + Br [ 3 O + ][Br ] K a = ~ 10 6 for Br, pka = - 5.8 [Br] 8
Need to know by next class: pka = -log 10 Ka STRONG ACID = LOW pka WEAK ACID = IG pka I, Cl, NO 3, 3 PO 4 pka -10 to -5 Super strong acids 3 O + pka 1.7 RCO 2 pka ~ 5 acids PhO pka ~ 10 get 2 O, RO pka ~ 16 weaker RCC (alkynes) pka ~ 26 RN 2 pka ~ 36 Extremely weak acid RC 3 pka ~ 60 Not acidic at all 9
1.14 What happened to pk b? A separate basicity constant K b is not necessary. Because of the conjugate relationships in the Brønsted-Lowry approach, we can examine acidbase reactions by relying exclusively on pk a values. C pka ~60 Essentially not acidic C Corresponding base Extremely strong 1.15 ow Structure Affects Acid/Base Strength Bond Strength Acidity of X increases (I>Br>Cl>F) down the periodic table as -X bond strength decreases and conjugate base (X: - anion) size increases (basic strength of anion decreases). strongest X bond weakest X bond Electronegativity Acidity increases across periodic table as the atom attached to gets more electronegative (F> 2 O> 2 N>C 4 ). least electronegative most electronegative 10
Inductive Effects Electronegative groups/atoms remote from the acidic can effect the pka of the acid. C 3 C 2 O CF 3 C 2 O pka = 16 pka = 11.3 O bond in CF 3 C 2 O is more polarized CF 3 C 2 O- is stabilized by EW fluorine atoms Resonance Stabilization in Anion Delocalization of charge in anion (resonance) makes the anion more stable and thus the conjugate acid more acidic e.g. (C 3 CO 2 > C 3 C 2 O). pka ~16 pka ~5 1.16 Acid-base reactions - equilibria The equilibrium will lie to the side of the weaker conjugate base 11
1.17 Lewis acids and Lewis bases C 2 C 3 F 3 B + O C 2 C 3 C 2 C 3 + F 3 B O C 2 C 3 Lewis acid Lewis base Product is a stable substance. It is a liquid with a boiling point of 126 C. Of the two reactants, BF 3 is a gas and C 3 C 2 OC 2 C 3 has a boiling point of 34 C. 12