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 YSU YSU Electronegativity the attraction of an atom for electrons YSU YSU 1
1.2 Ionic bonding Electrons Transferred Big differences in E.N. values; Metals reacting with non ‐ metals YSU YSU Important Electronegativity Values H 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 YSU YSU 2
1.3 Lewis Dot Structures of Molecules YSU YSU 1.4 Double bonds and triple bonds Double bonds ‐ alkenes H H H H C C C : : C H H H H Triple bonds ‐ alkynes H : C : : : C : H H C C H YSU YSU 3
1.5 Polar covalent bonds and electronegativity H 2 HF H 2 O CH 4 CH 3 Cl Based on electronegativity differences YSU YSU 1.6 Structural Formula - Shorthand in Organic Chemistry YSU YSU 4
1.6 Constitutional Isomers Same molecular formula, completely different chemical and physical properties YSU YSU 1.7 Formal Charge O H O N group number O ‐ number of bonds ‐ number of unshared electrons Formal charge YSU YSU 5
1.8 Resonance Structures - Electron Delocalization Table 1.6 – formal rules for resonance YSU YSU 1.9 Shapes of Molecules Shapes of molecules are predicted using VSEPR theory YSU YSU 6
1.9 Shape of a molecule in terms of its atoms Figure 1.9 Table 1.7 – VSEPR and molecular geometry YSU YSU Table 1.7 VSEPR & Molecular Geometry YSU YSU 7
Trigonal planar geometry of bonds to carbon in H 2 C=O Linear geometry of carbon dioxide YSU YSU 1.10 Molecular dipole moments Figure 1.7 YSU YSU 8
1.11 Curved Arrows – Extremely Important Curved arrows used to track flow of electrons in chemical reactions Consider reaction shown below which shows the dissociation of AB YSU YSU Curved Arrows to Describe a Reaction Many reactions involve both bond breaking and bond formation. More than one arrow may be required. YSU YSU 9
Curved Arrows to Describe a Reaction Many reactions involve both bond breaking and bond formation. More than one arrow may be required. YSU YSU 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. YSU YSU 10
1.13 A Brønsted-Lowry Acid-Base Reaction A proton is transferred from the acid to the base YSU YSU 1.13 A Brønsted-Lowry Acid-Base Reaction Definitions to describe species on each side of equation YSU YSU 11
1.13 A Brønsted-Lowry Acid-Base Reaction Curved arrows describe how bonds are formed and broken YSU YSU Proton Transfer from HBr to Water Hydronium ion YSU YSU 12
Proton Transfer from HBr to Water Hydronium ion YSU YSU Proton Transfer from HBr to Water Hydronium ion YSU YSU 13
Equilibrium Constants for Chemical Reactions Reactants Products Equilibrium Constant (K) = [Products] [Reactants] G = H – T S G = ‐ RTlnK YSU YSU Equilibrium Constants for Chemical Reactions Left-Side Right-Side Equilibrium Constant (K) = [Right ‐ Side] [Left ‐ Side] G = ‐ RTlnK K > 1, RHS favoured; K ~ 1, equal; K < 1, LHS favoured YSU YSU 14
Equilibrium Constant for Proton Transfer H H O + H Br O H + Br H H [H 3 O + ] [Br - ] K a = [HBr] p Ka = - log 10 K a YSU YSU Acids and Bases: Arrow Pushing YSU YSU 15
YSU YSU Need to know by next class: pKa = -log 10 Ka STRONG ACID = LOW pKa WEAK ACID = HIGH pKa HI, HCl, HNO 3 , H 3 PO 4 pKa ‐ 10 to ‐ 5 Super strong acids H 3 O + pKa – 1.7 RCO 2 H pKa ~ 5 acids PhOH pKa ~ 10 get H 2 O, ROH pKa ~ 16 weaker RCCH (alkynes) pKa ~ 26 RNH 2 pKa ~ 36 Extremely weak acid RCH 3 pKa ~ 60 Not acidic at all YSU YSU 16
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 acid ‐ base reactions by relying exclusively on p K a values H H H C H H C H H p K a ~60 Corresponding base Not at all acidic Extremely strong YSU YSU 1.15 How Structure Affects Acid/Base Strength HF HF HCl HCl HBr HBr HI HI 3.1 3.1 - -3.9 3.9 - -5.8 5.8 - -10.4 10.4 p K K a p a weakest acid strongest acid strongest H—X bond weakest H—X bond YSU YSU 17
Inductive Effects Electronegative groups/atoms remote from the acidic H can effect the p K a of the acid. CH 3 CH 2 O H CF 3 CH 2 O H p K a = 16 p K a = 11.3 O – H bond in CF 3 CH 2 OH is more polarized by EWG CF 3 CH 2 O ‐ anion is stabilized by EW fluorine atoms YSU YSU Resonance Stabilization in Anion Delocalization of charge in anion (resonance) makes the anion more stable and thus the conjugate acid more acidic e.g. (CH 3 CO 2 H > CH 3 CH 2 OH). p K a ~16 p K a ~5 YSU YSU 18
1.16 Acid-Base Reactions - Equilibria The equilibrium will lie to the side of the weaker conjugate base YSU YSU 1.17 Lewis acids and Lewis bases 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 CH 3 CH 2 OCH 2 CH 3 has a boiling point of 34 °C. YSU YSU 19
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