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
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
1.3 Covalent Bonding - Similar electronegativities Lewis dot representations of molecules H . + H . H : H B.D.E Hydrogen atoms Hydrogen molecule +104 kcal/mol H C H C + 4 H H H B.D.E +104 kcal/mol B.D.E. = bond dissociation energy
1.3 Lewis Dot Structures of Molecules
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
1.5 Polar covalent bonds and electronegativity H 2 HF H 2 O CH 4 CH 3 Cl Based on electronegativity δ− δ+ + δ+ δ− δ− − δ δ+ δ .. .. : F : H H Li H F H Li .. ..
1.6 Structural Formula - Shorthand in Organic Chemistry H H H H H CH 3 CH 2 CH 2 CH 3 H H H H H CH 3 CH 2 CH 2 CH 2 OH OH H Cl Cl H H H H H H H H H H
1.6 Constitutional Isomers H H H H H C C O H H C O C H H H H H Same molecular formula, completely different chemical and physical properties
1.7 Formal Charge O H O N O Formal charge = group number - number of bonds - number of unshared O O electrons O
1.8 Resonance Structures - Electron Delocalization O O O O O O O O C O C O CH 3 CH 3 Table 1.6 – formal rules for resonance
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
Trigonal planar geometry of bonds to carbon in H 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: B - A + + A B
Curved Arrows to Describe a Reaction Many reactions involve both bond breaking and bond formation. More than one arrow may be required. H H Br - H O + H C Br H O C H + H H
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. + A– . B . + + H . B . H A base acid conjugate conjugate acid base
Proton Transfer from HBr to Water O + + ) hydronium ion (H ion (H 3 ) hydronium 3 O H H H H .. .. + .. + .. – – . . . . . . . . . . . . H H . O . . . + . + O + . . . + Br . . . H Br Br O . H Br O .. .. .. .. H H H H base acid conjugate conjugate base acid conjugate conjugate acid base acid base
Equilibrium Constant for Proton Transfer H H .. .. – + . . . . . . + . + . H Br . O . . Br . H .. O .. H H [H 3 O + ][Br – ] K a = [HBr] p K a = – log 10 K a
Acids and Bases: Arrow Pushing + Br H O H + H Br H O H H ? + Br H O H + H Br H O H H + Br H O H + H Br H O H H [H 3 O + ][Br – ] ~ 10 6 for HBr, p K a = - 5.8 K a = [HBr]
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
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 Essentially not acidic Extremely strong
1.15 How Structure Affects Acid/Base Strength Bond Strength • Acidity of HX increases (HI>HBr>HCl>HF) down the periodic table as H-X bond strength decreases and conjugate base (X: - anion) size increases (basic strength of anion decreases). strongest H—X bond weakest H—X bond
Electronegativity Acidity increases across periodic table as the atom attached to H gets more electronegative (HF>H 2 O>H 2 N>CH 4 ). least electronegative most electronegative
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 • CF 3 CH 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. (CH 3 CO 2 H > CH 3 CH 2 OH). CH 3 CH 2 OH CH 3 CH 2 O p K a ~16 O O O C OH C O C O CH 3 CH 3 CH 3 p K a ~5
1.16 Acid-base reactions - equilibria + NaCl + H 2 O NaOH H Cl O O NaOH H 2 O + + H 3 C OH H 3 C ONa H 2 O CH 3 OH NaOH CH 3 ONa + + The equilibrium will lie to the side of the weaker conjugate base
1.17 Lewis acids and Lewis bases CH 2 CH 3 CH 2 CH CH 2 CH 3 CH 2 CH 3 3 + – + – • • • • O F 3 B + O F 3 B + • • O • F 3 B O • • F 3 B • • • CH 2 CH 3 CH 2 CH CH 2 CH 3 CH 2 CH 3 3 Lewis acid Lewis acid Lewis base 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 CH 3 CH 2 OCH 2 CH 3 has a boiling point of 34°C.
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