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1.3 Covalent Bonding - Electrons Shared

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

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1.3 Covalent Bonding - Electrons Shared

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  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 relativeelectronegativitiesof the elements involved

  2. Electronegativity the attraction of an atom for electrons

  3. 1.2 Ionic bonding Electrons Transferred Big differences in E.N. values Metals reacting with non-metals

  4. Important Electronegativity Values H 2.1 LiBe 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

  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 B.D.E +104 kcal/mol B.D.E. = bond dissociation energy

  6. 1.3 Lewis Dot Structures of Molecules

  7. 1.4 Double bonds and triple bonds Double bonds - alkenes Triple bonds - alkynes

  8. ..   : H F .. 1.5 Polar covalent bonds and electronegativity H2 HF H2O CH4 CH3Cl Based on electronegativity + - H Li

  9. 1.6 Structural Formula - Shorthand in Organic Chemistry

  10. 1.6 ConstitutionalIsomers Same molecular formula, completely different chemical and physical properties

  11. 1.7 Formal Charge Formal charge = group number - number of bonds - number of unshared electrons

  12. 1.8 Resonance Structures - Electron Delocalization Table 1.6 – formal rules for resonance

  13. 1.9 Shapes of Molecules Shapes of molecules are predicted using VSEPR theory

  14. 1.9 Shape of a molecule in terms of its atoms Figure 1.9 Table 1.7 – VSEPR and molecular geometry

  15. Trigonal planar geometry of bonds to carbon in H2C=O Linear geometry of carbon dioxide

  16. 1.10 Molecular dipole moments Figure 1.7

  17. 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:

  18. Curved Arrows to Describe a Reaction Many reactions involve both bond breaking and bond formation. More than one arrow may be required.

  19. 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.

  20. . . 1.13 A Brønsted-Lowry Acid-Base Reaction A proton is transferred from the acid to the base. + – . . + B H A B + A H base acid conjugate acid conjugate base

  21. . . . . . . . . Proton Transfer from HBr to Water hydronium ion (H3O+) H H .. – + .. . . . . O H Br H Br O + + .. .. H H base acid conjugate conjugate acid base

  22. H H – .. . . . . . . . . . . Br .. H H Equilibrium Constant for Proton Transfer .. . + . + O + H Br H O .. [H3O+][Br–] Ka = [HBr] pKa = – log10Ka

  23. Acids and Bases: Arrow Pushing [H3O+][Br–] Ka = ~ 106 for HBr, pKa = - 5.8 [HBr]

  24. Need to know by next class: pKa = -log10Ka STRONG ACID = LOW pKa WEAK ACID = HIGH pKa HI, HCl, HNO3, H3PO4pKa -10 to -5 Super strong acids H3O+pKa – 1.7 RCO2HpKa ~ 5 acids PhOHpKa ~ 10 get H2O, ROHpKa ~ 16 weaker RCCH (alkynes) pKa ~ 26 RNH2 pKa ~ 36 Extremely weak acid RCH3 pKa ~ 60 Not acidic at all

  25. 1.14 What happened to pKb? • A separate “basicity constant” Kb is not necessary. • Because of the conjugate relationships in the Brønsted-Lowry approach, we can examine acid-base reactions by relying exclusively on pKa values. pKa ~60 Essentially not acidic Corresponding base Extremely strong

  26. strongest H—X bond weakest H—X bond 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).

  27. least electronegative most electronegative Electronegativity Acidity increases across periodic table as the atom attached to H gets more electronegative (HF>H2O>H2N>CH4).

  28. Inductive Effects Electronegative groups/atoms remote from the acidic H can effect the pKa of the acid. pKa = 16 pKa = 11.3 • O – H bond in CF3CH2OH is more polarized • CF3CH2O- is stabilized by EW fluorine atoms

  29. Resonance Stabilization in Anion Delocalization of charge in anion (resonance) makes the anion more stable and thus the conjugate acid more acidic e.g. (CH3CO2H > CH3CH2OH). pKa ~16 pKa ~5

  30. 1.16 Acid-base reactions - equilibria The equilibrium will lie to the side of the weaker conjugate base

  31. CH2CH3 CH2CH3 + – O •• •• O F3B •• CH2CH3 CH2CH3 1.17 Lewis acids and Lewis bases F3B + Lewis acid Lewis base Product is a stable substance. It is a liquid with a boiling point of 126°C. Of the two reactants, BF3 is a gas and CH3CH2OCH2CH3 has a boiling point of 34°C.

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