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Chemistry 343—Summer 2006

Chemistry 343—Summer 2006. General Information (Grading, Policies, etc.) Syllabus (Lectures, Quizzes, Exams) Recommended Problems Study Tips Chapter One: Basically Review (I hope); Let’s Have at it…. Organic Chemistry: What and Why. Compounds Based on Carbon

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Chemistry 343—Summer 2006

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  1. Chemistry 343—Summer 2006 • General Information (Grading, Policies, etc.) • Syllabus (Lectures, Quizzes, Exams) • Recommended Problems • Study Tips • Chapter One: Basically Review (I hope); • Let’s Have at it…

  2. Organic Chemistry: What and Why • Compounds Based on Carbon • Biological Molecules • DNA • RNA • Amino Acids/Proteins • Photosynthesis • Pharmaceuticals • A #&*$ Load of Other Stuff

  3. Empirical vs. Molecular Formulas • Empirical Formula: Lowest whole number ratio of atoms in a given compound • Molecular Formula: Exact composition of a compound • Drawback: No Structural Information Provided by Either • Later on we will look at methods that provide structural detail

  4. Empirical & Molecular Formula Examples Consider 4 Hydrocarbons: Ethene, Cyclopentane, Cyclohexane, 2-Butene Empirical Formula: CH2 Molecular Formula: C2H4, C5H10, C6H12, C4H8

  5. Valence • Valence best described as # of bonds an atom can form • Related to # of valence electrons (Periodic Table)

  6. Valence and the Periodic Table • Valence Corresponds To Column (Group I, II, Nonmetals)

  7. Electronegativity and the Periodic Table Increasing Electronegativity • Know the electronegativity trends!!

  8. Lewis Structures • Use only valence (outer shell) electrons • Each atom acquires Noble gas configuration • Octet Rule exceptions: Ions, Radicals, 3rd row • and lower (S, P, etc.) • Sum # of valence electrons in atoms: this is the • number of electrons that should be represented • in the Lewis structure • ½S(valence electrons) = # shared + lone pairs

  9. Example: CH3Br

  10. Example: C2H4

  11. Example: CO32- • Place brackets around ions, indicate their charge • We could have just as easily placed the double bond at other 2 O’s

  12. Resonance: The Carbonate Ion • Double headed arrows indicate resonance forms • Red “Curved Arrows” show electron movement • Curved Arrow notation used to show electron flow in resonance • structures as well as in chemical reactions: we will use • this electron bookkeeping notation throughout the course

  13. Octet Rule Exceptions: SO42- • For now we focus on 3rd row atoms and beyond w/ ‘d’ orbitals • Consider the sulfate ion: Here’s one valid Lewis structure • THIS IS NOT THE BEST POSSIBLE LEWIS STRUCTURE!

  14. Formal Charge • Formal Charge = #Valence Electrons - #Assigned Electrons • We assign all electrons in a lone pair to an atom; • ½ bonded electrons Formal Charges S: 6 – 4 = +2 O: 6 – 7 = -1 • Lewis structures that minimize formal charge tend to be better • Note: Sum of formal charges = molecular or ionic charge

  15. d Orbitals & Minimizing Formal Charge _____Formal Charges_____ • Better Lewis structure with • minimized Formal Charge • Note: There are resonance • structures (draw these?) S 6 – 6 = 0 O(single) 6 – 7 = -1 O(double) 6 – 6 = 0

  16. More Formal Charge Examples _____Formal Charges_____ C: 4 – 4 = 0 O: 6 – 6 = 0 N: 5 – 5 = 0 H: 1 – 1 = 0 H: 1 – 1 = 0 N: 5 – 4 = 1

  17. Rules for Drawing Resonance Structures • Hypothetical Structures; “Sum” Makes Real Hybrid Structure • Must be Proper Lewis Structures • Can Only Generate by Moving Electrons (NO Moving Atoms) • Resonance Forms are Stabilizing • Equivalent Resonance Structures Contribute Equally to Hybrid

  18. Rules for Drawing Resonance Structures • More Stable Resonance Forms Contribute More to Hybrid • Factors Affecting Stability • Covalent Bonds • Atoms with Noble Gas (Octet) Configurations • Charge Separation Reduces Stability • Negative Charge on More Electronegative Atoms

  19. Isomerism: Structural • Structural Isomers: Same Molecular Formula; Different • Connectivity • Why Might This Be a Big Deal? Consider Properties: • C2H6O CH3CH2OH CH3OCH3 • BP 78.5 oC -24.9 oC • MP -117.3 oC -138 oC • Properties Can Differ Substantially Between Isomers!!

  20. Isomerism: Cis/Trans • Same Molecular Formula (C2Cl2H2) • Same Connectivity • Different Structures  Double Bonds Don’t Rotate

  21. Hybridization For now, worry only about Carbon hybridization Recall C’s valence configuration: 2s2 2p2 Will combine to form hybrid orbitals based on the valence of the carbon atom

  22. Hybridization (2) Hybrid orbitals form single (s) bonds; pure p form multiple (p)

  23. VSEPR Theory: What to Know You are responsible for these geometries (the most prevalent in Organic Chemistry): Linear (e.g. acetylene) Trigonal Planar (e. g. BF3, carbocations) Trigonal Pyramidal (e.g. NH3, carbanions) Tetrahedral (e.g. CH4, Ammonium Ion) Angular (Bent) (e.g. H2O)

  24. Representations of Organic Structures • Condensed Formula: CH3CH2OH, CH3CH2CH2CH3 • Dash Formula: • Bond-Line Formula

  25. Some Common Cyclic Structures Cyclopropane Cyclobutane Cyclopentane Cyclohexane Benzene

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