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 Biological Molecules DNA RNA Amino Acids/Proteins Photosynthesis Pharmaceuticals A #&*$ Load of Other Stuff

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

Empirical & Molecular Formula Examples Consider 4 Hydrocarbons: Ethene, Cyclopentane, Cyclohexane, 2-Butene Empirical Formula: CH 2 Molecular Formula: C 2 H 4, C 5 H 10, C 6 H 12, C 4 H 8

Valence Valence best described as # of bonds an atom can form AtomValenceExample CTetravalentCH 4, CBr 4 B, NTrivalentBH 3, NH 3 ODivalentH 2 O, H 3 C-O-CH 3 H, Cl, BrMonovalentHCl, HBr, H 2 CCl 2 Related to # of valence electrons (Periodic Table)

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

Electronegativity and the Periodic Table Know the electronegativity trends!! Increasing Electronegativity

Lewis Structures Use only valence (outer shell) electrons Each atom acquires Noble gas configuration Octet Rule exceptions: Ions, Radicals, 3 rd 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 ½  (valence electrons) = # shared + lone pairs

Example: CH 3 Br

Example: C 2 H 4

Example: CO 3 2- Place brackets around ions, indicate their charge We could have just as easily placed the double bond at other 2 O’s

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

Octet Rule Exceptions: SO 4 2- For now we focus on 3 rd 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!

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

d Orbitals & Minimizing Formal Charge S6 – 6 = 0 O(single)6 – 7 = -1 O(double)6 – 6 = 0 _____Formal Charges_____ Better Lewis structure with minimized Formal Charge Note: There are resonance structures (draw these?)

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

Rules for Drawing Resonance Structures 1.Hypothetical Structures; “Sum” Makes Real Hybrid Structure 2.Must be Proper Lewis Structures 3.Can Only Generate by Moving Electrons (NO Moving Atoms) 4.Resonance Forms are Stabilizing 5.Equivalent Resonance Structures Contribute Equally to Hybrid

Rules for Drawing Resonance Structures 6.More Stable Resonance Forms Contribute More to Hybrid Factors Affecting Stability 1.Covalent Bonds 2.Atoms with Noble Gas (Octet) Configurations 3.Charge Separation Reduces Stability 4.Negative Charge on More Electronegative Atoms

Isomerism: Structural Structural Isomers: Same Molecular Formula; Different Connectivity Why Might This Be a Big Deal? Consider Properties: C 2 H 6 OCH 3 CH 2 OHCH 3 OCH 3 BP 78.5 o C o C MP o C -138 o C Properties Can Differ Substantially Between Isomers!!

Isomerism: Cis/Trans Same Molecular Formula (C 2 Cl 2 H 2 ) Same Connectivity Different Structures  Double Bonds Don’t Rotate

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

Hybridization (2) Carbon TypeHybridizationHybrid Composition Geometry Alkanesp 3 25% s 75% p Tetrahedral Alkenesp 2 (one pure p left) 33% s 67% p Trigonal planar Alkynesp (two pure p left) 50% s 50%p Linear Hybrid orbitals form single (  ) bonds; pure p form multiple (  )

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. BF 3, carbocations) Trigonal Pyramidal (e.g. NH 3, carbanions) Tetrahedral (e.g. CH 4, Ammonium Ion) Angular (Bent) (e.g. H 2 O)

Representations of Organic Structures Condensed Formula: CH 3 CH 2 OH, CH 3 CH 2 CH 2 CH 3 Dash Formula: Bond-Line Formula

Some Common Cyclic Structures CyclopropaneCyclobutaneCyclopentane Cyclohexane Benzene