Chapter 2. Molecular Representations

Transcription

1 hapter 2. Molecular Representations 3 () 3 ( 3 ) Lewis (Kekule) structure ondensed and par6ally condensed structure Skeletal (bond- line) structure Molecular formula Amoxicillin a widely prescribed anti-bacterial agent Bond-Line Structures Bond-line (Skeletal) otation 1. arbon atoms are at the ends of lines and at the intersection of two lines 2. ydrogens on carbon atoms are not typically shown. 3. All non-carbon and non-hydrogen atoms (heteroatoms) are shown. 4. ydrogens on heteroatoms are shown 5. Formal charges are shown 6. This representation is intended to approximate the actual geometry of the molecule 2 S 28 14

2 2.3 Identifying Functional Groups There are between million organic compounds which are classified into families according to structure and reactivity. Functional Group (FG): a group of atoms, which are part of a larger molecule, that have characteristic chemical behavior. FG s behave similarly in every molecule they are part of. The chemistry of organic molecules are defined by the function groups they contain. Br 2 Br Br 3 3 Br Br Br 29 Alkanes arbon - arbon Multiple Bonds arbon-heteroatom single bonds basic Alkenes X X= F, l, Br, I Alkyl alide amines nitro alkane Alkynes Arenes alcohols thiols acidic ethers S S sulfides (thioethers) epoxide S S disulfide 30 15

3 arbon-oxygen arbonyl-oxygen double bonds (carbonyls) acidic aldehyde carboxylic acid l acid chloride arbon-nitrogen multiple bonds imine (Schiff base) basic ketones ester anhydrides amide arbon Atoms with Formal harge Formal charge is often associated with the chemistry of some functional groups and define the reactivity of many intermediates. (recall hapter 1.4) arbon usually has 4 covalent bonds and no lone pairs of electrons, and therefore, no charge. arbocations are carbon atoms that carry a +1 charge, with 3 bonds to the cationic carbon. arbanions are carbon atoms that carry a 1 charge and have a lone pair of electrons on the anionic carbon

4 2.5 Identifying Lone Pairs. eteroatoms can carry formal charges (,, S) the number of lone pairs of electrons is related to the charge. 2.6 Three-Dimensional Bond-Line Structures rganic chemistry is a 3-D subject Introduction to Resonance Resonance - two or more proper Lewis structures can be drawn for a molecule (or ions). itromethane 3 3 Benzene Which is the correct structure? Both are correct, and neither is correct

5 Drawing and Interpreting Resonance Forms 1. o one resonance forms accurately depicts the structure of the molecule. The real structure is a composite or hybrid of all resonance forms. 2. Resonance forms differ only by the placement of electrons, usually non-bonding electron pairs, or the π-electron pairs of double and triple bonds. either the position or hybridization of the atoms can change. 3. Resonance forms are not necessarily equivalent. While all resonance forms contribute to the actual structure (resonance hybrid), some forms may contribute more. 4. All resonance forms must be proper Lewis structures (including formal charges). 5. The actual resonance hybrid is more stable than any single resonance form. 6. In general, the greater the number of resonance forms, the more stable the resonance hybrid. 35 Allyl carbocation and carbanion: Allyl position is adjacent to a = bond vinyl hydrogen vinyl carbon (sp 2 hybridized) allylic hydrogen allylic carbon Allyl carbocation Allyl carboanion What are the hybridization of the carbons? 36 18

6 Molecular rbitals of the Allyl arbocation urved Arrows Arrows in organic chemistry Reaction arrow Equilibrium arrow Resonance arrow Mechanism (curved) arrows Double-headed arrow Single-headed arrow 38 19

7 urved Arrow onvention 1. urved arrows show the movement (flow) of electrons during bond breaking and/or bond making processes (mechanism). The foot of the arrow indicates where the electron or electron pair originates, the head of the arrow shows where the electron or electron pair ends up. A. The movement of a single electron is denoted by a curved single headed arrow (fishhook or hook) radical reactions. B. The movement of an electron pair is denoted by a curved double headed arrow polar reactions. double-headed arrow single-headed arrow 2. If an electron pair moves in on a new atom, another electron pair must leave so that the atom does not exceed a full valance of eight electrons. There are two common exceptions: A. When an atom already has an incomplete valance (R 3 +). B. With third row (or below) elements the octet rule may be violated. 3. The arrows completely dictate the Lewis structure of the product Formal harges in Resonance Structures 2.10 Drawing Resonance Structures via Pattern Recognition a. Recognize a lone pair of electrons at an allylic position (adjacent to a π-bond) 40 20

8 b. recognize positive charge at an allylic position c. a lone pair of electrons adjacent to a positive charge d. a = and = π-bonds e. systems with alternating σ- and π-bonds (conjugated), particularly in cyclic systems (aromatic) Assessing Relative Importance of Resonance Structures Resonance structure do not necessarily contribute equally to the overall resonance hybrid Delocalized and Localized Lone Pairs 42 21