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Chem 232 Lecture 5 CHEM 232 University of Illinois Organic Chemistry I at Chicago UIC Organic Chemistry 1 Lecture 5 Instructor: Prof. Duncan Wardrop Time/Day: T & R, 12:30-1:45 p.m. January 26, 2010 1 Self Test Question Which of the following best depicts a π-bond? A. a a. c. B. b e. C. c D. d b. d. E. e 2 University of Slide CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 2 The answer is A: A bonding interaction exists when two orbitals overlap “in phase” with each other. The electron density in π bonds lie above and below the plane of carbon and hydrogen atoms. B depicts a C-C sigma bond between two sp-hybridized carbon atoms. C represents a sigma bond formed via the head-to-head overlap of two p-orbitals. Summary of Bond Types π-bond C C C C σ-bond σ-bonds π-bonds bond (head-to-head) (side-to-side) single 1 0 double 1 1 triple 1 2 University of Slide 3 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 3 Self Test Question Rank the following hydrocarbons in order of increasing acidity. A. ethane, ethylene, ethyne ethane ethylene ethyne H H H H B. ethane, ethyne, ethylene H C C H C C H C C H H H H H C. ethyne, ethylene, ethane four 2sp3 one 2p two 2p D. ethyne, ethane, ethylene three 2sp2 two 2sp E. none of the above University of Slide 4 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 4 The answer is A. Hybridization and Acidity increased s-character = increased electronegativity of carbon = electrons closer to the nucleus = stronger acid pKa = 26 sp H C C H H C C + H H H H 2 pKa = 45 sp C C C C + H H H H H H H H Increasing Acid Strength Increasing 3 pKa = 62 sp H C C H H C C H + H H H H H University of Slide 5 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 5 CHEM 232 University of Illinois Organic Chemistry I at Chicago UIC Chapter 3: Conformational Isomers of Alkanes Sections: 3.1-3.3 6 Isomer Classi#cation University of Slide 7 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 7 Classi#cation of Isomers stereoisomers Can the molecules be interconverted by rotation around single bonds? yes no conformational con#gurational Me Me We will continue the H H Me H isomer tree in Chapters 5 & 7 where we’ll H H H H Me H encounter subdivisions anti butane gauche butane of con#gurational isomers University of Slide 8 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 8 Model Activity 1. Make a model of butane. 2. Make a separate model of isobutane. 3. Using a minimum number of changes, convert your model of isobutane into butane. University of Slide 9 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 9 Self Test Question What action did you have to perform to convert isobutane to butane? A. rotate around C2-C3 bond If you have to break bonds B. remove methyl group from C-2 to interconvert isomers, they are constitutional C. add one methyl group to C-1 (structural) isomers D. add one methyl group to C-2 E. rotate around C1-C2 University of Slide 10 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 10 Rotation Around Single Bonds conformations: different spatial arrangements of atoms generated by rotation around single bonds conformational analysis: comparison of the relative energies of different conformational isomers and how they in%uence properties and reactivity lowest energy University of Slide 11 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 11 Measuring Relative Positions of Atoms dihedral angle: angle between two intersecting planes; also called the torsion angle plane can be de#ned by: • 3 non-collinear points • a line & a point not on that line • two intersecting lines • two parallel lines University of Slide 12 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 12 Eclipsed Conformation of Ethane 0º H H • C–H bonds on adjacent carbons are parallel (same plane) • H–C–C–H angle (dihedral angle) = 0º • highest energy conformation University of Slide 13 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 13 Staggered Conformation of Ethane 60º H H • C–H bond bisects (cuts in half) H–C–H angle on adjacent carbon • H–C–C–H angle (dihedral angle) = 60º • lowest energy conformation for ethane University of Slide 14 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 14 Drawing Conformations: Wedge & Dash H H H H H H H H H H H H = group is pointing toward you, in front of the plane of paper rarely followed convention: = group is pointing away from you, behind the plane of paper thickest part of = group is either toward or wedge or dash is away from you, usually denotes mixtures always closest to viewer = group lies in the plane of the drawing surface University of Slide 15 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 15 Drawing Conformations: Sawhorse CH3 CH3 CH3 H3C H3C H3C H H H CH3 H H CH3 H H H H H3C H H H3C H H H • one C–C bond viewed “head-on” from oblique angle (acute or obtuse, but not 0º, 90º or 180º); skewed • all atoms on central C–C bond are shown University of Slide 16 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 16 Why a Sawhorse? University of Slide 17 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 17 Drawing Conformations: Newman Projection CH3 CH3 CH3 H3C H3C H3C H H H H H H3C H H H3C CH3 H H CH3 H H H H H • one C–C bond viewed “head-on” at 0º angle • all atoms on central C–C bond are shown • circle represents back carbon atom University of Slide 18 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 18 Spatial Relationships in Staggered Conformations: Anti & Gauche 0 ° 60 ° Y Y Y X X H H H 180 ° H H H H H H H H H X Torsion Angle = 0° Torsion Angle = 60° Torsion Angle = 180° Eclipsed Gauche Anti-Periplannar •anti: dihedral angle (torsion angle) = 180º •gauche: dihedral angle (torsion angle) = 60º •these relationships apply to any groups on adjacent carbon atoms University of Slide 19 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 19 Comparison of Conformational Drawings of Eclipsed and Staggered Ethane Ball & Stick Newman Sawhorse Dash & Wedge H H H H H H H H H H H H H H H H H H H HH H H H H H H H H H H H H H H H University of Slide 20 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 20 Self Test Question What is the IUPAC name for molecule below? A. 1,2,2,4,4-pentametnylhexane 1 CH3 H C H B. 3,3,5,5-tetramethylheptane 3 2 3 C. 2-ethyl-2,4,4-trimethylhexane H3C 5 7 D. 1,2,4,4-tetramethylhexane H 4 6 E. 3,3-dimethyl-5,5-dimethylheptane University of Slide 21 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 21 Self Test Question Which set of molecules are conformational isomers? CH CH H 3 3 H H C H a. H3C H H H c. 3 H3C H A. a H3C CH3 H3C H H CH H CH3 3 H CH3 B. b H H HO CH3 H C. c Cl H H Cl H O b. d. HO CH3 D. d H H H H H HBr HO Br H H H University of Slide 22 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 22 The answer is C. Pair A are diferent compounds and not isomers; Pair B are identical and therefore neither isomers or conformers of one another; Pair D share the same formula but difer in their constitution and therefore cannot be conformers of one another. Conformational Analysis of Ethane staggered conformation more stable than eclipsed torsional strain: torsion angles (dihedral angles) are other than 60º (gauche) University of Slide 23 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 23 Two Con$icting Arguments Explain Preference for Staggered Conformation 1. Steric Repulsion Electrons in vicinal (adjacent) bonds destabilize (raise energy) in eclipsed conformations due to repulsion; they are closer. 2. Hyperconjugation: Electrons in vicinal (adjacent) bonds are delocalized by overlap between bonding and anti-bonding orbitals University of Slide 24 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 24 1. Steric Repulsion *Read on...... maximum electron- electron repulsion H H H H H H H H H H H H eclipsed staggered • widely accepted explanation until 2001* • electron-electron repulsion is greatest in eclipsed conformation University of Slide 25 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 25 Brief Revision of Molecular Orbitals Molecular Orbitals Review: H C • node = where orbitals change σ∗ sign = no electrons found • number of atomic orbitals = number of molecular orbitals • bonding orbital (σ) is lower C y in energy than both atomic g r e orbitals n E 2sp3 • anti-bonding orbital (σ*) = H less electron density between nuclei than if no bond at all; 1s electrons from each atom repel each other • even though an anti-bonding σ (s*) orbital may not be filled H C with electrons; it is still present in the molecule University of Slide 26 CHEM 232, Spring 2010 Illinois at Chicago UIC Lecture 5: January 26 26 2.
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