CHEM Lecture 4
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1 CEM 494 Special Topics in Chemistry Illinois at Chicago CEM Prof. Duncan Wardrop October 1, 2012
2 Course Website Syllabus Course Policies Other handouts Announcements (Course News) Course Calendar Illinois at Chicago CEM 494, Fall 2012 Slide 2
3 CEM 494 Special Topics in Chemistry Illinois at Chicago Conformational Isomers of Alkanes
4 Isomer Classification Illinois at Chicago CEM 494, Fall 2012 Slide 4
5 Classification of Isomers stereoisomers Can the molecules be interconverted by rotation around single bonds? yes no conformational configurational Me Me anti butane Me Me gauche butane We will continue the isomer tree in Chapters 5 & 7 where we ll encounter subdivisions of configurational isomers Illinois at Chicago CEM 494, Fall 2012 Slide 5
6 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. Illinois at Chicago CEM 494, Fall 2012 Slide 6
7 Self Test Question What action did you have to perform to convert isobutane to butane? If you have to break bonds to interconvert isomers, they are constitutional (structural) isomers A. rotate around C2-C3 bond B. remove methyl group from C-2 C. add one methyl group to C-1 D. add one methyl group to C-2 E. rotate around C1-C2 Illinois at Chicago CEM 494, Fall 2012 Slide 7
8 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 influence properties and reactivity lowest energy Illinois at Chicago CEM 494, Fall 2012 Slide 8
9 Measuring Relative Positions of Atoms dihedral angle: angle between two intersecting planes; also called the torsion angle plane can be defined by: 3 non-collinear points a line & a point not on that line two intersecting lines two parallel lines Illinois at Chicago CEM 494, Fall 2012 Slide 9
10 Eclipsed Conformation of Ethane 0º C bonds on adjacent carbons are parallel (same plane) C C angle (dihedral angle) = 0º highest energy conformation Illinois at Chicago CEM 494, Fall 2012 Slide 10
11 Staggered Conformation of Ethane 60º C bond bisects (cuts in half) C angle on adjacent carbon C C angle (dihedral angle) = 60º lowest energy conformation for ethane Illinois at Chicago CEM 494, Fall 2012 Slide 11
12 Drawing Conformations: Wedge & Dash = group is pointing toward you, in front of the plane of paper = group is pointing away from you, behind the plane of paper = group is either toward or away from you, usually denotes mixtures = group lies in the plane of the drawing surface Illinois at Chicago CEM 494, Fall 2012 rarely followed convention: thickest part of wedge or dash is always closest to viewer Slide 12
13 Drawing Conformations: Sawhorse 3 C C 3 3 C C 3 3 C C 3 C 3 3 C C 3 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 3 C Illinois at Chicago CEM 494, Fall 2012 Slide 13
14 Why a Sawhorse? Illinois at Chicago CEM 494, Fall 2012 Slide 14
15 Drawing Conformations: Newman Projection 3 C C 3 3 C C 3 3 C C 3 C 3 3 C C 3 3 C one C C bond viewed head-on at 0º angle all atoms on central C C bond are shown circle represents back carbon atom Illinois at Chicago CEM 494, Fall 2012 Slide 15
16 Spatial Relationships in Staggered Conformations: Anti & Gauche Y X 0 60 X Y Y X 180 Torsion Angle = 0 Eclipsed Torsion Angle = 60 Gauche Torsion Angle = 180 Anti-Periplannar anti: dihedral angle (torsion angle) = 180º gauche: dihedral angle (torsion angle) = 60º these relationships apply to any groups on adjacent carbon atoms Illinois at Chicago CEM 494, Fall 2012 Slide 16
17 Illinois at Chicago CEM 494, Fall 2012 Slide Comparison of Conformational Drawings of Eclipsed and Staggered Ethane 17 Ball & Stick Newman Sawhorse Dash & Wedge
18 Self Test Question What is the IUPAC name for molecule below? 1 3 C 3 C C A. 1,2,2,4,4-pentametnylhexane B. 3,3,5,5-tetramethylheptane C. 2-ethyl-2,4,4-trimethylhexane D. 1,2,4,4-tetramethylhexane E. 3,3-dimethyl-5,5-dimethylheptane Illinois at Chicago CEM 494, Fall 2012 Slide 18
19 Self Test Question Which set of molecules are conformational isomers? a. b. 3 C 3 C Cl C 3 C 3 3 C C 3 C 3 Cl c. d. 3 C O C 3 O C 3 Br 3 C C 3 O O Br C 3 A. a B. b C. c D. d Illinois at Chicago CEM 494, Fall 2012 Slide 19
20 Conformational Analysis of Ethane staggered conformation more stable than eclipsed torsional strain: torsion angles (dihedral angles) are other than 60º (gauche) Illinois at Chicago CEM 494, Fall 2012 Slide 20
21 Two Conflicting 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. yperconjugation: Electrons in vicinal (adjacent) bonds are delocalized by overlap between bonding and anti-bonding orbitals Illinois at Chicago CEM 494, Fall 2012 Slide 21
22 maximum electronelectron repulsion 1. Steric Repulsion *Read on... edu. eclipsed staggered widely accepted explanation until 2001* electron-electron repulsion is greatest in eclipsed conformation Illinois at Chicago CEM 494, Fall 2012 Slide 22
23 Brief Revision of Molecular Orbitals Energy 1s σ C σ C 2sp 3 C Molecular Orbitals Review: node = where orbitals change sign = no electrons found number of atomic orbitals = number of molecular orbitals bonding orbital (σ) is lower in energy than both atomic orbitals anti-bonding orbital (σ*) = less electron density between nuclei than if no bond at all; electrons from each atom repel each other even though an anti-bonding (s*) orbital may not be filled with electrons; it is still present in the molecule Illinois at Chicago CEM 494, Fall 2012 Slide 23
24 2. Vicinal yperconjugation "Bond-No Bond Resonance" Energy C- antibonding (vacant) σ C- bonding (filled) A Stereoelectronic Effect yperconjugatively stabilized bond σ σ σ Stabilizing filled-empty orbital overlap only possible in staggered conformation Illinois at Chicago CEM 494, Fall 2012 Slide 24
25 2. yperconjugation: Role in Conformations hyperconjugation: donation (transfer) of electrons from a filled orbital to an empty orbital; orbitals must overlap to allow transfer Eclipsed Staggered σc σ*c σc σ*c σ & σ* weak overlap = no hyperconjugation = no electron delocalization = no additional stabilization σ & σ* strong overlap = hyperconjugation (σ σ*) = delocalized electrons = lower energy (more stable) Illinois at Chicago CEM 494, Fall 2012 Slide 25
26 Conformational Analysis of Butane Illinois at Chicago CEM 494, Fall 2012 Slide 26
27 Van der Waals Strain (Steric Strain) in Staggered Conformations of Butane Gauche Anti C 3 C 3 C 3 C 3 The gauche conformation of butane is 3 kj/mol less stable than the anti. The gauche conformation is destabilized by van der Waals strain (also called steric strain); repulsive van der Waals force between methyl groups van der Waals strain = destabilization that results from atoms being too close together; nuclear-nuclear & electron-electron repulsions dominate Illinois at Chicago CEM 494, Fall 2012 Slide 27
28 Van der Waals Strain (Steric Strain) conformation of butane with two methyl group eclipsed is the least stable (highest in energy) destabilized by both torsional strain (eclipsed vicinal bonds) and van der Waals strain (atoms close together) Illinois at Chicago CEM 494, Fall 2012 Slide 28
29 Conformation of igher Alkanes anti arrangements of C-C-C-C units all vicinal (adjacent) bonds = gauche or anti minimize torsional strain; minimize steric strain described as zig-zag backbone Illinois at Chicago CEM 494, Fall 2012 Slide 29
30 CEM 494 Special Topics in Chemistry Illinois at Chicago Conformational Isomers of 3-5 Carbon Cycloalkanes
31 Self Test Question Geometry: What are the angles in a regular pentagon? 3 x 180º 5 = 108º A. 110º B º C. 60º D. 120º E. 108º Illinois at Chicago CEM 494, Fall 2012 Slide 31
32 Self Test Question Geometry: What are the angles in a regular hexagon? 4 x 180º 6 = 120º A. 90º B. 120º C. 144º D. 150º E. 30º 32 Illinois at Chicago CEM 494, Fall 2012 Slide
33 eats of Combustion of Cycloalkanes Angle strain (Baeyer strain): increase in energy associated with bond angles that deviate from tetrahedral (109.5º) Cycloalkane Shape Geometric Angles Difference from 109.5º eat of Combustion (- ) per C 2 Group cyclopropane 60º 49.5º 167 kcal/mol cyclobutane 90º 19.5º 163 kcal/mol cyclopentane 108º 1.5º 157 kcal/mol cyclohexane 120º 10.5º 156 kcal/mol Illinois at Chicago CEM 494, Fall 2012 Slide 33
34 Cyclopropane & Banana Bonds! only planar cycloalkane bent C-C bonds: sp 3 orbitals unable to overlap along internuclear axis; weaker C-C σ-bonds angle strain: 60º is a large deviation from 109.5º torsional strain: all bonds are eclipsed Illinois at Chicago CEM 494, Fall 2012 Slide 34
35 Conformations of Cyclobutane Nonplanar puckered Conformation torsional strain reduced in puckered conformation less angle strain than cyclopropane Illinois at Chicago CEM 494, Fall 2012 Slide 35
36 Conformations of Cyclopentane Planar Envelope alf-chair planar conformation least stable; all bonds eclipsed some torsional strain relieved in envelope and half-chair envelope & half-chair have similar energies; interconvert rapidly Illinois at Chicago CEM 494, Fall 2012 Slide 36
37 Self Test Question Which of the following conformations of cyclohexane would you expect to have the highest heat of combustion? a. lowest NRG: all gauche & anti relationships b. c. highest NRG: 4 eclipsed relationships A. a B. b C. c Illinois at Chicago CEM 494, Fall 2012 Slide 37
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