CHEM 494 Lecture 4

CHEM 494 University of Illinois Special Topics in Chemistry at Chicago UIC

CHEM 494 - Lecture 4

Prof. Duncan Wardrop October 1, 2012 Course Website http://www.chem.uic.edu/chem494

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University of Slide 2 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 CHEM 494 University of Illinois Special Topics in Chemistry at Chicago UIC

Conformational Isomers of Alkanes Isomer Classiﬁcation

University of Slide 4 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Classiﬁcation 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 5 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 6 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 7 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 8 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 9 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Eclipsed Conformation of Ethane

0º 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 10 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 11 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 12 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 13 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Why a Sawhorse?

University of Slide 14 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 15 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 16 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 17 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Self Test Question

What is the IUPAC name for molecule below?

A. 1,2,2,4,4-pentametnylhexane 1 CH3 B. 3,3,5,5-tetramethylheptane H3C 2 H 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 18 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 19 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Conformational Analysis of Ethane

staggered conformation more stable than eclipsed

torsional strain: torsion angles (dihedral angles) are other than 60º (gauche) University of Slide 20 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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. Hyperconjugation: Electrons in vicinal (adjacent) bonds are delocalized by overlap between bonding and anti-bonding orbitals

University of Slide 21 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 1. Steric Repulsion *Read on.....www.chem.uic. maximum electron- electron repulsion H edu. 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 22 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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 in energy than both atomic y 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 23 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 2. Vicinal Hyperconjugation

"Bond-No Bond Resonance" H C-H H H H H antibonding (vacant) H H H H H H H σ∗ C-H bonding

y (filled) g r e n E A Stereoelectronic Eﬀect

σ H H Stabilizing !lled-empty σ H orbital overlap only σ∗ Hyperconjugatively H possible in staggered stabilized bond H H conformation

University of Slide 24 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 2. Hyperconjugation: Role in Conformations hyperconjugation: donation (transfer) of electrons from a !lled orbital to an empty orbital; orbitals must overlap to allow transfer Eclipsed Staggered

σC–H σ*C–H σC–H σ*C–H

σ & σ* weak overlap = σ & σ* strong overlap = no hyperconjugation = hyperconjugation (σ→σ*) = no electron delocalization = delocalized electrons = no additional stabilization lower energy (more stable)

University of Slide 25 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Conformational Analysis of Butane

University of Slide 26 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Van der Waals Strain (Steric Strain) in Staggered Conformations of Butane Gauche Anti

CH3 CH3

H CH3 H H

H H H H H CH3

• 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

University of Slide 27 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 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)

University of Slide 28 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Conformation of Higher 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

University of Slide 29 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 CHEM 494 University of Illinois Special Topics in Chemistry at Chicago UIC

Conformational Isomers of 3-5 Carbon Cycloalkanes Self Test Question

Geometry: What are the angles in a regular pentagon?

A. 110º B. 109.5º 3 x 180º = 108º C. 60º 5 D. 120º E. 108º

University of Slide 31 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Self Test Question

Geometry: What are the angles in a regular hexagon?

A. 90º B. 120º 4 x 180º = 120º C. 144º 6 D. 150º E. 30º

University of Slide CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Heats of Combustion of Cycloalkanes

Angle strain (Baeyer strain): increase in energy associated with bond angles that deviate from tetrahedral (109.5º)

Geometric Diﬀerence Heat of Combustion Cycloalkane Shape Angles from 109.5º (-∆H) per CH2 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

University of Slide 33 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Cyclopropane & Banana Bonds!

• only planar cycloalkane • bent C-C bonds: sp3 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

University of Slide 34 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Conformations of Cyclobutane

Nonplanar “puckered” Conformation

• torsional strain reduced in puckered conformation • less angle strain than cyclopropane

University of Slide 35 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Conformations of Cyclopentane

Planar Envelope Half-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

University of Slide 36 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4 Self Test Question

Which of the following conformations of cyclohexane would you expect to have the highest heat of combustion?

a. c. A. a B. b lowest NRG: highest NRG: all gauche & anti b. 4 eclipsed C. c relationships relationships

University of Slide 37 CHEM 494, Fall 2012 Illinois at Chicago UIC Lecture 4