STRUCTURE, CONTINUED
Dr. Clower
CHEM 2411
Spring 2014
McMurry (8th ed.) sections 3.5-3.7, 4.3-4.9, 7.2, 7.6
Topics
• Conformations of Alkanes and Cycloalkanes
• Unsaturation
• Alkene Stability
Molecular Model Kits
• How to use
• Make a model for ethane
• Make a model for butane
• Make a model for cyclohexane
• Use 6 white hydrogens and 6 green hydrogens
• Put 1 green and 1 white hydrogen on each carbon atom
• The green and white hydrogen atoms should alternate (so as you
look at the molecule from the top the H’s should alternate greenwhite-green-white-green-white around the ring)
Alkane Three-dimensional Structure
• Methane:
• With 2 or more carbons, 3D arrangement can change due
to C─C bond rotation
• Conformations
• Same molecular formula
• Same atom connectivity
• Different 3D arrangement due to rotation around single bond
• Ethane:
Newman Projections
• Used to better visualize conformations
• View the C─C from the end (look down the C─C bond)
• Represent the C atoms as a dot (front carbon) and circle
(back carbon)
• Show bonds coming out of the circle and dot
• Example:
Ethane Conformations
• Staggered vs. eclipsed
• Staggered is more stable (lower E) due to maximum
separation of electron pairs in covalent bonds
• Eclipsed is less stable (higher E) due to electron repulsions
Dihedral Angle
• The degree of rotation between C-H bonds on the front
and back carbons
• Torsional strain
• Accounts for energy difference between eclipsed and staggered
• Barrier to rotation
• Caused by electron repulsion
• Overcome by collisions of molecules
Butane Conformations
• Look down C2─C3 bond to draw Newman projections
• Each C has 2 H atoms and 1 CH3 group
• Dihedral angle is angle between CH3 groups
• There are six conformations of butane:
• How many staggered conformations? How many eclipsed?
Strain in Butane Conformations
• Torsional strain
• Barrier to rotation
• Example: eclipsed vs. staggered conformations
• Steric strain
• Repulsive interaction when atoms are forced close together
(occupy the same space)
• Example: CH3-H eclipsed vs. CH3-CH3 eclipsed conformations
• Example: Anti vs. gauche conformations
• So, which conformation is lowest in E? Highest in E?
Butane Conformations
Cycloalkane Three-dimensional Structure
• C atoms in cycloalkanes are sp3
• Bond angles are not always 109.5º
• Bond angles are dictated by the number of atoms in the ring
• Angle strain = Forcing angles smaller or larger than 109.5º
• Cycloalkanes can also have torsional strain (eclipsed H’s)
Strain in Cycloalkanes
Cycloalkane Conformations
• Cycloalkanes adopt more stable conformations to relieve
strain
• Cyclopropane
• “Bent” bonds
Cycloalkane Conformations
• Cyclobutane
• Puckered conformation
• Cyclopentane
• Envelope conformation
Cyclohexane
• Most stable cycloalkane
• Most abundant in nature
• No angle strain (109.5º)
• No torsional strain (all H’s staggered)
• Conformation = chair
Cyclohexane
• Axial and equatorial hydrogens
• Axial = parallel to axis through ring
• Equatorial = perpendicular to axis
• Each C has one axial H and one equatorial H
• Look at molecular model
Cyclohexane
Ring Flip
• Interconversion of two chair conformations
• Try this with your molecular model
• If no substituents, these conformations are equal in energy
Monosubstituted Cyclohexanes
• Two conformations
1. Substituent in axial position
2. Substituent in equatorial position
• These conformations are not equal in energy
• Example: methylcyclohexane
Steric strain =
1,3-diaxial interactions
Larger groups have
more steric strain
Disubstituted Cyclohexanes
• The most stable conformation has the most substituents
in the equatorial position
• Conformational analysis
• Look at all chair conformations (cis and trans) and analyze stability
• Example: 1,4-dimethylcyclohexane
Additional Cyclohexane Conformations
• Boat
• No angle strain
• High torsional strain
• High steric strain
• Very unstable
• Twist-boat
• Relieves some torsional and steric strain
• No angle strain
• Lower E than boat
• Higher E than chair
Conformations of Polycyclic Molecules
• Fused rings
• Typically adopt chair conformations
• Norbornane and derivatives locked in boat conformation
Degree of Unsaturation
• Unsaturated compounds
• Have less than (2n+2) H atoms for (n) C atoms
• Contain elements of unsaturation
• p bonds
• Rings
• Calculating degree of unsaturation
• Index of Hydrogen Deficiency (IHD)
• IHD = C - ½ (H + X) + ½ (N) + 1
• Ex: C6H14
IHD = 6 - ½(14) + 1 = 0
Alkane
• Ex: C6H12
IHD = 6 - ½(12) + 1 = 1
1 p bond or 1 ring
• Ex: C6H10
IHD = 6 - ½(10) + 1 = 2
2 p bonds, 2 rings, or 1 of each
C6H14
C6H12
C6H10
Alkene Stability
• Which alkene is more stable, cis or trans?
• Cis has steric strain between R groups
Alkene Stability
• Stability determined by heats of hydrogenation
• Heat of reaction for addition of H2 (with metal catalyst) to alkene
• Heat of reaction is proportional to energy of alkene
• Smaller magnitude DH = more stable alkene
Alkene Stability
• Trends in alkene stability
• Trans is more stable than cis
• More substituted C=C is more stable
• Why?
• Hyperconjugation
• Stabilizing effect of adjacent orbital overlap
• Bond strengths
• sp2-sp3 bond more stable than sp3-sp3