STRUCTURE, CONTINUEDDr. ClowerCHEM 2411Spring 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 green-white-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:

C

H

HHH

C C

H

H HH

C C

H H

HHH H

HH

Two conformations of 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:

C C

H

H HH

HH

LOOK C C

H H

HHH H

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 position2. 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 CH3CH3

CH3

H

CH3

CH3

H

H

CH3

H

cis

CH3

CH3

H

H

CH3

H

CH3

H

trans

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

CH3

H

H

H

H

H

H

CH3

H

H

H

H

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

CH3

HH

CH3

H

CH3H

CH3

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