STRUCTURE, CONTINUED Dr. Clower CHEM 2411 Spring 2014 McMurry (8 th ed.) sections , , 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:

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 CH 3 group Dihedral angle is angle between CH 3 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: CH 3 -H eclipsed vs. CH 3 -CH 3 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 sp 3 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  bonds Rings Calculating degree of unsaturation Index of Hydrogen Deficiency (IHD) IHD = C - ½ (H + X) + ½ (N) + 1 Ex: C 6 H 14 IHD = 6 - ½(14) + 1 = 0 Alkane Ex: C 6 H 12 IHD = 6 - ½(12) + 1 = 1 1  bond or 1 ring Ex: C 6 H 10 IHD = 6 - ½(10) + 1 = 2 2  bonds, 2 rings, or 1 of each C 6 H 14 C 6 H 12 C 6 H 10

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 H 2 (with metal catalyst) to alkene Heat of reaction is proportional to energy of alkene Smaller magnitude  H = 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 sp 2 -sp 3 bond more stable than sp 3 -sp 3