Sigma bonds are quite 'twistable'.

A-cyclic compounds have many different structural orientations.

Ethane

If you look at the molecule end on;

• if the hydrogen is lined up then the conformation is called eclipsed. 3 possible eclipsed conformations (each hydrogen lined up behind each other hydrogen).
• if the hydrogen is at 60 then it is called staggered conformation.
• the torsion angle is the angle at which the back hydrogen is set off from the 'top front' hydrogen
  • if you graph this (kcal/mole vs angle) then you end up with a sine wave with 60,180 and 300 degrees being 0 kcal and 0,120,240 being 3 kcal/mole.

torsional repulsion = mutual repulsion between sigma bond and electron

the ratchet motion occurs thousands of times per second at normal room temperature; if it is cooled to -100 C then the ratcheting will stop

1,2-dichloro ethane

Steric view

eclipse view

staggered view

• gauche: chlorines next to each other
• anti: chlorines opposite from each other
• 
  

energy diagram has many different states, depending on the orientation of the molecule. The anti has the lowest configuration; the gauche has the highest (due to the bumping of the Cl atoms, called steric interaction-the atom has to bend in order to get those alligned).

An important skill we need to develop is to take any molecule, pick any bond, draw Newmann projection and guess what the bond energies (spin) are.

The smallest ring is a cyclopropane.

• it is eclipsed, and is such a tight ring that it won't bend. It is completely flat
• cyclobutane can't bend either, but is slightly non-flat
• cyclopentane is almost flat.

• cyclohexane has quite a bit different than, say hexane (which is like a snake and has many different conformations). Cyclohexane only has 2 conformations: chair conformations ~gauche
  • cyclohexane is a puckered molecule; it is like a chair.
  • has zxial bonds which are vertical and are parrallel with each other.
  • equatorial bonds point out to the side.
  • ring flipping: every axial goes into an equatorial and every equatorial goes into an axial.
  • if you have a methyl group it is less stable in the 1,3 diaxial (steric) interaction (methyl interacts across the top of the atom with the opposing hydrogens) than the
  • 7,8,9 are quite 'floppy'. They have lots of different conformations. We won't consider these in any detail.

May have to guess the more stable chair of substate interactions in cyclohexane

1. draw 2 chairs
2. count 1,3 diaxial interactions
3. don't need to memorize actual bond energies.