Unit 9

Conformations

Different arrangements of atoms are a result of rotating about carbon-carbon single bonds (no change in atom connectivities)

Newman Projection

A representation of one conformation that looks directly down a bond to show the spatial arrangement of bonds on two adjacent atoms

Dihedral Angle

The angle created by the outer two bonds over three consecutive bonds

Temperature Dependence

The rotation of sigma bonds is temperature dependent

Increasing Size Effect on Rotation of Sigma Bonds

• Increases the energy barriers between the low and high energy conformations

• Bulky groups hinder rotation

Decreasing Temperature Effect on the Rotation of Sigma Bonds

Makes rotation about sigma bonds more difficult

Which term best describes the relationship between the pair of structures shown below?

Identical, but in different conformation

Cycloalkanes

• Hydrocarbons that contain one or more rings

• With the exception of cyclopropane, the carbons in these rings are not coplanar

Angle Strain

Strain due to deviations from ideal bond angles

Cis Substituents

Are on the same side

Trans Substituents

Are on opposite sides

Identify the whether the two fluorine atoms are cis or trans to each other:

trans

Identify the whether the two fluorine atoms are cis or trans to each other:

trans

Identify the whether the two fluorine atoms are cis or trans to each other:

cis

Substituted Cyclohexanes

• There is no energy difference between the two chair conformations of the unsubstituted cyclohexane

• For mono substituted cyclohexanes, there is an energy difference between the two chair conformations due to steric interactions

Which of the two chair conformations is favored at equilibrium?

A

Which of the following conformations is favored at equilibrium?

The left

How are these two structures related?

They are the same compound