FLO 5 Newman Projections and Chairs

Question 1

ring strain

Answer 1

strain on cycloalkanes, composed of:
- bond angle strain (not 109)
- torsional strain from repulsions

Question 2

3D structure of cyclopropane + strain

Answer 2

essentially planar
- angle strain: C-C bonds are bent (“banana bonds”)–weaker bonds and cause some polarity
- torsional strain: all C-H bonds are eclipsing

Question 3

cyclopropane stereoisomers example

Answer 3

• dimethyl cyclopropane has cis and trans stereoisomers
• cis: CH3 groups on same side; torsional and steric strain
• trans: one CH3 group flipped to opposite side; only torsional strain

Question 4

3D structure of cyclobutane + strain

Answer 4

puckered
- some angle strain (angles slightly greater)
- reduces torsional strain (no eclipsing)

Question 5

3D structure of cyclopentane + strain

Answer 5

envelope or half-chair: one corner raised
- adds some angle strain compared to planar
- way less torsional strain than planar

Question 6

3D structure of cyclohexane + strain

Answer 6

chair conformation
- almost no angle strain
- no torsional strain
- includes 6 axial bonds and 6 equatorial bonds

Question 7

chair flips in cyclohexane

Answer 7

• axial substituents become equatorial (different conformation)

Question 8

strain involved when there’s cyclohexane substituents

Answer 8

• axial substituents have 1,3 diaxial interactions (steric strain from gauche interaction)
• as R groups get bigger, equatorial is more favored
• however, bigger atoms (e.g. Iodine) don’t have much steric strain because bonds are longer!

Question 9

favorability of different 1,2-dimethyl-cyclohexanes

Answer 9

• most fav: trans equatorial (1 gauche between them)
• middle: cis (2 diaxial interactions for axial methyl and 1 with equatorial)
• least fav: trans axial (2 diaxial interactions for each methyl)