Chapter 4 - Organic compounds: Cycloalkanes and their stereochemistry

Ring angles

Cyclopropane (triangle): 60 degrees

Cyclobutane (square): 90 degrees

Cyclopentane: 108 degrees

Cyclohexane : 109.5 degrees

Angle strain

angle strain is highest in small rings and decreases as the ring size increases, as larger rings allow the bond angles to come closer to their ideal values.

Torsional strain

The strain due to eclipsing of bonds on neighboring atoms

Steric strain

The strain due to repulsive interactions when atoms approach each other too closely

Which ring size is the most stable and why

six carbons

• Adopts chair conformation (very stable structure in a ring) which means there is no:

  • angle strain (C-C-C bonds ~ 109 degrees)

  • no torsional strain (C-H bonds staggered)

How do you draw a cyclohexane ring?

The bold indicates that the bond is in the front

• This is a cyclohexane

Three types of cyclohexane conformations from most to least stable

1. Chair conformation

2. Twisted boat conformation

3. Boat conformation

Twist-boat conformation

• Stable

• Molecules adopt this geometry only under special circumstances

  • Angle strain-minimal

  • Torsional strain - large

  • Steric strain - large

When is the (twist) boat conformation used?

• As an intermediate during chair-to-chair interconversion in cyclohexane

• It is not a predominant conformation because it is much less stable than a chair conformation.

• Chair → 2. Half-chair → 3. Twist-boat → 4. Boat → 5. Twist-boat → 6. Chair.

What are all the positions for substituents on a ring?

6 axial positions - vertical C-H bond, three on the top three at the bottom

6 equatorial positions - orientation ~30 degrees from plane

Where are substituents more stable in a ring?

In equatorial position, this difference in energy between axial and equatorial conformation is due to steric strain caused by 1,3-diaxal interactions

if question says chair conformation

Draw cyclohexane with bold lines in weird bow shape

What does trans and cis mean

Trans: one substituent goes up axitorially and one goes down. For cis their both the same