W2.3_Conformation

Question 1

Explain the possible arrangements of ethane. What are the two main ways of presenting them?

Answer 1

• Inter-convertible by rotation about single bonds (staggered and eclipsed conformations)
• Wedge-dash structures and newman projections

Question 2

What are the two main conformaions of ethane? Define dihedral angle and its relation to its conformation and stability.

Answer 2

• θ: torsion/dihedral angle
• θ = 0: eclipsed
• θ ≠ 0: staggered OR eclipsed if same conformation as θ = 0
• 0/360: Syn, eclipsed
• 60/300: Gauche, staggered
• 120/240: Eclipsed
• 180: Anti, staggered
• Staggered is more stable as electrons as more further away to reduce repulsion
• Rotation about C-C bond is very fast -> very low energy barrier
• Ratio of eclipsed to staggered in ethane: 1:119
• Syn: least stable (-Me eclipsing)
• Anti: most stable (-Me furthest away)

Question 3

Where can cyclohexanes be seen? Define and explain the two conformations of cyclohexane.

Answer 3

• Common in steroids/cholesterol/testosterone
• Most stable: chair
• Bond angle = 111
• Axial:6 -H above or below the plane of ring
  (electrons are most furthest away due to parallel bonds)
• Equatorial: 6 -H on plane of ring
• Less stable: boat
  Flagpole hydrogens are too near -> increase repulsion

Question 4

Explain how methylcyclohexanes can be interconverted and which conformation is more stable.

Answer 4

• With methyl groups, two chair conformations can be interconverted by a boat
• Axial destabilised by 1,3-diaxial interactions in methylcyclohexane
• Equatorial conformation is more stable than axial confirmation