2'5 Conformations of Alkanes and Cycloalkanes 2'6 Isomerism in Cycloalkanes

1
2.5 Conformations of Alkanesand
Cycloalkanes2.6 Isomerism in Cycloalkanes
2
2.5 Key Terms

• Strain
• Conformation
• Dihedral angle
• Staggered vs. Eclipsed

3
2.5A Alkanes
Consider the 3D structure of the
molecule Alkanes can be twisted into many 3D
arrangements. Conformation A 3D arrangement
resulting from rotation about single bonds
4
Ethane
5
Newman Projections
6
Ethane Rotomers
7
Why is staggered more stable?

• Possibilities
• Eclipsed leads to H-H nuclear repulsion
• Repulsion between C-H bond electron clouds
• Real Answer
• Staggered is stabilized by orbital interactions
• Interaction is between filled C-H bonding MO and
  empty C-H antibonding MO.
• Interaction is only possible in staggered form

8
Propane Rotomers
9
Butane Gauche and Anti
10
Butane Rotomers
11
Cyclopropane
Bond Angles are 60 A lot of Angle Strain All
Hs are eclipsed
12
Cyclobutane
Planar Conformation would make Hs
eclipsed Favored form is the puckered
ring Molecule alternates between two puckered
structures.
13
Cyclopentane
Planar structure would lead to eclipsed
Hs Envelope structure is favored
14
Cyclopentane
15
Cyclohexane
Chair Conformer has no strain
16
Cyclohexane Newman Projections
17
Cyclohexane Boat Conformer
18
(No Transcript)
19
(No Transcript)
20
Diaxial Interactions
21
(No Transcript)
22
(No Transcript)