Chapter 3 Notes: Conformations of Alkanes and Cycloalkanes
Conformations
- different 3D structures of the same molecule, differing
only by rotations about single bonds
Ethane Conformations
C-H bonds on the two carbons may or may not align
eclipsed: C-H bonds are aligned
staggered: C-H bonds fit in between
Newman Projections
- view down a C-C bond to visualize orientations
- staggered conformation (more stable)
Potential Energy Diagrams
- view energy changes as the C-C bond rotates
Butane conformations
- visualize rotation about the C2 - C3 bond
- monitor the relative positions of the two terminal methyl
groups
- gauche - dihedral angle ~60°
- anti - dihedral angle ~180°
- identify highest energy eclipsed conformation (dihedral
angle 0°)
Types of strain:
- torsional strain (staggered vs. eclipsed)
- nonbonded strain (gauche vs. anti)
- angle strain (small rings)
Small cycloalkanes
- cyclopropane - planar
- cyclobutane - slightly puckered
- cyclopentane - envelope or half-chair conformations
Cyclohexane
- chair conformations
- boat and twist-boat conformations
- potential energy diagram
Monosubstituted cyclohexanes
- axial and equatorial substituent locations
PRACTICE DRAWING ACCURATE CHAIR FORMS WITH SUBSTITUENTS
- relative stabilities of substituted cyclohexanes
equatorial generally preferred over axial
see the table in the text with delta G for axial vs equatorial
Disubstituted cyclohexanes - cis & trans isomerism
- numbered as 1,n
- cis - both substituents on the same side of the ring
- trans - substituents on opposite sides of the ring
- practice locating substituents in all the various combinations:
- cis-1,2-
- trans-1,2-
- cis-1,3-
- trans-1,3-
- cis-1,4-
- trans-1,4-
Depending on the substitution pattern, disubstituted cyclohexanes
have different combinations of axial and equatorial substituents.
substitution |
cis |
trans |
1,2- |
e,a <==>
a,e |
e,e <==>
a,a |
1,3- |
e,e <==>
a,a |
e,a <==>
a,e |
1,4- |
e,a <==>
a,e |
e,e <==>
a,a |
Example - the two chair conformations for cis-1-chloro-3-methylcyclohexane
involve either two axial substituents or two equatorial substituents.
The latter is more stable.