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°)

• bond length
• bond angle
• dihedral angle
• van der Waals strain

Types of strain:

• torsional strain (staggered vs. eclipsed) - dihedral angle effects
• nonbonded strain (gauche vs. anti) - van der Waals effects
• angle strain (small rings) - bond angle effects

Ring strain

Small cycloalkane conformations

• 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.

All substituents prefer equatorial over axial