Cycloalkanes | Organic Chemistry 1

Cycloalkanes are studied in this chapter: introduction to stereoisomerism, ring strain, chair conformations (strain free conformations of cyclohexane), substituted cyclohexanes.

Names of Cycloalkanes

Naming: alkane names preceded by the prefix cyclo-.
Monosubstituted systems: the carbon with the substituent = carbon 1 of the ring (do not name it).
Polysubstituted systems: provide the lowest possible numbering sequence + alphabetical order of the substituent names (if 2 possible sequences).
In two cycloalkyl systems: treat the smaller unit as a substituent to the larger one.

 

Stereoisomers

Stereoisomers = molecules with same molecular formula, same connections but different 3D geometry.
We use hashed-wedged line structures to depict the 3D arrangement.

When 2 subst. on different carbons are on the same face: cis-isomer.
When 2 subst. on different carbons are on the opposite faces: trans-isomer.

 

2 chloro substituents are on the same face:
cis-1,2-dichlorocyclohexane

 

2 chloro substituents are on the opposite faces:
​​​​​trans-1,2-dichlorocyclohexane

 

Ring Strain

Ring strain = eclipsing (torsional) strain + bond-angle strain + angle strain.

Torsional strain is due to eclipsing of bonds on neighboring atoms.
Steric strain arises when two atoms try to occupy the same space.
Angle strain is due to bond angle deformation (free sp3 orbitals: 120° vs. cyclopropane: 60°).

Strain affects the structures of the smaller cycloalkanes.

Cyclohexane: a Strain Free Molecule

Cyclohexane is free of bond-angle or eclipsing strain.
This molecule is not planar and adopts several conformations.
The most stable is called chair conformation and the ring can flip to form 2 chairs.
In this conformation, each carbon atoms has an equatorial position and an axial position.

Substituted Cyclohexanes

Monosubstituted Cyclohexane:
The more stable conformer is the conformer with the substituent in the equatorial position.


 

Disubstituted Cyclohexane:
If there is more than one substituent, the more stable conformer is the one with the bulkier substituent in the equatorial position.

 

CH3 is bulkier than F, the more stable conformer is the second one (CH3 in the equatorial position).