Cycloalkanes | Organic Chemistry 1

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

Naming Cycloalkanes

Cycloalkanes: 

Saturated hydrocarbons arranged in a structure containing a single ring
General formula: CnH2n

 

Naming:

Substituents-Cyclo-Alkane name


Monosubstituted systems: carbon 1 in the ring = carbon with the substituent. Do not mention 1 in the name
Polysubstituted systems: choose the lowest possible numbering sequence. If 2 possible sequences: choose the alphabetical order of the substituents
In 2 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 is an instability that exists when bonds in a molecule form angles that are abnormal
It affects the structures of the smaller cycloalkanes

​​​​​​​Ring strain = eclipsing (torsional) strain + steric strain + angle strain
​​​​​​​Torsional strain is due to the eclipse 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. For example, free sp3 orbitals: 120° vs. cyclopropane: 60°

Cyclohexane: a Strain Free Molecule

Cyclohexane is free of bond-angle and 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 equatorial position
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Disubstituted Cyclohexane:

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

 

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