Cycloalkanes | Organic Chemistry 1

Cycloalkanes are studied in this chapter: nomenclature of cycloalkanes, types of strain, conformers, chair conformations (strain free conformations of cyclohexane), substituted cyclohexanes

Nomenclature of Cycloalkanes

Cycloalkanes: 

Saturated hydrocarbons arranged in a structure containing carbons joined in one or more rings. The general formula of cycloalkanes with one ring is CnH2n
 

How to name a cycloalkane:

Cycloalkanes are named by using similar rules than alkanes, but the prefix cyclo- precedes the name of the parent chain

  1. Find the parent cycloalkane

  2. Add the prefix cyclo- and the suffix -ane to the parent name

  3. Name and number the substituents
    - monosubstituted systems: no number needed to indicate the location of the substituent
    - polysubstituted systems: begin numbering at one substituent and choose the lowest possible numbering sequence. With 2 different substituents, number the ring to assign the smallest number to the substituents in alphabetical order
    - bicyclic compound: treat the smaller ring as a substituent to the larger one

 

Conformers

Conformers (or conformational isomers):

Different arrangements of atoms of the same molecule resulting from a rotation around a single bond. Eclipsed conformation is the conformation where the bonds on one carbon are directly aligned with the bond on adjacent carbon. Staggered conformation is the conformation in which the bonds on one carbon bisect the R-C-R bond on the adjacent carbon. The staggered conformations are more stable than the eclipsed conformations


Newman projection:

A representation of the conformation of a molecule. It shows the 3 groups bonded to each carbon atom in a particular C-C bond and the dihedral angle that separates them
 

How to draw a Newman projection:

  1. Choose a particular C-C bond

  2. Draw a circle with a dot in the center
    - the circle represents the back carbon
    - the dot represents the front carbon

  3. Identify the 3 groups connected to the front carbon atom
    Draw the bonds as 3 lines meeting at the center of the circle

  4. Identify the 3 groups connected to the back carbon atom
    Draw the bonds as 3 lines coming out the edge of the circle

  5. Add the atoms on each bond

 

Newman projection of the eclipsed and the staggered conformers of 1,2-dichloroethane resulting from a rotation around the C-C single bond

 

Types of Strain

Ring strain:

Instability that exists when bonds in a molecule form abnormal angles. It affects the structures of the smallest cycloalkanes. Ring strain = torsional strain + steric strain + angle strain
 

  • ​​​​​Torsional strain: an increase in energy caused by eclipsing interactions
  • Steric strain: an increase in energy produced when atoms are forced to be too close to each other and try to occupy the same space
  • Angle strain: an increase in energy produced when tetrahedral bond angles deviate from 109.5° (sphybridized atoms). For example, cyclopropane has bond angles equal to 60°

Conformations of Cyclohexane

Chair conformation:

A stable conformation adopted by cyclohexane. Its stability results from the elimination of angle strain (all bond angles are 109.5°) and torsional strain (all groups on adjacent carbon atoms are staggered, not eclipsed). There are 2 chair conformations in equilibrium at room temperature that are formed by a ring-flipping process

Axial and equatorial positions:

In this conformation, each carbon atoms has one axial and one equatorial position. Axial substituents are located above and below the ring along a perpendicular axis, while equatorial substituents are located in the plane of the ring around the equator. Axial and equatorial positions are interconverted during a ring flip
 

Substituted Cyclohexanes

Monosubstituted cyclohexane:

The more stable conformer is the conformer with the substituent in equatorial position: larger axial substituent creates unfavorable 1,3-diaxial interactions, destabilizing one of the chair conformations
 

 

​​​Disubstituted cyclohexane:

The more stable conformer is the conformer 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)