Cycloalkanes | Organic Chemistry 1

A cycloalkane is a saturated hydrocarbon that contains at least one ring in its carbon backbone

The general formula for one-ring cycloalkanes is C_nH_2n, the formula for two-ring compounds is C_nH_2n-2, and so on

Cycloalkanes are named using rules similar to those for alkanes, but the prefix cyclo- precedes the name of the parent chain

1. Find the ring with the highest number of carbons: this is the parent chain
2. Add the prefix cyclo- and the suffix -ane to the name of the corresponding linear parent chain
3. Name and number the substituents
   - monosubstituted systems: no number is needed to indicate the location of the substituent
   - polysubstituted systems: start the numbering at one substituent and choose the lowest possible numbering sequence. With 2 different substituents, number the ring to assign the lowest number to the first substituent in alphabetical order
   - bicyclic compound: treat the smaller ring as a substituent of the larger one

If the number of carbons in the ring is greater than or equal to the number of carbons in the longest chain, the compound is named as a cycloalkane. If it is less than the number of carbons in the longest chain, the compound is named as an alkane with a cycloalkyl substituent
 

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

Structural isomers are chemical species that have the same molecular formula but differ in the way the atoms are bonded together, while conformational isomers are isomers that have the same molecular formula, the same bonds but differ in the way the atoms are oriented in space (different 3D geometry)

A Newman projection is a representation of the conformation of a molecule. It shows the 3 groups attached to the 2 carbon atoms of a particular C-C bond and the dihedral angle between them
 

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

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 in 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 at the end of each bond

The staggered and eclipsed conformations of a molecule generally interconvert at room temperature, but each conformation, and thus its corresponding Newman projection, is not equally stable: the staggered conformations are more stable (lower in energy) than the eclipsed conformations

Ring strain is an instability that exists when the bonds in a molecule form abnormal angles, the ideal bond angle of sp³ hybridized carbon being 109.5^o. It affects the structures of the smaller cycloalkanes and results in an increase in the potential energy of these species. Ring strain = torsional strain + steric strain + angle strain

The smaller cycloalkanes, cyclopropane and cyclobutane, are the most strained because they have particularly high ring strains due to the deviation of their bond angles from 109.5° and the eclipse of their hydrogens

Ring strain = torsional strain + steric strain + angle strain

• ​​​​​Torsional strain: increase in energy caused by eclipsing interactions
• Steric strain: increase in energy produced when atoms are forced to be too close to each other and try to occupy the same space
• Angle strain: increase in energy produced when the tetrahedral bond angles deviate from 109.5° (sp³ hybridized atoms). For example, cyclopropane has bond angles equal to 60°

A chair conformation is 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

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

The most stable conformation of a monosubstituted cyclohexane is the one with the substituent in equatorial position: an axial substituent creates unfavorable 1,3-diaxial interactions, destabilizing one of the chair conformations
 

The conformation in which both substituents are equatorial will always be more stable than a conformation with both groups axial. If the 2 substituents are one axial and the other equatorial, the most stable conformation 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)