Lecture 3 - Cycloalkanes and their Stereochemistry Rev CME 2024

Lecture 3: Cycloalkanes and their Stereochemistry

Naming Cycloalkanes

  • Cycloalkanes or alicyclic compounds: Saturated cyclic hydrocarbons

  • Can be represented using skeletal drawings

  • Follow general rule of naming structures, however, use “cyclo” at the start of the suffix.

Cis-Trans Isomerism in Cycloalkanes

  • Cycloalkanes are less flexible than open-chain alkanes.

  • Significantly lesser conformational freedom in cycloalkanes

  • Cycloalkanes have two faces, when viewed edge-on, owing to their structure (top face and bottom face)

  • Isomerism is possible in substituted cycloalkanes

  • Stereoisomerism: Compounds which have their atoms connected in the same order but differ in 3-D orientation

  • Stereochemistry: Term used to refer to the 3-D aspects of chemical structure and reactivity.

  • Cis-trans isomers: Stereoisomers that differ in their stereochemistry about a ring or double bond.

Stability of Cycloalkanes: Ring Strain

  • Angle Strain:

    • Induced in a molecule when bond angles are forced to deviate from the ideal 190° tetrahedral value.

  • Cyclic molecules can assume nonplanar conformations to minimize angle strain and torsional strain by ring-puckering

  • Torsional Strain

    • Caused due to eclipsing of bonds between neighboring atoms

  • Steric Strain

    • Caused due to repulsive interactions when atoms approach each other too closely

  • Larger rings have many more possible conformations than smaller rings; more difficult to analyze.

Conformations

Cyclopropane

  • Most strained of all rings due to angle strain caused by its C-C-C bond angles of 60°.

  • Has considerable torsional strain.

  • Has bent bonds.

  • C-H bonds are eclipsed.

  • Weaker and more reactive than typical alkane bonds.

  • Cyclopropane

Cyclobutane

  • Has less angle strain than cyclopropane

  • More torsional strain because of larger number of ring hydrogens, and their proximity to each other.

  • Slightly bent out of plane, one carbon atom is about 25° above the plane.

  • Increases angle strain but decreases torsional strain.

  • Cyclobutane

Cyclopentane

  • No angle strain

  • Large torsional strain

  • Non planar conformation strike balance between increased angle strain and decreased torsional strain

  • Four carbon atoms are approximately in the same plane

  • Fifth carbon atom is bent out of the plane

  • Cyclopentane

Cyclohexane

  • Cyclic version of hexane, an alicyclic hydrocarbon with a ring of six carbon atoms

  • a colorless, mobile liquid with a mild, sweet odor

  • slightly soluble in water and soluble in alcohol, acetone, benzene, ethanol, ethyl ether, olive oil, and carbon tetrachloride; performs the function of a non-polar solvent

  • commonly used as a chemical intermediate such as utilized as a raw material in the creation of nylon

  • Melting point: 6.47 C

  • Boiling point: 80.7 C

  • Specific gravity: 0.779

  • Adopts chair conformation: Strain-free, 3-D shape

  • Has neither angle strain nor torsional strain

  • Boat cyclohexane

    • Alternate conformation of cyclohexane that bears a slight resemblance to a boat

    • No angle strain

    • Large number of eclipsing interactions

  • Twist-boat conformation

    • Alternate conformation of cyclohexane that is somewhat more stable than a pure boat conformation

    • Nearly free of angle strain

Axial and Equatorial Bonds

Cyclohexane

  • Chair conformation positions for substituents on the ring

    • Axial positions

    • Equatorial positions

  • Chair cyclohexane has six:

    • Axial hydrogens perpendicular to the ring

    • Equatorial hydrogens near the plane of the ring

  • Each carbon atom has one axial and one equatorial hydrogen

  • Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement.

  • Conformational Mobility of Cyclohexane

    • Ring-flip

      • Interconversion of chain conformations, resulting in the exchange of axial and equatorial positions

  • Conformations of Disubstituted Cyclohexane

    • Steric effects of both substituents is taken into account in both conformations

    • Isomers of 1,2-dimethylcyclohexane

      • Cis-1,2-dimethylcyclohexane

      • Trans-1,2-dimethylcyclohexane

    • Cis isomer

      • Both methyl groups are on the same face of the ring

      • Compound can exist in two chair conformations

    • Trans isomer

      • Methyl groups are on opposite faces of the ring

      • One trans conformation has both methyl groups equatorial and only a gauche butane interaction between methyl groups.

      • Ring-flipped conformation has both methyl groups axial with four 1,3-diaxial interactions

    • Will exist almost exclusively in di-equatorial conformation

1,3-Diaxial Interactions

  • Causes Steric Strain

    • difference between axial and equatorial conformers

  • Hydrogen atoms of the axial methyl group on C1 are too close to the axial hydrogen three carbons away on C3 and C5

  • Results in 7.6 kJ/mol of steric strain

Relationship to Gauche Butane Interactions

  • Gauche butane is less stable than anti butane by 3.8 kJ/mol due to steric interference between hydrogen atoms on the two methyl groups

  • Four-carbon fragment of axial methylcyclohexane and gauche butane have the same steric interaction

  • Equatorial methylcyclohexane has no such interactions

  • More stable