Ch04_Cycloalkanes_and_their_stereochemistry

4.1 Naming Cycloalkanes

• Cycloalkanes are saturated cyclic hydrocarbons, having the general formula CnH2n.

• Cyclical nature affects the naming of substituted cycloalkanes similarly to open-chain alkanes.

• Steps for Naming:

  • Step 1: Identify the parent cycloalkane by counting carbon atoms in the ring versus the substituents; name as alkyl- or cycloalkyl-substituted accordingly.

  • Step 2: Number substituents to give the lowest possible numbers, first to substituents, then resolving ties by the lowest numbers for later substituents.

4.2 Cis-Trans Isomerism in Cycloalkanes

• Cycloalkanes exhibit less flexibility compared to open-chain alkanes.

• Cis-Trans Isomerism:

  • Occurs due to the confined space of the ring, which creates two different molecular shapes for isomers like 1,2-dimethylcyclopropane (cis/trans).

• Cis: Both substituents on the same side of the ring.

• Trans: Substituents are on opposite sides.

4.3 Stability of Cycloalkanes: Ring Strain

• Ring Strain arises when bond angles deviate from idealized angles; especially relevant for three-membered (cyclopropane) and four-membered rings (cyclobutane).

• Three types of strain influence stability:

  • Angle Strain: Occurs when angles are compressed or expanded.

  • Torsional Strain: Results from eclipsing interactions.

  • Steric Strain: Occurs when atoms are forced too close to each other.

4.4-4.6 Conformations of Cycloalkanes and Cyclohexane

• Cyclohexane adopts various strain-free conformations, primarily the chair conformation, which has:

  • Both angles near 109 degrees and staggered bonds.

  • Two types of bonds: Axial and Equatorial.

  • Ring Flip: Allows easy interconversion between these positions.

4.7 Conformations of Monosubstituted and Disubstituted Cyclohexanes

• Monosubstituted cyclohexanes prefer equatorial positions for stability due to spatial requirements.

• Disubstituted Cyclohexanes: Stability varies more intricately depending on the positions of both substituents (axial/equatorial interactions).

4.9 Conformations of Polycyclic Molecules

• Decalin: Consists of two fused cyclohexane rings that can exist in either cis- or trans-form, with significant implications for physical properties.

• Norbornane: A bicyclic structure that illustrates steric interactions and stability when compared with simpler cyclic structures like cyclohexane.

Key Terms

• Alicyclic, Angle Strain, Axial Position, Boat Conformation, Cis-Trans Isomers, Cycloalkanes, Ring Flip, Steric Strain, Torsional Strain.

Summary

• Understanding cycloalkanes is crucial in organic and biological chemistry due to their prevalence.

• Principles like ring strain, conformational mobility, and cis/trans relationships influence the stability and behavior of these molecules in various contexts.