Chapter 2 Lecture 2

Chapter 2 - Cycloalkanes and Optical Activity

I. Introduction to Cycloalkanes

  • Focus on cycloalkane structures, their behavior, optical activity, and the resolution of chiral molecules.

II. Cycloalkanes

  • A. Stability of Rings

  • Heat of combustion is an indicator of the stability of cycloalkanes, measured as follows:

    • Cyclopropane > Cyclobutane > Cyclopentane > Cyclohexane

  • As the size of the ring increases, the heat of combustion per -CH2 unit decreases:

    • Cyclopropane: 120 kcal/mole

    • Cyclobutane: 28.7 kcal/mole

    • Cyclopentane: 100 kcal/mole

    • Cyclohexane: 23.9 kcal/mole

III. Combustion Data for Cycloalkanes

  • Data on the heat of combustion for different cycloalkanes and ring strain per CH2 unit:

    • Cyclopropane (n=3): AH(25°) = 468.7 kcal, Ring Strain = 27.6 kcal/mole

    • Cyclobutane (n=4): AH(25°) = 614.3 kcal, Ring Strain = 26.4 kcal/mole

    • Cyclopentane (n=5): AH(25°) = 741.5 kcal, Ring Strain = 6.5 kcal/mole

    • Cyclohexane (n=6): AH(25°) = 882.1 kcal, Ring Strain = 0.0 kcal/mole

IV. Analysis of Cycloalkanes

  • A. Cyclopropane

    1. Bond angle: 60° vs 109.5° (describes angle/ring strain)

    2. All C-H bonds are eclipsed, resulting in torsional strain.

  • B. Cyclobutane

    1. Bond angles of 88° vs 109.5°;

    2. Not planar: slight folding helps distribute strain.

  • C. Cyclopentane

    1. Bond angles are about 108°;

    2. Exists in different conformations (flat, envelope, twist).

    3. Less angle strain compared to a flat structure.

  • D. Cyclohexane

    1. Vital for various biological structures (carbohydrates, steroids).

    2. Chair conformation is the most stable:

      • No angle strain.

      • No eclipsed hydrogen atoms.

    3. Other conformations (half-chair, boat) are less stable.

    4. Planar cyclohexane is the least stable conformation.

V. Chair Conformation of Cyclohexane

  • A. Drawing the Chair Structure

    1. Visual representation of the chair conformation is achieved through specific steps involving parallel lines.

    2. Importance of axial (up/down) vs equatorial (sideways) hydrogens.

  • B. Ring-Flipping

    • Chairs can interchange between equatorial and axial positions for substituents during a ring-flip.

VI. Disubstituted Cyclohexanes

  • A. Relationship between cis-trans Isomers

    • The presence of a double bond or ring prevents rotation, leading to different isomeric forms.

    • A useful table categorizes substituents as cis or trans.

  • B. Stability of Isomers

    • Consideration needs to be made for steric hindrance and stability of each isomer's conformation.

VII. Optical Activity

  • A. Definitions

    • Absolute Configuration: Determined by X-ray crystallography (R/S).

    • Relative Configuration: Observational, based on a standard.

  • B. Racemic Mixtures

    • Result from chiral compounds synthesized from achiral reagents.

    • Example: Reduction of butan-2-one results in both enantiomers (R/S).

VIII. Resolution of Enantiomers

  • A. Importance in Drug Activity

    • Different enantiomers exhibit different biological activities; one may be effective while the other may cause adverse effects.

  • B. Separation Techniques

    1. Make Diastereomers: React a racemic mixture with a pure chiral compound.

    2. Column chromatography using a chiral stationary phase for separation.

IX. Conclusion

  • Understanding the structures, stability, and interactions of cycloalkanes and their optical properties is critical for various applications, especially in drug design and biological systems.