ORGANIC CHEMISTRY - CHAPTER 4: CYCLOALKANES

IMPORTANT CONCEPTS

1. Cycloalkane nomenclature is derived from that of the straight-chain alkanes.

2. All but the 1,1-disubstituted cycloalkanes exist as two isomers: If both substituents are on the same face of the molecule, they are cis; if they are on opposite faces, they are trans. Cis and trans isomers are stereoisomers—compounds that have identical connectivities but differ in the arrangement of their atoms in space.

3. Some cycloalkanes are strained. Distortion of the bonds about tetrahedral carbon introduces bond-angle strain. Eclipsing (torsional) strain results from the inability of a structure to adopt staggered conformations about C – C bonds. Steric repulsion between atoms across a ring leads to transannular strain.

4. Bond-angle strain in the small cycloalkanes is largely accommodated by the formation of bent bonds.

5. Bond-angle, eclipsing, and other strain in the cycloalkanes larger than cyclopropane (which is by necessity flat) can be accommodated by deviations from planarity.

6. Ring strain in the small cycloalkanes gives rise to reactions that result in opening of the ring.

7. Deviations from planarity lead to conformationally mobile structures, such as chair, boat, and twist-boat cyclohexane. Chair cyclohexane is almost strain free.

8. Chair cyclohexane contains two types of hydrogens: axial and equatorial. These interconvert rapidly at room temperature by a conformational chair–chair (“flip”) interconversion, with an activation energy of 10.8 kcal mol^-1 (45.2 kJ mol^-1).

9. In monosubstituted cyclohexanes, the ΔG^o of equilibration between the two chair conformations is substituent dependent. Axial substituents are exposed to 1,3-diaxial interactions.

10. In more highly substituted cyclohexanes, substituent effects are often additive, the bulkiest substituents being the most likely to be equatorial.

11. Completely strain-free cycloalkanes are those that can readily adopt an all-anticonformation and lack transannular interactions.

12. Bicyclic ring systems may be fused or bridged. Fusion can be cis or trans.

13. Natural products are generally classified according to structure, physiological activity, taxonomy, and biochemical origin. Examples of the last class are the terpenes, of the first the steroids.

14. Terpenes are made up of isoprene units of five carbons.

15. Steroids contain three angularly fused cyclohexanes (A, B, C rings) attached to the cyclopentane D ring. Beta substituents are above the molecular plane, alpha substituents below.

16. An important class of steroids are the sex hormones, which have a number of physiological functions, including the control of fertility.

From "Organic Chemistry" Textbook of VOLLHARDT & SCHORE