Cycloalkanes

1. Cycloalkanes • Some two dimensional representations of cycloalkanes cyclopropane cyclobutane cyclopentane cyclohexane

2. Stereoisomers • The main difference between cycloalkanes and noncyclic alkanes is the lack of complete rotation about the carbon to carbon ring bond. This decrease in freedom of rotation about the carbon to carbon bond is most obvious in the small ring cycloalkanes • Rotation about C-C bonds in cycloalkanes is limited by the ring structure • There are two different 1,2-dimethyl-cyclopropane isomers, one with the two methyls on the same side (cis) of the ring and one with the methyls on opposite sides (trans)

3. Stability of Cycloalkanes: The Baeyer Strain Theory • In 1885, Baeyer proposed a theory to explain the apparent lack of cyclic alkanes having certain ring sizes. • More specifically only 5 and 6 membered cycloalkane rings were known but smaller and larger rings could not be prepared. • Baeyer theorized that these could not be prepared because their bond angles would necessarily deviate from the preferred sp3 bond angle of 109.5 degrees. • This deviation would cause such angle strain that the rings would be too unstable to exist.

4. Bayer’s Proposed Bond Angle Strain Baeyer concluded that cyclopropane would be the most strained followed by cyclobutane. Cyclopentane would be strain free while cyclohexane would show a substantial amount of strain energy. Rings larger than cyclohexane would be impossibly strained and not capable of existing.

5. Conformations of some Cycloalkanes • 3-membered ring must have planar structure • Symmetrical with C–C–C bond angles of 60° • Requires that sp3based bonds are bent (and weakened) • All C-H bonds are eclipsed

6. Conformations of Cyclobutane and Cyclopentane • Flat cyclobutane has less angle strain than cyclopropane but much more torsional strain because of its larger number of ring hydrogens • Consequently, cyclobutane is slightly bent out of plane - one carbon atom is about 25° above • The bend increases angle strain but decreases torsional strain

7. Cyclopentane • Planar cyclopentane would have no angle strain but very high torsional strain • Actual conformations of cyclopentane are nonplanar because this reduces the torsional strain somewhat. • Four carbon atoms are in a plane • The 5-carbon atom is above or below the plane – looks like an envelope

8. Conformations of Cyclohexane • Cyclohexane compounds are the most important of all cycloalkanes because of their wide occurrence in nature. • Cyclohexane is a strain free, because of a 3-D conformation that relieves all strain. • The C,C bond angles of this chair confirmation are all 109.5 degrees and the conformation displays no H,H eclipsing strain

9. How to Draw Cyclohexane

10. Axial and Equatorial Bonds in Cyclohexane • The chair conformation has two kinds of positions for substituents on the ring carbons: axialand equatorial positions • Chair cyclohexane has six axial hydrogens perpendicular to the ring (parallel to the ring axis) and six equatorial hydrogens near the plane of the ring. • Each carbon atom in cyclohexane has one axial and one equatorial hydrogen • Each face of the ring has three axial and three equatorial hydrogens in an alternating arrangement

11. Conformational Mobility of Cyclohexane • Chair conformations readily interconvert, resulting in the exchange of axial and equatorial positions by a ring-flip

12. Bromocyclohexane • When bromocyclohexane ring-flips the bromine’s position goes from equatorial to axial and so on

13. Conformations of Monosubstituted Cyclohexanes • The two conformers of a monosubstituted cyclohexane are not equal in energy • The equatorial conformer of methyl cyclohexane is more stable than the axial by 7.6 kJ/mol  CH3 in axial position CH3 in equatorial position is more stable

14. Conformational Analysis of Disubstituted Cyclohexanes • In disubstituted cyclohexanes the steric effects of both sbstituents must be taken into account in both conformations • There are four conformational isomers of dimethyl cyclohexane. There are two cis conformers related by a ring flip and two trans related by a ring flip.The most stable one • The most stable conformer is the one that has the largest group in the equatorial position.