CHAPTER 4  Cyclic Alkanes

4-1      Names and Physical Properties of Cycloalkanes

Cyclic hydrocarbons are named as are non-cyclicstructures, with the prefix cyclo- added to the name corresponding to the number of carbon atoms in the ring.

4-2      Ring Strain and the Structure of Cycloalkanes

Constraining atoms within a ring can cause ring strain.

Cyclopropane has 600 angles and eclipsing of all bonds.

Cyclobutane has 900 angles if planar and eclipsing. One atom moves out of the plan of the other three, the decrease in the bond angles is off set by reduced eclipsing.

Cyclopentane if planar would have 1080 angles, close to tetrahedral but with eclipsing of all bonds. One or two atoms move out of the plane of the others, decreasing both the angle and the eclipsing interactions. Cyclopentane and substituted cyclopentanes exist as a mixture of rapidly interconverting conformations that differ little in energy.

4-3      Cyclohexane: A Strain-Free Cycloalkane

If planar, cyclohexane would have bond angles of 1200 and eclipsing of all bonds. Moving two atoms on opposite sides of the ring, one above and one below the plane of the remaining four decreases the bond angle to 109.50 and moves all bonds to staggered.

4-4      Substituted Cyclohexanes

Substituents on a cyclohexane ring can be either equatorial or axial. Axial substituents experience steric interaction with axial hydrogens on the third carbon away. An axial methyl group experiences two 1,3-diaxial interactions very similar to the steric interaction in guache butane.

Disubstituted cyclohexanes are either cis or trans. For 1,2- and 1,4-substitution, diaxial and diequitorial are trans; for 1,3-, diaxial and diequitorial are cis.

4-5      Larger Cycloalkanes

4-6      Polycyclic Alkanes

The number of rings in a polycyclic compound is determined by the number of pairs of hydrogens the formula is missing compared to CnH2n+2.

4-7      Carbocyclic Products in Nature