# 12: Cycloalkanes, Cycloalkenes and Cycloalkynes

Many important hydrocarbons, known as cycloalkanes, contain rings of carbon atoms linked together by single bonds. The simple cycloalkanes of formula $$(CH_2)_n$$, make up a particularly important homologous series in which the chemical properties change in a much more dramatic way with increasing n than do those of the acyclic hydrocarbons $$\ce{CH_3(CH_2)}_{n-1} \ce{H}$$. The cycloalkanes with small rings ($$n = 3$$-$$6$$) are of special interest in exhibiting chemical properties intermediate between those of alkanes and alkenes. In this chapter we will show how this behavior can be explained in terms of angle strain and steric hindrance, concepts that have been introduced previously and will be used with increasing frequency as we proceed further. We also discuss the conformations of cycloalkanes, especially cyclohexane, in detail because of their importance to the chemistry of many kinds of naturally occurring organic compounds. Some attention also will be paid to polycyclic compounds, substances with more than one ring, and to cycloalkenes and cycloalkynes.

• 12.1: Nomenclature and Physical Properties of Cycloalkanes
The nomenclature and physical properties of cycloalkanes covers the the IUPAC systems of naming cycloalkanes. The physical properties of cycloalkanes can explain each cycloalkane molecular structure and the relative size from simple propane to multiple carbon containing cycloakane like cyclononane.
• 12.2: Spectroscopic Properties of Cyclohexanes
The spectroscopic properties of cycloalkanes are considerably similar to those of alkanes. We mentioned previously the main features of their infrared spectra, and that their lack of ultraviolet absorption at wavelengths down to 200 nm makes them useful solvents for the determination of ultraviolet spectra of other substances. Some spectoscopic properties of cycloalkanes has been notable in the proton NMR spectra.
• 12.3: Conformations of Cycloalkanes
The equilibria (relative stabilities) and equilibration (rate of interconversion) of the rotational conformations of ethane and butane were discussed previously. If you review this material, it will be clear that forming a ring from a hydrocarbon chain will greatly reduce the number of possible staggered and eclipsed conformations. We will begin our discussion with cyclohexane because of its special importance, proceed to smaller rings, then give a brief exposition of the conformations of the l
• 12.4: Strain in Cycloalkane Rings
Many of the properties of cyclopropane and its derivatives are similar to the properties of alkenes.
• 12.5: Chemical Properties
Strain in small-ring cycloalkanes has a profound influence on their heats of combustion. We reasonably expect that other chemical properties also will be affected. Indeed, like alkenes, cyclopropane and cyclobutane undergo C−C bond cleavage reactions that are not observed for cyclopentane and cyclohexane, or for saturated, open-chain hydrocarbons.
• 12.6: The Larger Cycloalkanes and their Conformations
The Baeyer strain theory suggested that the larger cycloalkanes ring are difficult to synthesize because of angle strain associated with planar rings. We now know that, except for cyclopropane, none of the cycloalkanes have planar carbon rings and that the higher cycloalkanes have normal or nearly normal bond angles.
• 12.7: Cycloalkenes and Cycloalkanes
The C−C=C angle in alkenes normally is about 22°, which is 10° larger than the normal C−C−C angle in cycloalkanes. This means that we would expect about 20° more angle strain in small-ring cycloalkenes than in the cycloalkanes with the same numbers of carbons in the ring. Comparison of the data for cycloalkenes and for cycloalkanes  reveals that this expectation is realized for cyclopropene, but is less conspicuous for cyclobutene and cyclopentene.
• 12.8: Nomenclature of Polycycloalkanes
There are many hydrocarbons and hydrocarbon derivatives with two or more rings having common carbon atoms. Compounds of this type usually are named by attaching the prefix bicyclo to the name of the open-chain hydrocarbon with the same total number of carbon atoms as in the rings.
• 12.9: Conformations of Decalin
The six-membered rings of decalin, like those of cyclohexane, are expected to be most stable in the chair form. However, there are two possible ways in which two chairs can be joined. The ring-junction hydrogens may be either on the same side of the molecule (cis-decalin) or on opposite sides (trans-decalin). When the two rings are joined through two equatorial-type bonds, trans-decalin results, whereas an axial-equatorial union gives cis-decalin.
• 12.E: Cycloalkanes, Cycloalkenes, and Cycloalkynes (Exercises)
These are the homework exercises to accompany Chapter 12 of the Textmap for Basic Principles of Organic Chemistry (Roberts and Caserio).
• 12.10: Strain in Polycyclic Molecules
Knowing the importance of angle and eclipsing strain in the small-ring cycloalkanes, we should expect that these strains would become still more important in reactions.