Cycloalkanes

Countless organic compounds are known in which a sequence of carbon atoms, rather than being connected in a chain, closes to form a ring. Saturated hydrocarbons that contain one ring are referred to as cycloalkanes. With a general formula of C_nH_2n (n is an integer greater than 2), they have two fewer hydrogen atoms than an alkane with the same number of carbon atoms. Cyclopropane (C₃H₆) is the smallest cycloalkane, whereas cyclohexane (C₆H₁₂) is the most studied, best understood, and most important. It is customary to represent cycloalkane rings as polygons, with the understanding that each corner corresponds to a carbon atom to which is attached the requisite number of hydrogen atoms to bring its total number of bonds to four.

In naming cycloalkanes, alkyl groups attached to the ring are indicated explicitly and listed in alphabetical order, and the ring is numbered so as to give the lowest locant to the first-appearing substituent. If two different directions yield equivalent locants, the direction is chosen that gives the lower number to the substituent appearing first in the name.

The three carbon atoms of cyclopropane define the corners of an equilateral triangle, a geometry that requires the C―C―C angles to be 60°. This 60° angle is much smaller than the normal tetrahedral bond angle of 109.5° and imposes considerable strain (called angle strain) on cyclopropane. Cyclopropane is further destabilized by the torsional strain that results from having three eclipsed C―H bonds above the plane of the ring and three below.

Cyclopropane is the only cycloalkane that is planar. Cyclobutane (C₄H₈) and higher cycloalkanes adopt nonplanar conformations in order to minimize the eclipsing of bonds on adjacent atoms. The angle strain in cyclobutane is less than in cyclopropane, whereas cyclopentane and higher cycloalkanes are virtually free of angle strain. With the exception of cyclopropane, all cycloalkanes undergo rapid internal motion involving interconversion of nonplanar “puckered” conformations.

Many of the most important principles of conformational analysis have been developed by examining cyclohexane. Three conformations of cyclohexane, designated as chair, boat, and skew (or twist), are essentially free of angle strain. Of these three the chair is the most stable, mainly because it has a staggered arrangement of all its bonds. The boat and skew conformations lack perfect staggering of bonds and are destabilized by torsional strain. The boat conformation is further destabilized by the mutual crowding of hydrogen atoms at carbons one and four. The shape of the boat brings its two “flagpole” hydrogen atoms to within 1.80 angstroms of each other, far closer than the 2.20-angstrom distance at which repulsive forces between hydrogen atoms become significant. At room temperature, 999 of every 1,000 cyclohexane molecules exist in the chair form (the other being skew).

There are two orientations of carbon-hydrogen bonds in the chair conformation of cyclohexane. Six bonds are parallel to a vertical axis passing through the centre of the ring and are called axial (a) bonds. The directions of these six axial bonds alternate up and down from one carbon to the next around the ring; thus, the axial hydrogens at carbons one, three, and five lie on one side of the ring and those at carbons two, four, and six on the other. The remaining six bonds are referred to as equatorial (e) because they lie in a region corresponding to the approximate “equator” of the molecule. The shortest distances between nonbonded atoms are those involving axial hydrogens on the same side of the molecule.

A rapid process of chair-chair interconversion (called ring-flipping) interconverts the six axial and six equatorial hydrogen atoms in cyclohexane. Chair-chair interconversion is a complicated process brought about by successive conformational changes within the molecule. It is different from simple whole-molecule motions, such as spinning and tumbling, and because it is a conformational change only, it does not require any bonds to be broken.

Chair-chair interconversion is especially important in substituted derivatives of cyclohexane. Any substituent is more stable when it occupies an equatorial rather than an axial site on the ring, since equatorial substituents are less crowded than axial ones. In methylcyclohexane, the chair conformation in which the large methyl group is equatorial is the most stable and, therefore, the most populated of all possible conformations. At any instant, almost all the methylcyclohexane molecules in a given sample exist in chair conformations, and about 95 percent of these have the methyl group in an equatorial orientation.

The highly branched tert-butyl group (CH₃)₃C― (tert-butyl) is even more spatially demanding than the methyl group, and more than 99.99 percent of tert-butylcyclohexane molecules adopt chair conformations in which the (CH₃)₃C― group is equatorial.

Conformational analysis of six-membered rings, especially the greater stability of chair conformations with equatorial substituents, not only is important in the area of hydrocarbons but also is essential to an understanding of the properties of biologically important molecules, especially steroids and carbohydrates. Odd Hassel of Norway and Derek H.R. Barton of England shared the Nobel Prize for Chemistry in 1969 for their important discoveries in this area. Hassel’s studies dealt with structure, while Barton showed how conformational effects influence chemical reactivity.

The most stable structures of cycloalkanes and compounds based on them have been determined by a number of experimental techniques, including X-ray diffraction and electron diffraction analyses and infrared, nuclear magnetic resonance, and microwave spectroscopies. These experimental techniques have been joined by advances in computational methods such as molecular mechanics, whereby the total strain energies of various conformations are calculated and compared (see also chemical bonding: Computational approaches to molecular structure). The structure with the lowest total energy is the most stable and corresponds to the best combination of bond distances, bond angles, and conformation. One benefit of such calculations is that unstable conformations, which are difficult to study experimentally, can be examined. The quality of molecular mechanics calculations is such that it is claimed that many structural features of hydrocarbons can be computed more accurately than they can be measured.

The conformations of rings with 7–12 carbons have been special targets for study by molecular mechanics. Unlike cyclohexane, in which one conformation (the chair) is much more stable than any other, cycloalkanes with 7–12 carbons are generally populated by several conformations of similar energy. Rings with more than 12 carbons are sufficiently flexible to adopt conformations that are essentially strain-free.

Polycyclic hydrocarbons are hydrocarbons that contain more than one ring. They are classified as bicyclic, tricyclic, tetracyclic, and so forth, according to the number of formal bond disconnections necessary to produce a noncyclic carbon chain. Examples include trans-decalin and adamantane—both of which are present in small amounts in petroleum—and cubane, a compound synthesized for the purpose of studying the effects of strain on chemical reactivity.