3.3: Conformational analysis of cyclohexanes

Chair conformations of cyclohexane

The chair conformation is the most stable conformation of cyclohexane, as illustrated in the 3D interactive model in Figure \(\PageIndex{1}\) and depicted in the figure on the right margin. Cyclohexane is six-\(\ce{C}\) cyclic structure. Every ring \(\ce{C}\) has two bonds in addition to the two \(\ce{C-C}\) bonds in the ring. One is pointing perpendicular to the ring along the axis of the ring, called axial, and labeled 'a', and the other is almost along the equator of the ring, labeled 'e' in the figure on the right margin.

Model, top view

Model, edge-on view

Sketch, chair conformation

Model, Newman projection

Sketch, Newman projection

Figure \(\PageIndex{1}\): Models and sketches of a chair conformation of cyclohexane (Copyright; Public domain)

All \(\ce{C-C-C}\) bond angles in cyclohexane are 111\(^\circ\), i.e., very close to tetrahedral value 109.5\(^\circ\), and all six pairs of \(\ce{H's}\) are staggered as shown in Figure \(\PageIndex{9}\). Three alternate axial bonds are on the top face, pointing up, and the other three axial bonds are on the bottom face, pointing down. The \(\ce{C}\) on which an axial bond is pointing up, the equatorial is pointing down, and vice versa.

Drawing chair conformations of cyclohexane

1. Draw two parallel lines slanting up, separated by about half their length, and the top line starting from about the middle of the first as shown in Figure \(\PageIndex{2}\) (a).
2. Start a line from the top end of the top line, making a wide V shape, and extend it to about parallel to the bottom end of the bottom line. Draw a line parallel to it, starting from the bottom end of the bottom line and extending to about the top end of the top line. Figure \(\PageIndex{2}\) shows these two lines in blue.
3. Connect the open ends of the lies of step#2 with the nearest end of the other line of step#1. These are shown in red in Figure \(\PageIndex{2}\). The chair skeleton of the cyclohexane ring is now complete.
4. Draw lines starting from the corners of the ring, going straight upwards from the corners pointing up, and going straight downwards from the corners pointing down. These are axial bonds, shown in green in Figure \(\PageIndex{2}\).
5. Draw lines starting from the corners, making a big V-shape to the axial bond and approximately parallel to one bond away in the ring. These are equatorial bonds, shown in black, blue, and red, corresponding to the color of the bonds one bond away to which they are drawn parallel in Figure \(\PageIndex{2}\).

a)

b)

Figure \(\PageIndex{2}\): Illustration of steps to draw chair conformation of cyclohexane (right) and its flipped form (left). (Copyrights; Public domain)

The flipped form of the chair conformation in Figure \(\PageIndex{2}\) (a) is shown on the Figure \(\PageIndex{2}\) (b). To draw the flipped form, follow the same five steps but start with two parallel lines slanting downwards and replace up with down and vice versa in the instructions, as illustrated in Figure \(\PageIndex{2}\) (b).

Ring flipping in cyclohexane

There is limited rotation around \(\ce{C-C}\) bond in a cyclohexane chain that allows flipping of the cyclohexane chair conformation, converting all the axial bonds into equatorial and all the equatorial bonds to axial as illustrated by animation and drawing in Figure \(\PageIndex{3}\).

Figure \(\PageIndex{3}\): Animation of cyclohexane ring flipping (left) and sketch showing all axial bonds become equatorial and vice versa upon flipping the ring. (The animation is derived from the CheMagic app, Copyright; Public Domain)

The energy versus reaction time curve for ring flipping reaction of cyclohexane is illustrated in Figure \(\PageIndex{4}\). The ring flipping, starting from one bending one corner of a chair conformation labeled A1, proceeds to a transition state labeled D1 and then to a twist-boat conformation labeled B1. The twist boat B1 transforms into another twist boat conformation, B2, via a boat transition state labeled C. The twist boat B2 converts into a flipped chair conformation A2 via transition state D2. Proceeds through twist boat conformation and half-chair and boat transition states, with higher energy conformations, as shown in Figure \(\PageIndex{4}\).

Figure \(\PageIndex{4}\):Cyclohexane chair flip (ring inversion) reaction plotted against their energy differences. Inversion happens quickly & constantly at room temperature. A1 & A2: chair; B1 & B2: twist-boat; C: boat; and D1 & D2: half-chair. (Copyright; Keministi, Public domain via Wikimedia commons)

The two chair conformations are of equal energy in the case of cyclohexane and predominant in the mixture due to their low energy. For every 10,000 molecules in chair conformations, there is no more than one molecule in the twist boat conformation. All other conformations are higher energy than the two chair conformations due to angle strain, eclipsing strain, trans-annular strain, or a combination of these. For example, the boat conformation, labeled C in the figure, has four \(\ce{H}\) to \(\ce{H}\) eclipsing interactions and a trans-annular interaction between two \(\ce{H}\) pointing up from the top corners, like flagpoles. The latter is a trans-annular steric strain, referred to as a flagpole interaction in a boat conformation, as illustrated by red lines in Figure \(\PageIndex{4}\).

Model, showing flagpole \(\ce{H's}\)

Sketch, showing flagpole interaction by red lines

Model, showing eclipsing

Sketch, showing eclipsing

Figure \(\PageIndex{5}\): Boat conformation of cyclohexane shown in different views to highlight flagpole strains (two figures on the left) and eclipsing strains (two figures on the right) (Copyright: Public Domain).

Conformations of monosubstituted cyclohexanes

When one \(\ce{-H}\) of cyclohexane is replaced with another group, e.g., \(\ce{-CH3}\), it becomes a monosubstituted cyclohexane. Two chair conformations of cyclohexane are of equal energy, but in a monosubstituted cyclohexane, the bulky group like \(\ce{-CH3}\) is less stable at axial postion by 7.1 kJ/mol than at equatorial position, as illustrated in Figure \(\PageIndex{6}\). The bulky group at the axial position has steric repulsion with the other two axial \(\ce{H's}\) on the same face of the ring, called 1,3-diaxial interaction. The bulky group at the equatorial position is in free space with no steric interaction.

Equilibrium between chair conformations of methylcyclohexane

Diaxial interaction highlighted in red lines

Figure \(\PageIndex{6}\): Equilibrium between axial and equatorial conformations of methylcyclohexane with 1,3-Diaxial interaction illustrated by green lines (left) and electron cloud model of axial methylcyclohexane with 1,3-diaxial interaction illustrated by red lines (right). (Copyright; Public domain)

Each of the two 1,3-diaxial interactions in methylcyclohexane is similar to a gauche interaction in butane, as illustrated in Figure \(\PageIndex{7}\).

Gauche conformation of butane

Chair conformations of axial and equatorial methylcyclohexane

Anti conformation of butane

Figure \(\PageIndex{7}\): Comparing Newman projections of the gauche conformation of butane with the axial conformation of methylcyclohexane and the anti conformation of butane with the equatorial conformation of methylcyclohexane (Copyright; Public domain).

Equilibrium constant \(K\) of any chemical reaction is related with the free energy change \(\Delta G\), by the following realation: \[\Delta G = -RT\;\text{ln}\;K\nonumber\], where \(R\) is gas constant equal to 8.314 J.K^-1.mol^-1., and \(T\) is absolute temperature equal to 298 K at room temperature. It rearranges to: \[\text{ln}\;K = -\frac{\Delta G}{RT}\nonumber\] Pluging in \(\Delta G = -7100 \;J/mol\) for axial to equatorial methylcyclohexane conversion, gives the value of \(K\):

\[\text{ln}\;K = -\frac{-7100 \;\frac{J}{mol}}{8.314 \;\frac{J}{K.mol}\times 298\;K} = 2.9 \nonumber\]

\[K = \frac{18.2\text { equatorial}}{1\text { axial}}\nonumber\]

It means the percentage of equatorial methylcyclohexane at equilibrium \(=\frac{18.2}{18.2+1}\times 100 = 95\text %\)

The 1,3 diaxal strain varies with the size of the substituent -the larger the substituent, the higher the strain, as shown in Table \(\PageIndex{8}\).

Table \(\PageIndex{8}\): 1,3-diaxial strain, also called A-values, of some substituents on a cyclohexane ring. (taken from https://en.Wikipedia.org/wiki/A_value, 07/10/2025)

Group:	\(\ce{-F}\)	\(\ce{-Cl}\)	\(\ce{-Br}\)	\(\ce{-I}\)	\(\ce{-C\equiv N}\)	\(\ce{-CH3}\)	\(\ce{-CH2CH3}\)	\(\ce{-CH(CH3)2}\)	\(\ce{-C(CH3)3}\)
Strain (\(\frac{kJ}{mol}\))	0.63	1.8	1.6	1.9	0.71	7.1	7.3	9.9	23
Group:	\(\ce{-Ph}\)	\(\ce{-COOH}\)	\(\ce{-COOCH3}\)	\(\ce{-OH}\)	\(\ce{-OCH3}\)	\(\ce{-NH2}\)	\(\ce{-NO2}\)	\(\ce{-COCH3}\)	\(\ce{-SH}\)
Strain (\(\frac{kJ}{mol}\))	12.6	5.6	5.3	3.6	2.5	6.7	4.6	4.9	3.8

The bulky substituents nearly lock the substituted cyclohexanes in one chair conformation in which the bulky substituent is in the equatorial position. For example, the above calculations for \(\ce{-CH3}\) substituent show that only five in a sample of a hundred molecules are expected to have a methyl substituent in the axial position. When the same calculation is repeated for bulky \(\ce{-C(CH3)3}\) substituent, the results show that only one in a sample of ten thousand molecules is expected to have the substituent in the axial position.

Disubstituted cyclohexane

Stereoisomers have the same atom connectivity, but the atoms are oriented differently. There are two major classes of stereoisomers: i) conformational isomers in which the different orientation of atoms is a result of rotation around single bonds, and ii) configurational isomers in which the different orientation of atoms is not interconvertible by simple rotation around bonds, it requires making and reforming some bonds. Gauche and anti-conformations of cyclobutane are examples of conformational isomers. Axial and equatorial conformations of monosubstituted cyclohexanes, described in the previous section, are also examples of conformational isomers. One type of configurational isomers, called cis-trans isomerism, is described in the next seciton. Other types of configurational isomerism are described in a later section.

Cis-trans isomerism

Two substituents on two \(\ce{C's}\) of a double bond or on two \(\ce{C's}\) of a cycloalkane exhibit a subclass of configurational isomers called cis-trans isomerism. Cis- means the two substituents are pointing on the same side, and trans- means they are pointing on the opposite side. For example, two \(\ce{-CH3}\) substituents on different \(\ce{C'}\) of a \(\ce{C=C}\) are pointing on the same side in cis-but-2-ene and pointing on the opposite side in trans- but-2-ene, as shown in Figure \(\PageIndex{9}\)

cis-but-2-ene

trans-but-2-ene

cis-1,2-dimethylcyclopentane

trans-1,2-dimethylcyclopentane

cis-1,3-dimethylcyclopentane

trans-1,3-dimethylcyclopentane

Figure \(\PageIndex{9}\): Examples of cis-trans isomers of alkenes and cycloalkanes (Copyright: Public domain).

When an acycloalkane is shown in a planar configuration, the two substituents pointing in the same direction can either point up towards the viewer, as indicated by a solid wedge, or point down away from the viewer, as shown by a hashed wedge. For example, cis-1,2-dimethylcyclopentane has both methyl substituents on a solid wedge, and trans-1,2-dimethylcyclopentane has one on a solid wedge and the other on a hashed wedge, as shown in Figure \(\PageIndex{9}\). The two substituents need not be on adjacent \(\ce{C's}\) for cis-trans isomerism, they can be on any two different \(\ce{C's}\) of cycloalkane structure. For example, the last two structures in Figure \(\PageIndex{9}\) show cis- and trans- isomers of 1,3-dimethylcyclopentane.

Usually, the cis-isomer of an alkene or cycloalkane is less stable due to the steric strain of the bulky substituents on the same side as the trans-isomer. For example, cis-but-2-ene is 5.0 kJ/mol higher in energy than trans-but-2-ene, and cis-1,2-dimethylcyclopentane is 7.1 kJ/mol higher in energy than trans-1,2-dimethylcyclopentane.

Cis-trans isomerism in cyclohexane

Cis-trans isomerism in cyclohexane can be shown by drawing the ring in a planar configuration as a hexagon and the substituents on solid wedge or hashed wedge. For example, the structure on the left margin is cis-1,2-dimethylcyclohexane, and the one on the right margin is trans-1,2-dimethylcyclohexane. However, drawing it on a chi plane is more realistic and can also show the ring flipping, resulting in conformational isomers.

In the chair conformation of cyclohexane, every \(\ce{C}\) has two bonds outside the ring, an axial and an equatorial, one pointing up and the other pointing down, as illustrated in the figure on the right margin. Suppose axial is pointing up in a \(\ce{C}\)#1, the axial on the neighboring \(\ce{C}\)#2 and #6 are pointing down, the axial on one \(\ce{C}\) away that is \(\ce{C}\)#3 and #5 are pointing up, and axial on the \(\ce{C}\) across, i.e., \(\ce{C}\)#6 is pointing down, and vice versa. The same is true for equatorial bonds. This information allows the drawing of cis-trans isomers of disubstituted cyclohexanes.

Two groups on two different rings \(\ce{C,s}\) pointing in the same direction, both up or both down, is cis, irrespective of whether they are both on axial or equatorial or one on axial and the other on equatorial. The opposite, i.e., one pointing up and the other down, is a trans isomer. For example, two substituents,

1. one on \(\ce{C}\)#1 and the other on #2 or on #6, are cis- if one is on axial and the other on equatorial, and
   • trans- if both are axial or both are equatorial.
   • The same is true for two substituents, one on \(\ce{C}\)#1 and the other on #4.
2. The opposite is true for two substituents, one on \(\ce{C}\)#1 and the other on #3 or on #5, i.e., they trans- if one is on axial and the other on equatorial, and
   • cis- if both are axial or both are equatorial.

These rules are explained with examples in the following sections.

1,2-Disubstitued cyclohexanes

Cis 1,2 substituetns are one axial and the other equatorial, or the other way around. For example, cis-1,2-dimethylcyclohexane has one \(\ce{-CH3}\) axial and the other equatorial, as shown in the figure below. The ring flip reaction converts every axial bond into equatorial and vice versa. Both chair conformations of cis-1,2-dimethylcyclohexane are of equal energy. When the two substituents are not the same, the bulky substituent in the equatorial position is a more stable configuration. For example, the methyl group in the equatorial position of cis-1-chloro-2-methylcyclohexane (1.8 kJ/mol for axial \(\ce{-Cl}\)) is more stable than the chloride in the equatorial position (7.1 kJ/mole for axial \(\ce{-CH3}\)), as shown in the figure below.

cis-1,2-dimethylcyclohexane

cis-1-chloro-2-methylcyclohexane

trans-1,2-dimethylcyclohexane

A trans-1,2-dimethylcyclohexane has both methyl substituents in axial positions in the less stable conformation or both methyl groups in equatorial positions in the more stable conformation, as shown in the figure above. The less stable conformation has two 1,3-diaxial interactions, whereas the more stable conformation features only one gauche interaction between methyl groups. The \(\Delta G\) for diaxial to diequatorial conversion reaction is 10.4 kJ/mol (two diaxial interactions (\(7.1\text{ kJ/mol} + 7.1 \text{ kJ/mol} = 14.2 \text{ kJ/mol}\)) - one gauche interaction (3.8 kJ/mol) = 10.4 kJ/mol).

1,3-Disubstitued cyclohexanes

Cis-1,3 substituents are either both axial or both equatorial. For example, cis-1,3-dimethylcyclohexane has both \(\ce{-CH3}\) axial in less stable or both equatorial in more stable conformation as shown in the figure below. Both chair conformations of cis-1,2-dimethylcyclohexane are of equal energy. The \(\Delta G\) for diaxial to diequatorial conversion reaction is 14.2 kJ/mol (two diaxial interactions (\(7.1\text{ kJ/mol} + 7.1 \text{ kJ/mol} = 14.2 \text{ kJ/mol}\)) - 0 strain in diequatorial = 14.2 kJ/mol).

cis-1,3-dimethylcyclohexane

trans-1,3-dimethylcyclohexane

trans-1-tert-butyl-3-methylcyclohexane

Trans-1,3 substituetns are one axial and the other equatorial, or the other way around. For example, trans-1,3-dimethylcyclohexane has one \(\ce{-CH3}\) axial and the other equatorial, as shown in the figure above. Both chair conformations of trans-1,3-dimethylcyclohexane are of equal energy. When the two substituents are not the same, the bulky substituent in the equatorial position is a more stable configuration. For example, tert-butyl group in the equatorial position of trans-1-tert-butyl-3-methylcyclohexane ((\(27.1\text{ kJ/mol}\) for \(\ce{-CH3}\) axial) is more stable than the methyl in the equatorial position ((\(23\text{ kJ/mol}\) for \(\ce{-C(CH3)3}\) axial), as shown in the figure above.

1,4-Disubstituted cyclohexanes

Cis 1,4 substituents are one axial and the other equatorial, or the other way around. For example, cis-1,4-dimethylcyclohexane has one \(\ce{-CH3}\) axial and the other equatorial, as shown in the figure below. Both chair conformations of cis-1,4-dimethylcyclohexane are of equal energy. When the two substituents are not the same, the bulky substituent in the equatorial position is a more stable configuration. For example, the methyl group in the equatorial position of cis-4-methylchclohexan-1-ol is more stable (3.6 kJ/mol for axial \(\ce{-OH}\)) than the hydroxyl group in the equatorial position (7.1 kJ/mol for axial \(\ce{-CH3}\)), as shown in the figure below.

cis-1,4-dimethylcyclohexane

cis-4-methylcyclohexan-1-ol

trans-1,4-dimethylcyclohexane

A trans-1,4-dimethylcyclohexane has both methyl substituents in axial positions in the less stable conformation or both methyl groups in equatorial positions in the more stable conformation, as shown in the figure above. The less stable conformation has two 1,3-diaxial interactions, whereas the more stable conformation has no such destabilizing interaction. The \(\Delta G\) for diaxial to diequatorial conversion reaction is 14.2 kJ/mol (two diaxial interactions (\(7.1\text{ kJ/mol} + 7.1 \text{ kJ/mol} = 14.2 \text{ kJ/mol}\)) - 0 strain in diequatorial isomer = 14.2 kJ/mol).

Polysubstituted cyclohexanes

Polysubstituted cyclohexanes also have two chair conformations, in which one is usually more stable. If the given structure is in a planar configuration, the direction of the substituents, up or down, is indicated by the use of solid or hashed wedges. The following rules convert the planar to a chair conformation.

1. Number the cyclohexane chain. The numbers could be IUPAC numbering of the parent chain, or they can be arbitrary numbers beginning from any one of the substituents.
2. Draw a cyclohexane chair and place the first substituent, axial or equatorial, at its designated number.
3. Place the second and the third substituents at their designated numbers, but the direction, axial or equatorial, is determined by the cis- or trans-relationship to the first substituent.
4. Draw a flipped cyclohexane chain and place the substituents at their designated numbers such that all axial substituents become equatorial and vice versa in the flipped conformation.
5. Calculate the strain, i.e., 1,3-diaxial interaction energies and gauche interactions for each conformation.
   • The confirmation that has less strain is more stable.
6. Subtract the larger strain value from the smaller to calculate the \(\Delta G\) for conversion from more stable to less stable conformation.

Note: If the value of gauche interaction is not known, it can be ignored because, usually, it is common in the two flipped conformations and cancels out when the overall strain energy of one is subtracted from the other to calculate the \(\Delta G\) for conversion from more stable to less stable conformation.

Application of these rules is demonstrated below with the applied examples.

Example \(\PageIndex{1}\)

Convert the following structure of cyclohexane into two chair conformations and calculate which one is more stable.

Solution

The given structure is 4-bromo-2-ethyl-1-methylcyclohexane. The steps to convert it into chair conformations are shown below.

Step 1	Step 2	Step 3	Step 4
IUPAC numbering	Methyl drawn equatorial at #1	Ethyl at #2 is cis and bromo at #4 is trans to methyl	All axial substituents become equators and vice versa in the flipped conformation
Step 5: Strain in conformation I = (7.3 kJ/mol for \(\ce{-CH2CH3}\) axial + gauche for \(\ce{-CH3}\) to \(\ce{-CH2CH3}\)) Strain in conformation II = (7.1 kJ/mol for \(\ce{-CH3}\) axial + 1.6 kJ/mole for \(\ce{-Br}\) axial + gauche for \(\ce{-CH3}\) to \(\ce{-CH2CH3}\)). Confirmation I with one axial substitute is more stable because the diaxial strain is less in it, and the gauche interaction is the same in the two. Step 6: \(\Delta G\) released for II to I conversion = (7.1 kJ/mol for \(\ce{-CH3}\) axial + 1.6 kJ/mole for \(\ce{-Br}\) axial + \(\cancel{\text{gauche for } \ce{-CH3} \text{ to } \ce{-CH2CH3}}\)) - (7.3 kJ/mol for \(\ce{-CH2CH3}\) axial + \(\cancel{\text{gauche for } \ce{-CH3} \text{ to } \ce{-CH2CH3}}\)) = 1.4 kJ/mole

Note that in the above example, confirmation I with fewer axial substituents is more stable, but this is not always the case. This is because the 1,3-Diaxial strain is a steric strain dependent on the bulk of the substituent -the more bulky the substituent, the larger the strain. The following example proves the above point.

Example \(\PageIndex{2}\)

Convert the following structure of cyclohexane into two chair conformations and calculate which one is more stable.

Solution

The given structure is 4-tert-butyl-1-ethyl-2-methylcyclohexane. The steps to convert it into chair conformations are shown below.

Step 1	Step 2	Step 3	Step 4
IUPAC numbering	Ethyl drawn equatorial at #1	Methyl at #2 is trans and tert-butyl at #4 is cis to ethyl	All axial substituents become equators and vice versa in the flipped conformation
Step 5: Strain in conformation I = (23 kJ/mol for \(\ce{-C(CH3)3}\) axial + gauche for \(\ce{-CH3}\) to \(\ce{-CH2CH3}\)) Strain in conformation II = (7.1 kJ/mol for \(\ce{-CH3}\) axial + 7.3 kJ/mole for \(\ce{-CH2CH3}\) axial + gauche for \(\ce{-CH3}\) to \(\ce{-CH2CH3}\)). Confirmation II with \(\ce{-C(CH3)3}\) is more stable because the diaxial strain is less in it, and the gauche interaction is the same in the two. Step 6: \(\Delta G\) released for I to II conversion = (23 kJ/mol for \(\ce{-C(CH3)3}\) axial + \(\cancel{\text{gauche for } \ce{-CH3} \text{ to } \ce{-CH2CH3}}\)) - (7.1 kJ/mol for \(\ce{-CH3}\) axial + 7.3 kJ/mole for \(\ce{-CH2CH3}\) axial + \(\cancel{\text{gauche for } \ce{-CH3} \text{ to } \ce{-CH2CH3}}\)) = 8.6 kJ/mole

Caution

The descriptor cis or trans used to identify stereoisomers, e.g., cis-1,2-dimethylcyclohexane, trans-1,3-dimethylcyclohexane, etc., in the previous section is not an IUPAC name of these compounds. IUPAC uses stereodescriptors, like R, S, E, Z, etc., that are described in a later section.

Preference for equatorial over axial substitutions in nature

Six-membered rings are the most common cyclic structures found in nature. Those with all or most of the bulky groups in equatorial positions are significantly more common because of the stability of the equatorial relative to the axial bulky groups. For example, the majority of monosaccharides (sugars) exist as six-membered cyclic structures having more equatorial than axial substituents. Glucose -the most common monosaccharide exists primarily as a six-membered ring structure with all the bulky groups at the equatorial positions, as shown in Figure \(\PageIndex{10}\). Glucose is synthesized during photosynthesis, and either consumed as an energy source or for the synthesis of other biochemicals. For example, glucose units combined through an equatorial group form a polymer called cellulose, the major component of wood. Starch is also a polymer of glucose, but with one connecting bond axial. This change of the connecting bond from equatorial to axial changes it from cellulose, a hard structural material, to starch. This carbohydrate is an energy storage compound in animals and plants.

Glucose

Starch

Cellulose

Figure \(\PageIndex{10}\): Structure of glucose, starch, and cellulose. (Copyright; Public domain)

Fused ring systems also exist in nature; most have six- and five-membered rings. They also show a preference for bulky substituents on equatorial positions. Some of these fused ring systems are described in the next section.

Cyclohexanes in polycyclic systems

Cyclohexane is commonly found as a part of polycyclic systems in biochemicals. One example is decalin (bicyclo[4,4,0]decane), which has two cyclohexane rings in chair conformations fused by sharing one bond. It has two isomeric forms: cis-decalin, which is less stable, and trans-decalin, which is more stable and more common. The trans-decalin model from the top view appears to resemble the planar conformation. Still, its actual 3D structure with two cyclohexane rings in chair conformation reveals, when viewed from edge-on, as shown in Figure \(\PageIndex{11}\).

trans-decalin, top view, and sketch

trans-decalin, edge-on view, and sketch

cis-decalin, model and sketch

trans-decalin-2-ol, model and sketch

Figure \(\PageIndex{11}\): Decalin (bicyclo[4,4,0]decane) systems, models of cis and trans isomers, and an example trans-decalin-2-ol used in fragrance compositions (Copyright; Public domain).

The trans relationship of trans-decalin can be easily observed by viewing the orientation of two \(\ce{H}\) atoms at ring junctions, which are 1,2-axial. The bulky groups, i.e., the alkyl parts of the second ring, are 1,2-diequatorial, the more stable trans-conformation. Cis-decaline has one alkyl substituent on each cyclohexan ring in an unstable axial position. An example is trans-decalin-2-ol used in fragrance compositions. It has a trans-decalin backbone, as shown in Figure \(\PageIndex{11}\).

Another common fused cyclohexane system is norbornane (bicyclo[2,2,1]heptane), as shown in Figure \(\PageIndex{12}\). It is a cyclohexane locked in boat conformation by the one \(\ce{C}\) bridge. Although the boat conformation of cyclohexane is an unstable transition state due to flagpole interaction, the flagpole \(\ce{H's} \) are replaced by \(\ce{>\!CH2}\) bridge in norbornane, which removes the strain and keeps the cyclohexane locked in boat conformation.

Norbornane model

Sketch of norbornane (bicyclo[2,2,1]heptane)

Camphor

Camphene

Figure \(\PageIndex{12}\): Norbornane (bicyclo[2,2,1]heptane) model, sketch, and examples: camphor, a strongly centered natural product, and camphene, a constituent of many natural oils (Copyright; Public domain).

Examples of natural products having a norbornane backbone are camphor, a strongly scented compound isolated from evergreen trees, and camphene, which is a component in several natural oils, such as pine oil and ginger oil.

Steroids are a class of biologically active compounds that are derivatives of the gonane structure, also called sterane, which is composed of three cyclohexane rings in chair conformation and one cyclopentane ring in envelope conformation, fused in a specific way, as shown in Figure \(\PageIndex{13}\). All four rings are trans-fused, except for a few examples where only the first cyclohexane ring is cis-fused.

Gonane (sterane)

Gonan-3-ol, the simplest sterol

Cholesterol

Figure \(\PageIndex{13}\): Models and sketches of steroids based on fused ring system of three cyclohexane and one cyclopentane in a specific way as in gonane, gonan-3-ol, the simplest sterol, cholesterol, a well know sterol, and cholestanol, a derivative of sterol (Copyright; Public domain).

Sterols, a subclass of steroids, have an \(\ce{-OH}\) group at equatorial position of \(\ce{C}\)#3 of gonane, as in gonan-3-ol shown in Figure \(\PageIndex{13}\). Cholesterol, shown in a planar configuration in the figure on the right margin and in cyclic chair conformations of six membered rings and envelop conformation of cyclopentane in Figure \(\PageIndex{13}\), is a well-known sterol present in blood, brain, spinal cord, fats, oils, and cell membranes of all higher animals.