Cyclic Aliphatic Hydrocarbons

CYCLIC ALIPHATIC HYDROCARBON

Cyclic aliphatic hydrocarbons are a type of hydrocarbon in which the carbon atoms are arranged in a closed ring structure, and all the carbon-carbon bonds are single bonds (also known as alkanes). These compounds are also known as cycloalkanes and are classified according to the number of carbon atoms in the ring, such as cyclopropane, cyclobutane, cyclopentane, cyclohexane, etc.

Some key properties of cyclic aliphatic hydrocarbons are:

  • They have the same general formula as acyclic alkanes (CnH2n), but the number of hydrogen atoms is reduced by two due to the formation of a ring structure.
  • The carbon atoms in the ring are sp3 hybridized and form tetrahedral structures.
  • Cyclic aliphatic hydrocarbons are non-polar and have low reactivity due to the stability of the ring structure.
  • They are often used as solvents and in the production of other chemicals.
  • The physical properties of cycloalkanes, such as boiling points and melting points, increase with the size of the ring.

Cyclic aliphatic hydrocarbons also have some unique characteristics compared to their acyclic counterparts. For example, they can undergo conformational isomerism, where the ring can adopt different conformations due to the rotation of carbon-carbon bonds. Additionally, the strain energy of a cyclic alkane can vary depending on the size of the ring and the specific conformation it adopts. This makes cyclic aliphatic hydrocarbons an interesting class of compounds for the study of structure-property relationships in organic chemistry.

Stability of cycloalkane

The stability of cycloalkanes depends on their ring strain, which is caused by the angle strain and torsional strain present in the cyclic structure. Angle strain arises from the deviation of the bond angles from their ideal values, while torsional strain arises from the eclipsing interactions between the C-H bonds of the adjacent carbons in the ring.

As the size of the ring increases, the number of carbons in the ring also increases, and the angle strain decreases due to the decrease in bond angles. Additionally, the torsional strain decreases as the eclipsing interactions between the C-H bonds decrease.

Thus, larger cycloalkanes are more stable than smaller ones. Cyclohexane is the most stable cycloalkane due to its low ring strain resulting from its ideal bond angles and low torsional strain from its chair conformation, which allows for the staggered arrangement of the C-H bonds. In contrast, smaller cycloalkanes such as cyclopropane and cyclobutane are highly strained and relatively unstable.

Baeyer's Strain Theory

Baeyer's strain theory is a concept that explains the relative stability of cyclic compounds based on their ring strain. According to this theory, when atoms in a ring structure are forced into a non-optimal arrangement, they experience a strain that destabilizes the molecule. This strain arises due to the deviation from the ideal bond angles and bond distances that occur in the planar structure of the ring. The greater the deviation from ideal bond angles and bond distances, the higher the ring strain.

Baeyer's theory also suggests that smaller rings, such as cyclopropane and cyclobutane, have a higher ring strain due to their high angle strain and eclipsing interactions. As the size of the ring increases, the ring strain decreases, resulting in a more stable structure. Cyclohexane, with its ideal bond angles and bond distances, is the most stable of all cycloalkanes.

Conformations of cycloalkane

 

Cyclohexane has a non-polar structure that makes it almost free from ring strain. The most important conformations that it can have included chain conformation and boat conformation. The chair conformation is more stable than the boat conformation. The boat conformation can sometimes be more stable than it is usually, by a slight rotation in the C-C bonds and is called the skew boat conformation. Nevertheless, the chair conformation is the most stable cyclohexane form.

A Conformation of cyclohexane can refer to many 3-Dimensional shapes assumed by a cyclohexane molecule without disturbing the integrity of the chemical bonds in it.

A regular hexagon shape contains internal angles of 120o. However, the carbon-carbon bonds belonging to the cyclohexane ring have a tetrahedral symmetry, with the bond angles corresponding to 109.5o.

This is the reason why the cyclohexane ring has a tendency to take up several warped conformations (so that the bond angles are brought closer to the tetrahedral angle (109.5o) and there is reduced overall strain energy).

Examples of common conformations of cyclohexane include the boat, the twist-boat, the chair, and the half-chair conformations, which are named based on the shape that the cyclohexane molecule assumes in them.

These four cyclohexane conformations have been illustrated below along with some insight into their stability.

It can be noted that the cyclohexane molecule has the ability to switch between the conformations listed above and that only the chair and the twist-boat conformations can be isolated into their respective pure forms.

Due to hydrogen-hydrogen interactions in these conformations, the bond length and the bond angle vary slightly from their nominal values.

The chair conformations of cyclohexane have lower energies than the boat forms. However, the rather unstable boat forms of cyclohexane undergo rapid deformation to give twist-boat forms which are the local minima corresponding to the total energy.

The hydrogen atoms belonging to the carbon-hydrogen bonds that are at a perpendicular angle to the mean plane are called axial hydrogens, whereas those belonging to the carbon-hydrogen bonds which are parallel to the mean plane are called equatorial hydrogens. These bonds are also referred to as axial and equatorial bonds respectively.

Boat and Chair Forms of Cyclohexane

Cyclohexane is the most widely occurring ring in compounds of natural origin. Its prevalence, undoubtedly a consequence of its stability, makes it the most important of the cycloalkanes. The deviation of bond angle in cyclohexane molecules is more than in cyclopentane, it should be more strained and less reactive than cyclopentane. But actually, it is less strained and more stable than cyclopentane.

In order to avoid the strain, cyclohexane does not exist as a planar molecule as expected. It exists as a puckered ring which is non-planar and the bond angles are close to tetrahedral bond angles. Two such puckered rings for cyclohexane are called the boat and chair conformations.

Stability of conformations of cyclohexane

Generally, in the chair-shaped conformation of cyclohexane, there are two carbon-hydrogen bonds of each of the following types:

        • Axial ‘up’
        • Axial ‘down’
        • Equatorial ‘up’
        • Equatorial ‘down’

This geometry of chair cyclohexane conformations is generally preserved when the hydrogen atoms are replaced by halogen atoms such as fluorine, chlorine, bromine, and iodine. The phenomenon wherein the cyclohexane molecule undergoes a conversion from one chair form to a different chair form is called chair flipping (or ring flipping).

When chair flipping occurs, axial carbon-hydrogen bonds become equatorial and the equatorial carbon-hydrogen bonds become axial. However, they retain the corresponding ‘up’ or ‘down’ positions.

It can be noted that at a temperature of 25o Celsius, 99.99% of the molecules belonging to a given cyclohexane solution would correspond to a chair-type conformation.

The boat conformation of cyclohexane is not a very stable form due to the torsional strain applied to the cyclohexane molecule. The stability of this form is further affected by steric interactions between the hydrogen atoms. Owing to these factors, these conformations are generally converted into twist-boat forms which have a lower torsional strain and steric strain in them.

These twist-boat conformations of cyclohexane are much more stable than their boat-shaped counterparts. This conformation has a concentration of less than 1% in a solution of cyclohexane at 25o. In order to increase the concentration of this conformation, the cyclohexane solution must be heated to 1073K and then cooled to 40K.

Conformation of a substituted cyclohexane

Cis and Trans Substituted System

On conformationally limited systems, the labels cis/trans describe the relative orientations of substituents. When two substituents are oriented on the same side of a planar ring, they are called cis, and when they are on opposing sides, they are called trans. Note that the designations cis/trans and axial/equatorial refer to different features of cyclohexane skeleton substituents.

·         Consider 1-chloro-2-methylcyclohexane, a common disubstituted cyclohexane. One stereoisomer of this molecule has both the chloro and methyl groups at equatorial locations.

·         With a conformational diastereomer in which both the chloro and methyl groups are oriented axially, this chemical quickly achieves conformational equilibrium.

·         Trans-1-chloro-2-methylcyclohexane refers to either conformation (or a mixture of both).

·         When two substituents have an up–down relationship in cyclic compounds, the name “trans” is employed.

·         The chloro and methyl groups occupy neighbouring equatorial and axial locations in a distinct stereoisomer of 1-chloro-2-methylcyclohexane.

·         Cis-1-chloro-2-methylcyclohexane is a conformational diastereomer combination that quickly equilibrates. In a cis-disubstituted cycloalkane, the substituents are connected up–up or down–down.

The chair interconversion has no effect on the cis connection. The same cis and trans substitution definitions apply to substituent groups in additional cyclohexane ring positions. The terms cis and trans define the relative stereochemical configurations of the two asymmetric carbons when the substituents in a disubstituted cyclohexane are on asymmetric carbons. Still, they say nothing about the absolute configurations of these carbons. As a result, cis-1-chloro-2-methylcyclohexane has two enantiomers.

Conformational Analysis

Conformational analysis can be performed on disubstituted cyclohexanes, much like monosubstituted cyclohexanes. The 1,3-diaxial interactions (or gauche-butane interactions) in each conformation are compared to assess the relative stability of the two chair conformations.

When two groups on substituted cyclohexane disagree over the equatorial position, the preferable conformation can typically be inferred from the groups’ relative conformational preferences. Take the chair interconversion of cis1-tert-butyl-4-methylcyclohexane, for example.

The van der Waals repulsions of the tert-butyl group are so strong that they regulate the conformational equilibrium. As a result, the tert-butyl group that adopts the equatorial position in the chair conformation is overwhelmingly preferred. Based on this, the methyl group is forced to take up an axial position.

Stability of conformation of substituted cyclohexane

Cyclohexane substituents can be found in either axial or equatorial positions. However, in general, equatorial substituents tend to be preferred because they are more stable because of reduced steric interactions.

Let's consider an example, methylcyclohexane.
In the equatorial system, the methyl group has space around it as it is pointed away from the rest of the ring. The C-C bond that connects the methyl group is anti to the two C-C bonds in the rest of the ring system which means there is minimal torsional strain.

However, in the axial conformation, the methyl group is closer to the rest of the ring. 

There is an unfavourable steric interaction between the methyl group with the two axial hydrogen atoms on the same face of the ring. This destabilises the axial conformation.