Name: cyclohexane chair axial vs equatorial
Uploaded: 2023-04-30T18:28:33.000Z
Duration: 2 min 30 s
Channel: JoVE

The two chair conformations of cyclohexane, at equilibrium, have identical energies and stabilities, with each conformer representing about 50% of the equilibrium mixture.
Replacing a hydrogen atom with an alkyl group makes the two conformations energetically nonequivalent.
For instance, in methylcyclohexane, the CH3 group occupies an axial position in one chair conformation and an equatorial position in another.
The axial conformation has an elevated energy of approximately 7.6 kJ mol&#8722;1 compared to the equatorial conformation, making the latter more stable.
Consequently, the equatorial conformation comprises about 95% of the equilibrium mixture. The question is &#8212; what is the reason for such variations in energy and stability?
Studies reveal that in an axial conformation, the methyl hydrogens experience repulsive dispersion interactions with the two parallel and closely positioned axial hydrogens on the same side of the ring.
This unfavorable steric strain between groups on C1 and C3 or C5 is called 1,3-diaxial interaction, which is a gauche interaction.
Each gauche interaction contributes approximately 3.8 kJ of additional energy.
Such gauche interactions are absent in an equatori

stability of substituted cyclohexanes

Stability of Substituted Cyclohexanes

This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial interactions.
The two chair conformations of cyclohexanes undergo rapid interconversion at room temperature. Both forms have identical energies and stabilities, each comprising equal amounts of the equilibrium mixture. Replacing a hydrogen atom with a functional group makes the two conformations energetically non-equivalent.
For example, in methylcyclohexane, the CH3 group occupies an axial position in one chair conformation and an equatorial position in another. This leads to an increase in energy of the axial conformation to approximately 7.6 kJ mol&#8722;1, making the equatorial conformation more stable with an abundance of 95%.
The reason for such variations in energy and stability is that the methyl hydrogens experience repulsive dispersion interactions with the two parallel and closely positioned axial hydrogens on the same side of the ring. Since the steric strain originates between groups on C1 and C3 or C5, it is called a 1,3-diaxial interaction. These interactions, when shown with the Newman projection, exhibit a gauche relationship. However, if the methyl group is positioned equatorially, it is placed anti to C3 and C5, minimizing the steric repulsion.
As the size of a functional group increases, 1,3-diaxial interactions become more pronounced, increasing the energy difference between the two conformations.

This lesson discusses the stability of substituted cyclohexanes with a focus on energies of various conformers and the effect of 1,3-diaxial 
...

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JoVE Core

Organic Chemistry

Unit 1

Alkanes and Cycloalkanes

KEY TERMS AND CONCEPTS

cyclohexane chair axial vs equatorial

Cyclohexane chair conformations are shaped by the position of a substituent, and that position can change how stable the molecule is. The two chair forms of cyclohexane interconvert rapidly at room temperature. For unsubstituted cyclohexane, both chairs have the same energy and the same stability, so each appears in equal amount at equilibrium.

When a hydrogen atom is replaced by a functional group, the two chair forms are no longer equal. In methylcyclohexane, the CH3 group is axial in one chair and equatorial in the other. The axial form is higher in energy by about 7.6 kJ mol−1, so the equatorial form is more stable and makes up about 95% of the mixture.

The difference comes from 1,3-diaxial interactions. These are repulsive dispersion interactions between the axial substituent and the two nearby axial hydrogens on the same side of the ring at C1 and C3 or C5. In a Newman projection, this strain appears as a gauche relationship. When the methyl group is equatorial, it is anti to C3 and C5, so steric repulsion is reduced.

Larger functional groups make these 1,3-diaxial interactions stronger. As group size increases, the energy gap between the two chair conformations also increases. That is why the axial and equatorial positions can lead to very different stability in substituted cyclohexanes.

Cyclohexane Chair Axial vs Equatorial

structure of alkanes

Alkane Bonding and Formula Patterns

Alkanes are the simplest hydrocarbons, and their structure starts with carbon-carbon bond formation. That idea helped shape organic chemistry, as August Kekulé and Archibald Scott Couper independently proposed carbon chain formation. Hydrocarbons are organic compounds made only of carbon and hydrogen. Based on the bond order between carbon atoms, hydrocarbons are classified as alkanes, alkenes, and alkynes. Alkanes contain sp3 hybrid carbon atoms. These carbons form sigma bonds with other sp3...

constitutional isomers of alkanes

Alkane Branching and Isomer Counts

Alkanes can form constitutional isomers, which are compounds with the same molecular formula but different structural formulas. This idea is called constitutional isomerism. It becomes important for alkanes with four or more carbons, because they can have more than one structure with the same formula. A straight-chain alkane is named with the prefix n, so the linear form of pentane is n-pentane. Branched isomers use prefixes such as iso and neo. Pentane has two branched isomers, iso-pentane...

nomenclature of alkanes

Naming Alkane Chains and Substituents

Alkane names follow IUPAC rules, a system used to name organic compounds in a clear and standard way. These rules became important in the late 19th century as new chemical compounds were discovered at a fast pace. IUPAC, the International Union for Pure and Applied Chemistry, was established in 1919 to set these naming rules. The naming process for alkanes depends on the length of the carbon chain, the number of substituents, and where those substituents are placed. The first step is to find...

physical properties of alkanes

How Alkane Chain Length Changes Properties

Alkane properties change with chain length and molecular shape. Alkanes contain only carbon and hydrogen, so they are nonpolar molecules. The electronegativity difference between carbon and hydrogen is very small, and alkanes therefore have a zero dipole moment. As a result, the only forces between alkane molecules are dispersion forces. Dispersion forces depend on surface area. In straight-chain alkanes, surface area increases as the carbon chain gets longer. That means dispersion forces also...

newman projections

Molecule Rotations in Newman Projections

Newman projections show molecule rotations around a single bond in a clear two-dimensional view. Chemists use several notations to show the three-dimensional shape of a molecule on a flat surface. The dash-wedge formula is one common way to do this. In that notation, dashed wedges show groups behind the plane, solid wedges show groups coming out of the plane, and plain lines show groups in the plane. Organic molecules can rotate around single bonds. This rotation creates many temporary...

conformations of ethane and propane

Ethane Rotation: Staggered vs Eclipsed

Ethane and propane show how rotation around a carbon-carbon single bond changes a molecule’s shape and energy. This internal rotation creates conformers, which are different arrangements of the same molecule. For ethane, the main conformations are staggered and eclipsed. Staggered ethane is the lower-energy, more stable form. In this shape, the C-H bonds on the front carbon sit 60° away from the C-H bonds on the back carbon. This spacing reduces torsional strain, which is the repulsion that...

conformations of butane

Butane’s Anti, Gauche, and Eclipsed Forms

Butane has several conformations, or shapes made by rotation around a carbon-carbon bond. Unlike ethane and propane, which have only two major conformations, butane shows more than two conformers. The lowest-energy form is the staggered anti conformer. In this shape, the two bulky methyl groups sit opposite each other at a dihedral angle of 180°. This spacing reduces steric repulsion, so the anti form is the most stable. Butane also has two other staggered conformations. These are degenerate,...

cycloalkanes

Cycloalkane Rings and Naming Rules

Cycloalkanes are saturated hydrocarbons with carbon atoms arranged in rings. Because they have two fewer hydrogen atoms than the matching open-chain alkane, their general formula is CnH2n. These ring structures are a common topic in organic chemistry. Chemists often draw cycloalkanes with line-angle formulas. In this shortcut, regular polygons stand for the ring, and each side shows a carbon-carbon bond. The number of corners in the polygon matches the number of carbons in the ring. Naming...

conformations of cycloalkanes

Cycloalkane Ring Strain and Shapes

Cycloalkane ring strain depends on ring shape, bond angles, and bond rotation. Adolf von Baeyer first tried to explain the stability of cycloalkanes with angle strain, which is the strain caused when bond angles differ from the ideal 109.5° tetrahedral angle for sp3-hybridized carbons. His idea worked well for some small rings, but not for every cycloalkane. Cyclopropane and cyclobutane are strained, as expected from their compressed bond angles. Cyclopentane is more strained than Baeyer...

conformations of cyclohexane

Cyclohexane Chair, Boat, and Twist-Boat Forms

Cyclohexane adopts non-planar shapes because a flat ring would create high angle strain and torsional strain. The molecule instead bends into chair and boat conformations. These shapes lower the strain and make the ring more stable. The chair conformation is the most stable form. It is named for its easy-chair shape. In this form, two carbon atoms sit out of the plane, with one above and one below. This arrangement minimizes torsional strain. The bond angles are also very close to the ideal...

chair conformation of cyclohexane

Cyclohexane Ring Flip and Axial Bonds

Cyclohexane is most stable in the chair conformation. In this shape, the ring avoids angle strain because its bond angles are close to the ideal tetrahedral angle of 109.5°. It also avoids torsional strain because the bonds are arranged in a perfectly staggered way. In the chair form, the hydrogen atoms attached to cyclohexane carbons take two positions: axial and equatorial. Axial bonds point straight up or down and run parallel to the ring axis. Equatorial bonds point outward, roughly along...

Cyclohexane chair conformations are shaped by the position of a substituent, and that position can change how stable the molecule is. The two chair forms of cyclohexane interconvert rapidly at room temperature. For unsubstituted cyclohexane, both chairs have the same energy and the same stability, so each appears in equal amount at equilibrium. When a hydrogen atom is replaced by a functional group, the two chair forms are no longer equal. In methylcyclohexane, the CH3 group is axial in one...

disubstituted cyclohexanes: cis-trans isomerism

disubstituted-cyclohexanes-cis-trans-isomerism

Cyclohexane Chair Stability and Bulky Groups

Disubstituted cyclohexanes can differ in both cis-trans arrangement and chair stability. In a cyclohexane ring, the substituents may sit on the same side of the ring or on opposite sides. These cis and trans stereoisomers have different physical properties, and they cannot switch into each other without breaking carbon-carbon bonds. Cyclohexane also has chair conformers, which place substituents in axial or equatorial positions. For 1,4-dimethylcyclohexane, the cis isomer has two conformers...

combustion energy: a measure of stability in alkanes and cycloalkanes

combustion-energy-a-measure-of-stability-in-alkanes-and-cycloalkanes

Why Branched Alkanes Burn More Cleanly

Alkanes are low-reactivity hydrocarbons because their C–C and C–H sigma bonds are non-polar. That is why they were once called paraffins, from the Latin words parum, meaning “too little,” and affinis, meaning “affinity.” In the presence of excess oxygen and high heat, alkanes undergo combustion to form carbon dioxide and water. Combustion is the energy source for natural gas, liquified petroleum gas (LPG), fuel oil, gasoline, diesel fuel, and aviation fuel. The heat released in this reaction,...

key terms and concepts

See why axial and equatorial positions change cyclohexane stability, how 1,3-diaxial interactions raise energy, and why methylcyclohexane favors the equatorial chair.

Cyclohexane Chair Axial vs Equatorial | JoVE K12

organic chemistry

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Cyclohexane Chair Axial vs Equatorial

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