Name: cyclohexane chair stability and bulky groups
Uploaded: 2023-04-30T18:28:33.000Z
Duration: 2 min 37 s
Channel: JoVE

A disubstituted cycloalkane can have two distinct stereoisomers &#8212; cis and trans &#8212; depending on the relative spatial orientation of the two substituents.
While the cis isomer has both substituents on the same side of the ring, the trans isomer has its substituents on the opposite sides.
The isomers have different physical properties and are not interconvertible by rotation about the C-C bond. As such, they are never in equilibrium.
Consider the stereoisomers of 1,4-dimethylcyclohexane.
The two chair conformations of the cis isomer have equal energies and equilibrate in equal proportions.
Both conformations of the cis isomer have one axial methyl group and one equatorial methyl group, and their positions switch when the ring flips.
In contrast, trans-1,4-dimethylcyclohexane exists in two non-equivalent chair forms with the methyl groups facing opposite directions.
One chair conformer has both methyl groups in the axial position, whereas the alternative conformer has the methyl groups equatorially arranged.
Due to strong steric repulsions, the trans-diaxial form has higher energy than the trans-diequatorial form, making the latter more stable and abundant at equilibrium.
Now consider

disubstituted cyclohexanes: cis-trans isomerism

Disubstituted Cyclohexanes: cis-trans Isomerism

Depending upon the different spatial orientation of the substituents, the disubstituted cycloalkanes exhibit two types of stereoisomers. The cis isomers have the substituents on the same side of the ring, whereas the trans isomers have the substituents on the opposite sides. These stereoisomers exhibit different physical properties and cannot be interconverted without breaking the carbon-carbon bonds.
In cyclohexane, the substituents can occupy different positions generating distinct isomers. For instance, in the case of 1,4-dimethylcyclohexane, the cis isomer has two conformers equilibrating in equal proportions. Both conformers have one methyl group in the axial and the other in the equatorial positions. However, the conformers of the trans isomer are energetically non-equivalent. The trans conformer having both the methyl groups in the axial position experiences strong steric repulsion and is less stable than the diequatorial form. Hence the trans-diequatorial form is most abundant at equilibrium.
The isomers of 1,2-dimethylcyclohexane are similar to that of 1,4 counterparts. The trans-1,2-dimethylcyclohexane has two energetically non-equivalent conformers. Due to the absence of 1,3-diaxial interactions, the diequatorial form is more stable and abundant than the diaxial conformer. In contrast, both conformers of the cis-isomer have equivalent energies with one axial and one equatorial methyl group.
The presence of bulky substituents alters the equilibrium because the bulky substituent preferentially occupies the equatorial position to avoid the strong steric repulsion in the axial position.

Depending upon the different spatial orientation of the substituents, the disubstituted cycloalkanes exhibit two types of stereoisomers. The cis isomers ...

disubstituted-cyclohexanes-cis-trans-isomerism

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JoVE Core

Organic Chemistry

Unit 1

Alkanes and Cycloalkanes

KEY TERMS AND CONCEPTS

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 that are present in equal amounts. In both forms, one methyl group is axial and the other is equatorial. The trans isomer has two conformers that are not equal in energy. The form with both methyl groups axial is less stable because it has strong steric repulsion, so the diequatorial form is most common at equilibrium.

A similar pattern appears in 1,2-dimethylcyclohexane. The trans isomer has two conformers with different energies. The diequatorial conformer is more stable and more abundant than the diaxial conformer because it avoids 1,3-diaxial interactions. In the cis isomer, both conformers have the same energy, and each one has one axial methyl group and one equatorial methyl group.

Bulky substituents can shift this balance even more. A bulky group usually prefers the equatorial position because the axial position creates stronger steric repulsion. This preference helps explain which cyclohexane conformer is most stable at equilibrium.

Cyclohexane Chair Stability and Bulky Groups

cyclohexane-chair-stability-and-bulky-groups

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...

stability of substituted cyclohexanes

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...

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

Explore how cis and trans disubstituted cyclohexanes differ in chair forms, axial and equatorial positions, and stability at equilibrium.

Cyclohexane Chair Stability and Bulky Groups | JoVE K12

organic chemistry

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Cyclohexane Chair Stability and Bulky Groups

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