Name: cycloalkane ring strain and shapes
Uploaded: 2023-04-30T18:28:33.000Z
Duration: 2 min 29 s
Channel: JoVE

The relative stabilities of cycloalkanes vary with their ring sizes. The variations arise from the combined effects of angle strain and torsional strain, together known as the ring strain of cyclic compounds.
Angle strain is introduced in a cycloalkane when the C-C-C bond angle deviates from the regular tetrahedral bond angle of 109.5&#176;, as predicted for all sp3 &#160;hybridized carbons of alkanes.
On the other hand, the torsional strain exists between the eclipsing bonds&#160;and is a result of repulsive dispersion forces.
In cyclopropane, the internal angle of 60&#176;, which is significantly smaller than the ideal angle, introduces a severe angle strain in the molecule, forcing the sp3 orbitals to overlap at an angle to give weaker &#8220;bent&#8221; carbon-carbon bonds.
Cyclopropane, owing to its planar nature, also experiences considerable torsional strain from the six pairs of fully eclipsed C-H bonds.
In essence, cyclopropane is a highly strained molecule with strain energy as high as 116 kJ/mol. For this reason, cyclopropanes are highly reactive.
Unlike cyclopropane, cyclobutane is non-planar and takes up a folded conformation. Folded cyclobutane is more stable than its hypothetical planar form.
Altho

conformations of cycloalkanes

Conformations of Cycloalkanes

Adolf von Baeyer attempted to explain the instabilities of small and large cycloalkane rings using the concept of angle strain &#8212; the strain caused by the deviation of bond angles from the ideal 109.5&#176; tetrahedral value for sp3 &#160;hybridized carbons. However, while cyclopropane and cyclobutane are strained, as expected from their highly compressed bond angles, cyclopentane is more strained than predicted, and cyclohexane is virtually strain-free. Hence, Baeyer&#8217;s theory that was based on the assumption that all cycloalkanes are flat was wrong, and, in reality, most cycloalkanes adopt a non-planar structure.
Cyclopropane, the three-carbon cyclic alkane, has the highest angle strain since its planar structure is highly compressed, deviating by 49.5&#176; from the ideal value. Additionally, cyclopropane has a torsional strain due to the eclipsing interaction between six C-H bonds. Hence, cyclopropane has an overall ring strain of 116 kJ/mol. Unlike cyclopropane, which is planar, cyclobutane takes up a more stable, folded non-planar conformation. Folding causes the angle strain to be slightly elevated compared to the hypothetical planar cyclobutane, but the torsional strain from the ten eclipsing hydrogens is greatly relieved. Cyclobutane has an overall strain of 110 kJ/mol. Cyclopentane also adopts a non-planar conformation known as envelope conformation. Compared to the hypothetical planar form of cyclopentane, the envelope form has its bond angles slightly reduced, which marginally increases the angle strain. However, it significantly alleviates the torsional strain from ten eclipsing C-H bonds. Hence, the overall strain in cyclopentane is 27 kJ/mol.

Adolf von Baeyer attempted to explain the instabilities of small and large cycloalkane rings using the concept of angle strain &#8212; the strain caused ...

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Organic Chemistry

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Alkanes and Cycloalkanes

KEY TERMS AND CONCEPTS

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 predicted, and cyclohexane is almost strain-free. This shows that Baeyer’s theory, which assumed all cycloalkanes were flat, was wrong.

Most cycloalkanes are not planar in real life. Cyclopropane, the three-carbon ring, has the highest angle strain because its flat shape is tightly compressed, with bond angles that deviate by 49.5° from the ideal value. It also has torsional strain, which comes from eclipsing interactions between six C-H bonds. Its overall ring strain is 116 kJ/mol.

Cyclobutane is more stable when it folds into a non-planar shape. Folding slightly increases angle strain compared with a hypothetical flat cyclobutane, but it greatly relieves torsional strain from ten eclipsing hydrogens. Its overall strain is 110 kJ/mol.

Cyclopentane also bends out of plane and forms an envelope conformation. In this shape, the bond angles are slightly reduced compared with a flat ring, so angle strain rises a little. At the same time, the envelope form strongly reduces torsional strain from ten eclipsing C-H bonds. The overall strain in cyclopentane is 27 kJ/mol.

Cycloalkane Ring Strain and Shapes

structure of alkanes

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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...

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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...

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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

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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...

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: cis-trans isomerism

disubstituted-cyclohexanes-cis-trans-isomerism

Cyclohexane Chair Stability and Bulky Groups

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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

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key terms and concepts

Learn how ring strain, angle strain, and torsional strain affect cyclopropane, cyclobutane, and cyclopentane, and why non-planar shapes are more stable.

Cycloalkane Ring Strain and Shapes | JoVE K12

organic chemistry

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Cycloalkane Ring Strain and Shapes

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