CYCLOPENTANE (C5H10): STRUCTURE, PROPERTIES AND USES - CHEMISTRY - 2025

The cyclopentane is cyclic hydrocarbon, a cycloalkane specifically. In turn, it is an organic compound whose molecular formula is C ₅ H _{10. It} can be visualized as the closed version of n -pentane, with an open chain, in which its ends have been joined by the loss of two hydrogen atoms.

The lower image shows the skeleton of cyclopentane. Notice how geometric its skeleton looks, forming a pentagonal ring. However, its molecular structure is not flat, but rather presents folds that seek to stabilize and reduce the stress within the ring. Cyclopentane is a highly volatile and flammable liquid, but not as flammable as n -pentane.

Carbon skeleton of cyclopentane. Source: Ccroberts

Due to its solvent capacity, cyclopentane is one of the most widely used solvents in the chemical industries. It is not surprising that many products with strong odors contain it among their composition, being therefore flammable. It is also used as a blowing agent for polyurethane foams used in refrigerators.

Structure of cyclipentane

Intermolecular interactions

Molecular structure of cyclopentane represented by a sphere and rod model. Source: Jynto

In the first image the cyclopentane skeleton was shown. Above we now see that it is more than a simple pentagon: hydrogen atoms (white spheres) protrude at its edges, while carbon atoms make up the pentagonal ring (black spheres).

By having only CC and CH bonds, their dipole moment is negligible, so cyclopentane molecules cannot interact with each other through dipole-dipole forces. Instead, they are held together thanks to London's dispersive forces, with the rings trying to stack on top of each other.

This stacking offers some greater contact area than is available between linear n-pentane molecules. As a result of this, cyclopentane has a higher boiling point than n -pentane, as well as a lower vapor pressure.

Dispersive forces are responsible for cyclopentane forming a molecular crystal when frozen at -94 ºC. Although there is not much information regarding its crystalline structure, it is polymorphic and has three phases: I, II and III, with phase II being a disorderly mixture of I and III.

Conformations and ring stress

The cyclopentane ring is not totally flat. Source: Edgar181

The image above gives the false impression that cyclopentane is flat; but it's not like that. All its carbon atoms have sp ³ hybridization, so their orbitals are not in the same plane. Also, as if this were not enough, hydrogen atoms are very close to each other, strongly repelled when they are eclipsed.

Thus, we speak of conformations, one of them being that of a half chair (top image). From this perspective, it is clearly appreciated that the cyclopentane ring has bends, which help to reduce its ring tension due to its carbon atoms so close to each other.

This tension is due to the fact that the CC bonds present angles smaller than 109.5º, the ideal value for the tetrahedral environment as a result of their sp ³ hybridizations.

However, despite this stress, cyclopentane is a more stable and less flammable compound than pentane. This can be verified by comparing their safety diamonds, in which the flammability of cyclopentane is 3, while that of pentane, 4.

Properties of cyclipentane

Physical appearance

Colorless liquid with a mild petroleum-like odor.

Molar mass

70.1 g / mol

Melting point

-93.9 ºC

Boiling point

49.2 ºC

Flash point

-37.2 ºC

Autoignition temperature

361 ºC

Heat of vaporization

28.52 kJ / mol at 25 ºC

Viscosity

0.413 mPa s

Refractive index

1,4065

Vapor pressure

45 kPa at 20 ° C. This pressure corresponds to around 440 atm, however lower than that of n-pentane: 57.90 kPa.

Here the effect of the structure is manifested: the cyclopentane ring allows more effective intermolecular interactions, which binds and retains more of its molecules within the liquid compared to the linear molecules of n -pentane. Therefore, the latter has a higher vapor pressure.

Density

0.751 g / cm ³ at 20 ° C. On the other hand, its vapors are 2.42 times more dense than air.

Solubility

Only 156 mg of cyclopentane dissolves in one liter of water at 25ºC, due to its hydrophobic nature. However, it is miscible in nonpolar solvents such as other paraffins, ethers, benzene, carbon tetrachloride, acetone, and ethanol.

Octanol / water partition coefficient

3

Reactivity

Cyclopentane is stable when stored properly. It is not a reactive substance because its CH and CC bonds are not easy to break, even though it would lead to the release of energy caused by the tension of the ring.

In the presence of oxygen, it will burn in a combustion reaction, be it complete or incomplete. As cyclopentane is a very volatile compound, it must be stored in places where it cannot be exposed to any heat source.

Meanwhile, in the absence of oxygen, cyclopentane will undergo a pyrolysis reaction, decomposing into smaller and unsaturated molecules. One of them is 1-pentene, which shows that heat breaks the cyclopentane ring to form an alkene.

On the other hand, cyclopentane can react with bromine under ultraviolet radiation. In this way, one of its CH bonds is replaced by C-Br, which in turn can be replaced by other groups; and thus, cyclopentane derivatives are emerging.

Applications

Industrial solvent

The hydrophobic and apolar character of cyclopentane makes it a degreasing solvent, along with other paraffinic solvents. Because of this, it is often part of the formulations of many products, such as glues, synthetic resins, paints, adhesives, tobacco, and gasoline.

Ethylene source

When cyclopentane is subjected to pyrolysis, one of the most important substances it generates is ethylene, which has countless applications in the world of polymers.

Insulating polyurethane foams

One of the most notable uses of cyclopentane is as a blowing agent for the manufacture of insulating polyurethane foams; that is, the cyclopentane vapors, due to their great pressure, expand the polymeric material until it forms a foam with beneficial properties to be used in the framework of refrigerators or freezers.

Some companies have chosen to substitute HFCs for cyclopentane in the manufacture of insulating materials, since it does not contribute to the deterioration of the ozone layer, and also reduces the release of greenhouse gas into the environment.

References

1. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. (10 ^th edition.). Wiley Plus.
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5. Elsevier BV (2020). Cyclopentane. ScienceDirect. Recovered from: sciencedirect.com
6. GE Appliances. (2011, January 11). Reducing greenhouse gas emissions at GE's refrigerator manufacturing facilities. Recovered from: pressroom.geappliances.com