CYCLOBUTANE: STRUCTURE, PROPERTIES, USES AND SYNTHESIS - CHEMISTRY - 2025

The cyclobutane is a hydrocarbon consisting of a cycloalkane four carbons, with the molecular formula C ₄ H ₈. It can also be called tetramethylene, considering that they are four CH ₂ units that make up a ring with square geometry, although the name cyclobutane is more accepted and known.

At room temperature it is a colorless, flammable gas that burns with a bright flame. Its most primitive use is as a source of heat when burning; however, its structural base (the square) encompasses deep biological and chemical aspects, and contributes in some way to the properties of these compounds.

Cyclobutane molecule in a strained conformation. Source: Jynto.

The upper image shows a cyclobutane molecule with a square structure. In the next section, we will explain why this conformation is unstable, since its links are tight.

After cyclopropane, it is the most unstable cycloalkane, since the smaller the ring size, the more reactive it will be. Accordingly, cyclobutane is more unstable than the pentane and hexane cycles. However, it is curious to witness in its derivatives a nucleus or square heart, which, as will be seen, is dynamic.

Structure of cyclobutane

In the first image, the structure of cyclobutane was addressed as a simple carbonized and hydrogenated square. However, in this perfect square the orbitals suffer a severe twist from their original angles: they are separated by an angle of 90º compared to 109.5º for a carbon atom with sp ³ hybridization (angular tension).

Sp ³ carbon atoms are tetrahedral, and for some tetrahedra it would be difficult to flex their orbitals so much to create a 90º angle; but it would be even more so for carbons with sp ² (120º) and sp (180º) hybridizations to deviate their original angles. For this reason cyclobutane has essentially sp ³ carbon atoms.

Also, hydrogen atoms are very close to each other, eclipsed in space. This results in an increase in steric hindrance, which weakens the supposed square due to its high torsional stress.

Therefore, angular and torsional stresses (encapsulated in the term 'ring stress') make this conformation unstable under normal conditions.

The cyclobutane molecule will seek to reduce both stresses, and to achieve this it adopts what is known as the butterfly or puckered (in English, puckered) conformation.

Butterfly or wrinkled conformations

Conformations of cyclobutane. Source: Smokefoot.

The true conformations of cyclobutane are shown above. In them the angular and torsional stresses decrease; since, as can be seen, now not all hydrogen atoms are eclipsed. However, there is an energy cost: the angle of its bonds is sharpened, that is, it goes down from 90 to 88º.

Note that it can be compared to a butterfly, whose triangular wings are made up of three carbon atoms; and the fourth, positioned at an angle of 25º with respect to each wing. Two-way arrows indicate that there is an equilibrium between both conformers. It is as if the butterfly goes down and up its wings.

In cyclobutane derivatives, on the other hand, this flapping would be expected to be much slower and spatially impeded.

Intermolecular interactions

Suppose you forget the squares for a moment, and they are replaced by carbonated butterflies instead. These in their wing can only be held together in the liquid by London dispersion forces, which are proportional to the area of ​​their wings and their molecular mass.

Properties

Physical appearance

Colorless gas.

Molecular mass

56.107 g / mol.

Boiling point

12.6 ° C. Therefore, in cold conditions it could be handled in principle like any liquid; with the only detail, that it would be highly volatile, and its vapors would still represent a risk to consider.

Melting point

-91 ° C.

ignition point

50ºC in a closed glass.

Solubility

Insoluble in water, which is not surprising given its nonpolar nature; but, it is slightly soluble in alcohols, ether, and acetone, which are less polar solvents. It is expected to be soluble (although not reported) logically in nonpolar solvents such as carbon tetrachloride, benzene, xylene, etc.

Density

0.7125 at 5 ° C (relative to 1 of the water).

Vapor density

1.93 (in relation to 1 of the air). This means that it is denser than air, and therefore, unless there are currents, its tendency will not be to rise.

Vapor pressure

1,180 mmHg at 25 ° C.

Refractive index

1.3625 at 290 ° C.

Enthalpy of combustion

-655.9 kJ / mol.

Heat of formation

6.6 Kcal / mol at 25 ° C.

Synthesis

Cyclobutane is synthesized by hydrogenation of cyclobutadiene, whose structure is almost the same, with the only difference that it has a double bond; and therefore it is even more reactive. This is perhaps the simplest synthetic route to obtain it, or at least only to it and not to a derivative.

Obtaining it in crude oil is unlikely since it would end up reacting in such a way as to break the ring and form the line chain, that is, n-butane.

Another method to obtain the cyclobutane consists in striking ultraviolet radiation on the ethylene molecules, CH ₂ = CH ₂, which dimerize. This reaction is favored photochemically, but not thermodynamically:

Synthesis of cyclobutane by ultraviolet radiation. Source: Gabriel Bolívar.

The image above sums up very well what was said in the paragraph above. If instead of ethylene one had, for example, any two alkenes, a substituted cyclobutane would be obtained; or what is the same, a derivative of cyclobutane. In fact, many derivatives with interesting structures have been synthesized by this method.

Other derivatives, however, involve a series of complicated synthetic steps. Therefore, cyclobutanes (as their derivatives are called) are the object of study for organic syntheses.

Applications

Cyclobutane alone has no other use than serving as a heat source; but, its derivatives, enter intricate fields in organic synthesis, with applications in pharmacology, biotechnology and medicine. Without delving into overly complex structures, penitrems and grandisol are examples of cyclobutanes.

Cyclobutanes generally have properties that are beneficial for the metabolisms of bacteria, plants, marine invertebrates, and fungi. They are biologically active, and that is why their uses are very varied and difficult to specify, since each one has its particular effect on certain organisms.

Grandisol - an example of a cyclobutane derivative. Source: Jynto.

Grandisol, for example, is a pheromone from the weevil (a type of beetle). Above, and finally, its structure is shown, considered a monoterpene with a square base of cyclobutane.

References

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