CYCLOALKANES: PROPERTIES, REACTIONS, USES, EXAMPLES - CHEMISTRY - 2024

The cycloalkanes are a family of saturated hydrocarbons with a general formula of C _n H _2n coinciding with alkenes; with the difference that the apparent unsaturation is not due to a double bond, but to a ring or cycle. That is why they are considered isomers of alkenes.

These are formed when linear alkanes join the ends of their chains to create a closed structure. As with alkanes, cycloalkanes can exhibit different sizes, molecular masses, substitutions, or even systems composed of more than one ring (polycyclic).

Some monocyclic cycloalkanes. Source: Mephisto spa via Wikipedia.

Otherwise, chemically and physically they are similar to alkanes. They have only carbons and hydrogens, are neutral molecules and therefore interact by means of Van der Walls forces. They also serve as fuels, releasing heat when they burn in the presence of oxygen.

Why are cycloalkanes more unstable than their open chain counterparts? The reason can be suspected from a bird's eye view of the examples of cycloalkanes represented in the image above: there are steric (spatial) tensions and impediments.

Note that the fewer carbons there are (listed in blue), the more closed the structure; and the opposite occurs when they increase, becoming like a necklace.

Small cycloalkanes are gaseous, and as their sizes increase, so do their intermolecular forces. Consequently, they can be liquids capable of dissolving fats and apolar molecules, lubricants, or solids that sport dark colors and qualities like those of asphalt.

Physical and chemical properties

Polarity

By being composed only of carbons and hydrogens, atoms that in themselves do not differ too much in electronegativity, this makes the cycloalkane molecules apolar and therefore lack dipole moment.

They cannot interact through dipole-dipole forces, but depend specifically on London forces, which are weak but increase with molecular mass. That is why small cycloalkanes (with less than five carbons) are gaseous.

Intermolecular interactions

On the other hand, since they are rings, cycloalkanes have a greater contact area, which favors the London forces between their molecules. Thus, they group and interact in a better way compared to alkanes; and hence its boiling and melting points are higher.

Also, since they have two fewer hydrogen atoms (C _n H _2n for cycloalkanes and C _n H _{2n + 2} for alkanes), they are lighter; and adding to this the fact of its greater contact area, the volume occupied by its molecules decreases, and therefore, they are more dense.

Saturation

Why are cycloalkanes classified as saturated hydrocarbons? Because they don't have a way to incorporate a hydrogen molecule; unless the ring is opened, in which case they would become simple alkanes. For a hydrocarbon to be considered saturated it must have the maximum possible number of CH bonds.

Stability

Chemically they are very similar to alkanes. Both have CC and CH bonds, which are not so easy to break to produce other products. However, their relative stabilities differ, which can be verified experimentally by measuring their heats of combustion (ΔH _comb).

For example, when comparing the ΔH _comb for propane and cyclopropane (represented by a triangle in the image), you have 527.4 kcal / mol and 498.9 kcal / mol, respectively.

The detail is that the cyclopropane, based on the heats of combustion of the alkanes, should have a lower ΔH _comb (471 kcal / mol) because they are three methylene groups, CH ₂; but in reality, it releases more heat, reflecting greater instability than estimated. This excess energy is then said to be due to the stresses within the ring.

And in fact, these tensions govern and differentiate the reactivity or stability of cycloalkanes, with respect to alkanes, against specific reactions. As long as the stresses are not very high, cycloalkanes tend to be more stable than their respective alkanes.

Nomenclature

Some examples of substituted cycloalkanes to test the nomenclature rules. Source: Gabriel Bolívar.

The IUPAC nomenclature for cycloalkanes does not differ much from that for alkanes. The simplest rule of all is to prefix cyclo- to the name of the alkane from which the cycloalkane is formed.

Thus, for example, from n-hexane, CH ₃ CH ₂ CH ₂ CH ₂ CH ₂ CH ₃, cyclohexane is obtained (represented by a hexagon in the first image). The same happens with cyclopropane, cyclobutane, etc.

However, these compounds can undergo substitutions of one of their hydrogens. When the number of carbons in the ring is greater than that of the alkyl substituents, the ring is taken as the main chain; this is the case of a) for the image above.

Note that in a) the cyclobutane (the square) has more carbons than the propyl group attached to it; then this compound is named as propylcyclobutane.

If there is more than one substituent, they should be named in alphabetical order and in such a way that they have as few locator numbers as possible. For example, b) is called: 1-bromo-4-fluoro-2-butylcycloheptane (and not 1-bromo-5-fluoro-7-butylcycloheptane, which would be incorrect).

And finally, when the alkyl substituent has more carbons than the ring, the latter is then said to be the substituent group of the main chain. Thus, c) is called: 4-cyclohexylnonane.

Structure

Leaving aside the substituted cycloalkanes, it is convenient to focus only on their structural bases: the rings. These were depicted in the first image.

Observing them can lead to the false idea that such molecules are flat; but with the exception of cyclopropane, its surfaces are "zigzagging", with carbons rising or falling in relation to the same plane.

This is due to the fact that to begin with, all the carbons have sp ³ hybridization, and therefore present tetrahedral geometries with bond angles of 109.5º. But, if the geometry of the rings is carefully observed, it is impossible that their angles are these; for example, the angles within the cyclopropane triangle are 60º.

This is what is known as angular stress. The larger the rings, the angle between the CC bonds is closer to 109.5 °, which causes a decrease in said tension and an increase in stability for the cycloalkane.

Another example is observed in cyclobutane, whose bond angles are 90º. Already in cyclopentane its angles are 108º, and from cyclohexane it is then said that the angular stress ceases to exert such a marked destabilizing effect.

Conformations

In addition to angular stress, there are other factors that contribute to stress experienced by cycloalkanes.

The CC bonds cannot simply rotate, as this would imply that the entire structure would “shake”. Thus, these molecules can adopt very well defined spatial conformations. The purpose of these movements is to reduce the stresses caused by the eclipse of the hydrogen atoms; that is, when they are opposite each other.

For example, the conformations for cyclobutane resemble a butterfly flapping its wings; those of cyclopentane, an envelope; those of cyclohexane, a boat or chair, and the larger the ring, the greater the number and shapes they can take in space.

Inversions between chair-like and boat-like conformation for cyclohexane. Source: Keministi.

The top image shows an example of such conformations for cyclohexane. Note that the supposed flat hexagon actually looks more like a chair (on the left of the image) or a boat (on the right). One hydrogen is represented by a red letter, and another by a blue letter, to indicate how their relative positions change after the inversions.

In (1), when hydrogen is perpendicular to the plane of the ring, it is said to be in the axial position; and when it is parallel to it, it is said to be in the equatorial position.

Reactions

The reactions that cycloalkanes can undergo are the same as for alkanes. Both burn in the presence of excess oxygen in typical combustion reactions to produce carbon dioxide and water. Likewise, both can undergo halogenations, in which a hydrogen is replaced by a halogen atom (F, Cl, Br, I).

Reactions of cyclopentane. Source: Gabriel Bolívar.

The combustion and halogenation reactions for cyclopentane are shown by way of example above. One mole of it burns in the presence of heat and 7.5 moles of molecular oxygen to decompose into CO ₂ and H ₂ O. On the other hand, in the presence of ultraviolet radiation and bromine, it substitutes one H for one Br, releasing a gaseous molecule by HBr.

Applications

The use of cycloalkanes is highly dependent on their carbon number. The lightest, and therefore gaseous, were once used to power gas lamps in public lighting.

Liquids, on the other hand, have utilities as solvents for oils, fats or commercial products of a nonpolar nature. Among these, mention may be made of cyclopentane, cyclohexane and cycloheptane. They are also used very frequently in routine operations in oil laboratories, or in the formulation of fuels.

If they are heavier, they can be used as lubricants. On the other hand, they can also represent the starting material for the synthesis of drugs; like carboplatin, which includes a cyclobutane ring in its structure.

Examples of cycloalkanes

Finally, we return to the beginning of the article: the image with several unsubstituted cycloalkanes.

To memorize cycloalkanes, just think about the geometric figures: triangle (cyclopropane), square (cyclobutane), pentagon (cyclopentane), hexagon (cyclohexane), heptagon (cycloheptane), decagon (cyclodecane), pentadecagon (cyclopentadecane), and so on..

The larger the ring, the less it resembles its respective geometric figure. It has already been seen that cyclohexane is anything but a hexagon; the same is more evident with cyclotetradecane (fourteen carbons).

There comes a point where they will behave like necklaces that can be folded to minimize the stresses of their links and eclipsing.

References

1. Morrison, RT and Boyd, R, N. (1987). Organic Chemistry. 5th Edition. Editorial Addison-Wesley Interamericana.
2. Carey F. (2008). Organic Chemistry. (Sixth edition). Mc Graw Hill.
3. Graham Solomons TW, Craig B. Fryhle. (2011). Organic Chemistry. Amines. (10th edition.). Wiley Plus.
4. Chemistry LibreTexts. (June 02, 2019). Naming Cycloalkanes. Recovered from: chem.libretexts.org
5. Wikipedia. (2019). Cycloalkane. Recovered from: en.wikipedia.org
6. Clark Jim. (2015). Introducing alkanes and cycloalkanes. Recovered from: chemguide.co.uk
7. James Ashenhurst. (2019). Conformations and Cycloalkanes. Master Organic Chemistry. Recovered from: masterorganicchemistry.com
8. Fernández Germán. (sf). Cycloalkanes-theory. Organic Chemistry. Recovered from: quimicaorganica.org