12 julio, 2024

Cycloalkanes: properties, reactions, uses, examples

The cycloalkanes they are a family of saturated hydrocarbons with a general formula of CnH2n that coincides with that of 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 (polycyclic) ring.

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

Why are cycloalkanes more unstable than their open-chain counterparts? The reason can be suspected by looking from a bird’s eye view to the examples of cycloalkanes represented in the image above: there are stresses and steric (spatial) hindrances.

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

Small cycloalkanes are gases, and as their size increases, so does their intermolecular strength. Consequently, they can be liquids capable of dissolving fats and nonpolar molecules, lubricants, or solids that display dark colors and asphalt-like qualities.

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Physical and chemical properties

Polarity

Being composed only of carbons and hydrogens, atoms that by themselves do not differ too much in electronegativity, this makes the cycloalkanes molecules nonpolar and therefore lacking a 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, as 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 their boiling and melting points are higher.

Also, since they have two less hydrogen atoms (CnH2n for cycloalkanes and CnH2n+2 for alkanes), they are lighter; and adding to this the fact of their greater contact area, the volume occupied by their 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 as easy to break to produce other products. However, their relative stabilities differ, which can be checked experimentally by measuring their heats of combustion (ΔHcomb).

For example, when you compare the ΔHcomb for propane and cyclopropane (represented by a triangle in the image), you get 527.4 kcal/mol and 498.9 kcal/mol, respectively.

The detail is that cyclopropane, based on the heats of combustion of alkanes, should have a lower ΔHcomb (471 kcal/mol) because it consists of three methylene groups, CH2; but in reality, it releases more heat, reflecting a greater instability than estimated. This excess energy is then said to be due to 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 too high, cycloalkanes tend to be more stable than their respective alkanes.

Nomenclature

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, CH3CH2CH2CH2CH2CH3, 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 upper image.

Note that in a) 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 the lowest possible locating number. 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, then it is said that the latter is 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 represented in the first image.

Observing them may give rise to the false idea that such molecules are flat; but with the exception of cyclopropane, their surfaces are «zigzagging», with carbons going down or up in relation to the same plane.

This is due to the fact that to begin with, all the carbons are sp3 hybridized, and therefore present tetrahedral geometries with bond angles of 109.5º. But, if the geometry of the rings is carefully observed, it is impossible for their angles to be these; for example, the angles inside 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 109.5º, 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 said then that the angular tension ceases to exert such a remarkable destabilizing effect.

conformations

In addition to angle strain, there are other factors that contribute to the strain experienced by cycloalkanes.

CC links cannot simply rotate, as this would cause the entire structure to “shudder”. Thus, these molecules can adopt very well-defined spatial conformations. The purpose of these movements is to reduce the tensions caused by the eclipsing of the hydrogen atoms; that is, when they are facing 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 forms they can take in space.

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 inversions.

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

Above is shown as an example the combustion and halogenation reactions for cyclopentane. One mole of it burns in the presence of heat and 7.5 moles of molecular oxygen to break down into CO2 and H2O. On the other hand, in the presence of ultraviolet radiation and bromine, it replaces an H with a Br, releasing a gaseous molecule of HBr.

Applications

The use of cycloalkanes depends to a large extent on their number of carbons. The lightest, and therefore gaseous, were once used to power gas lamps for public lighting.

Liquids, for their part, have uses as solvents for oils, fats or commercial products of an apolar nature. Among these, mention may be made of cyclopentane, cyclohexane and cycloheptane. Likewise, they are often 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; such as carboplatin, which includes a cyclobutane ring in its structure.

Examples of cycloalkanes

To finish, go back to the beginning of the article: the image with several non-substituted cycloalkanes.

To memorize cycloalkanes, it is enough to think of 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 occurs more obviously with cyclotetradecane (of fourteen carbons).

There comes a point where they will behave like necklaces that can be folded to reduce the tension of their links and eclipsing as much as possible.

References

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

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