ALIPHATIC HYDROCARBONS: PROPERTIES, NOMENCLATURE, REACTIONS, TYPES - CHEMISTRY - 2025

The aliphatic hydrocarbons are those lacking aromaticity, not in an olfactory sense, but as regards the chemical stability. Classifying hydrocarbons in this way is nowadays too ambiguous and imprecise, since it does not discriminate between various types of hydrocarbons that are not aromatic.

Thus, we have aliphatic hydrocarbons and aromatic hydrocarbons. The latter are recognized by their basic unit: the benzene ring. The others, on the other hand, can adopt any molecular structure: linear, branched, cyclic, unsaturated, polycyclic; as long as they don't have conjugated systems like benzene.

The liquid fuel in lighters consists of a mixture of aliphatic hydrocarbons. Source: Pixnio.

The term 'aliphatic' was born from the Greek word 'aleiphar', which means fat, also used to refer to oils. Therefore, in the 19th century this classification was assigned to the hydrocarbons obtained from oily extracts; while aromatic hydrocarbons were extracted from tree and fragrant resins.

However, as the foundations of organic chemistry were consolidated, it was discovered that there was a chemical property that differentiated hydrocarbons, even more important than their natural sources: aromaticity (and not fragrance).

In this way, aliphatic hydrocarbons stopped being only those obtained from fats, to be all those lacking aromaticity. Within this family we have alkanes, alkenes and alkynes, regardless of whether they are linear or cyclic. That is why 'aliphatic' is considered imprecise; although it is useful to address some general aspects.

For example, when you say aliphatic 'ends' or 'chains', you are referring to molecular regions where aromatic rings are absent. Of all the aliphatic hydrocarbons, the simplest by definition is methane, CH ₄; while benzene is the simplest of the aromatic hydrocarbons.

Properties of aliphatic hydrocarbons

The properties of aliphatic hydrocarbons vary to different degrees depending on which ones are considered. There are those of low and high molecular masses, as well as linear, branched, cyclic or polycyclic, even those with amazing three-dimensional structures; as with the Cuban, cube-shaped.

However, there are some generalities that can be mentioned. Most aliphatic hydrocarbons are hydrophobic and apolar gases or liquids, some being more apolar than others, since even those in whose carbon chains contain halogen, oxygen, nitrogen or sulfur atoms are included in the list.

Likewise, they are flammable compounds, since they are susceptible to oxidation in the air with a minimal source of heat. This characteristic becomes more dangerous if we add its high volatility, due to the weak dispersive interactions that hold aliphatic molecules together.

We see this for example in butane, a gas that can liquefy relatively easily like propane. Both are highly volatile and flammable, making them active components in cooking gas or pocket lighters.

Of course, this volatility tends to decrease as the molecular mass increases and the hydrocarbon creates increasingly viscous and oily liquids.

Nomenclature

The nomenclature of hydrocarbons differs even more than their properties. If they are alkanes, alkenes or alkynes, the same rules stipulated by the IUPAC are followed: select the longest chain, assigning the lowest indicator numbers to the most substituted end or to the most reactive heteroatoms or groups.

In this way it is known on which carbon each substituent is found, or even the unsaturations (double or triple bonds). In the case of cyclic hydrocarbons, the name is preceded by the substituents listed in alphabetical order, followed by the word 'cycle', counting the carbon numbers that comprise it.

For example, consider the following two cyclohexanes:

Two cyclohexanes, which are classified as aliphatic hydrocarbons. Source: Gabriel Bolívar.

Cyclohexane A is called 1,4-dimethylcyclohexane. If the ring were five carbons, it would be 1,4-dimethylcyclopentane. Meanwhile, cyclohexane B is called 1,2,4-trimethylcyclohexane, and not 1,4,6-cyclohexane, since it is intended to use the lowest indicators.

Now, the nomenclature can get very complicated for hydrocarbons with bizarre structures. For them there are more specific rules, which must be explained separately and carefully; just like dienes, terpenes, polyenes, and polycyclic compounds.

Reactions

Combustion

Fortunately, the reactions are less varied for these hydrocarbons. One of them has already been mentioned: they burn easily, producing carbon dioxide and water, as well as other oxides or gases depending on the presence of heteroatoms (Cl, N, P, O, etc.). However, CO ₂ and H ₂ O are the main products of combustion.

Addition

If they present unsaturations, they can undergo addition reactions; that is, they incorporate small molecules into their backbones as substituents following a specific mechanism. Among these molecules we have water, hydrogen and halogens (F ₂, Cl ₂, Br ₂ and I ₂).

Halogenation

On the other hand, aliphatic hydrocarbons under the incidence of ultraviolet radiation (hv) and heat can break CH bonds to change them for CX bonds (CF, C-Cl, etc.). This is the halogenation reaction, which is observed in very short-chain alkanes, such as methane or pentane.

Cracking

Another reaction that aliphatic hydrocarbons, especially long-chain alkanes, can undergo is thermal cracking. It consists of supplying intense heat so that the thermal energy breaks the DC bonds, and thus small molecules are formed, which are more highly valued in the fuel market, from large molecules.

The four reactions above are the main ones that an aliphatic hydrocarbon can undergo, combustion being the most important of all, since it does not discriminate against any compound; all will burn in the presence of oxygen, but not all will add molecules or break down into small molecules.

Types

Aliphatic hydrocarbons group a myriad of compounds, which in turn are classified in a more specific way, indicating the degree of their unsaturations, as well as the type of structure they have.

According to how unsaturated they are, we have alkanes (saturated), alkenes and alkynes (unsaturated).

Alkanes are characterized by having single CC bonds, while in alkenes and alkynes we observe C = C and C≡C bonds, respectively. A very general way of visualizing them is to think of the carbon skeletons of alkanes as zigzagging and bent chains, being "squares" for alkenes, and "straight lines" for alkynes.

This is due to the fact that the double and triple bonds present energy and steric restriction in their rotations, "hardening" their structures.

Alkanes, alkenes, and alkynes can be branched, cyclic, or polycyclic. That is why cycloalkanes, cycloalkenes, cycloalkines, and compounds such as decalin (with a bicyclo structure), adamantane (similar to a baseball cap), heptalene, gonan, among others, are also considered aliphatic hydrocarbons.

Other types of hydrocarbons arise from alkenes, such as dienes (with two double bonds), polyenes (with many alternating double bonds), and terpenes (compounds derived from isoprene, a diene).

Applications

Again, the uses for these hydrocarbons can vary depending on which one is considered. However, in the properties and reactions sections it was made clear that they all combustion, not only to release gaseous molecules, but also light and heat. Thus, they are energy reservoirs, useful to serve as fuels or heat sources.

That is why they are used as part of the composition of gasoline, for natural gas, in Bunsen burners, and in general to be able to start fires.

One of the most notable examples is that of acetylene, HC≡CH, whose combustion enables the metal ions of a sample to be excited in the atomic absorption spectrometry carried out in analytical tests. Also, the resulting fire can be used for welding.

Liquid aliphatic hydrocarbons, such as paraffinics, are frequently used as extraction solvents for fats. In addition, its solvent action can be used to remove stains, enamels, paints, or simply to prepare solutions of a certain organic compound.

Those with the highest molecular mass, whether viscous or solid, are used for the production of resins, polymers or medicines.

As regards the term 'aliphatic', it is often used to refer to those regions, in a macromolecule, that lack aromaticity. For example, asphaltenes are superficially described as an aromatic nucleus with aliphatic chains.

Examples

At first it was said that methane is the simplest of the aliphatic hydrocarbons. This is followed by propane, CH ₃ CH ₂ CH ₃, butane, CH ₃ CH ₂ CH ₂ CH ₃, pentane, CH ₃ CH ₂ CH ₂ CH ₂ CH ₃, octane, nonane, decane, and so on, having alkanes each time longer.

The same applies to ethylene, CH ₂ = CH ₂, propene, CH ₃ CH = CH ₂, butene, CH ₃ CH ₂ CH = CH ₃, and for the rest of the alkynes. If there are two double bonds, they are dienes, and if there are more than two, polyenes. Likewise, there may be double and triple bonds in the same skeleton, increasing the structural complexity.

Among the cycloalkanes we can mention cyclopropane, cyclobutane, cyclopentane, cyclohexane, cycloheptane, cyclooctane, as well as cyclohexene and cyclohexine. Branched derivatives are in turn obtained from all these hydrocarbons, the available examples (such as 1,4-dimethylcyclohexane) multiplying even more.

Of the most representative terpenes we have limonene, menthol, pinene, vitamin A, squalene, etc. Polyethylene is a polymer saturated with -CH ₂ -CH ₂ - units, so it is also an example of these hydrocarbons. Other examples have already been cited in the previous sections.

References

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