What are hydrocarbons and how are these technically classified?

Hydrocarbons are organic compounds consisting solely of carbon and hydrogen atoms; they represent the fundamental basis of organic chemistry; these compounds constitute the formulation of countless materials, chemical products and energy sources of everyday use.

Understanding what are hydrocarbons? and their technical classification allows us to analyze their molecular properties, chemical behavior, industrial applications and environmental effects. From their molecular formula to their physicochemical properties, these compounds are grouped into categories according to their molecular structure, bond type and atomic arrangement.

What are hydrocarbons?

Hydrocarbons are acyclic or cyclic organic compounds formed only by bonds between carbon and hydrogen atoms. Their existence is due to the multiple forms of carbon hybridization, which allows the formation of saturated or unsaturated, linear, branched and aromatic structures.

From the structural point of view, they are the basis for energetic and synthetic systems, being studied for their behavior in addition, substitution or combustion reactions, as well as for their participation in industrial processes of controlled molecular transformation.

The carbon and hydrogen molecule in hydrocarbons

The core of hydrocarbons is their carbon and hydrogen molecule; their structure is based on the unique ability of the carbon atom to bond, generating stable chains and rings; each carbon atom, tetravalent, can establish up to four covalent bonds with other carbon or hydrogen atoms, which saturate the remaining valences; generating different structural forms: linear, branched, cyclic and, finally, aromatic molecules.

The general hydrocarbon formula is expressed as CnHm, where the subscripts “n” and “m” vary depending on the specific type and size of the hydrocarbon. However, it is the nature of the carbon-carbon bonds, whether single (C-C), double (C=C) or triple (C≡C) that dictates their macroscopic properties and hence their classification.

This structural hydrocarbon composition directly influences specific characteristics such as reactivity, combustion and solubility, fundamental to their chemical and industrial behavior.

Chemical classification: Aliphatic and aromatic hydrocarbons

The technical classification of hydrocarbons is based on structural criteria such as the nature of the carbon-carbon bonds and the molecular topology of the atoms. They are therefore separated into two major groups: aliphatic hydrocarbons and aromatic hydrocarbons, each with a markedly different profile of chemical properties and industrial applications.

Aliphatic hydrocarbons: Saturated and unsaturated

Aliphatic hydrocarbons (derived from the Greek aleiphar, meaning “fat”, alluding to their origin by chemical decomposition of fats or oils) are organic compounds characterized by their lack of benzene rings in their molecular architecture. Their subdivision (alkanes, alkenes and alkynes) is established based on the multiplicity of the carbon-carbon bonds that join their atoms.

Saturated hydrocarbons: Alkanes

Alkanes represent the most elemental and least reactive class of hydrocarbons. Their structural distinctiveness lies in the exclusive possession of single bonds between their carbon atoms, resulting in a complete saturation of valences with hydrogen atoms. The hydrocarbon formula for open-chain alkanes is CnH₂n+₂; chemically stable, they are major components of natural gas and gasoline.

The following Cognito video explains the physical properties of alkanes and their behavior in complete combustion reactions, highlighting how they change with increasing carbon chain length.

Unsaturated hydrocarbons: Alkenes and alkynes

These compounds are characterized by the presence of multiple bonds (double or triple), which leads to an unsaturation in the number of hydrogen atoms.

• Alkenes: These have carbon-carbon (C=C) double bonds. Their general formula is CnH₂n; these demonstrate greater reactivity compared to alkanes, being widely used in the petrochemical industry for the synthesis of polymers, such as polyethylene.
• Alkynes: These stand out for the presence of carbon-carbon triple bonds (C≡C). Their general formula is CnH₂n-2; and they exhibit the highest reactivity within the aliphatic hydrocarbons, being employed in advanced organic syntheses and in industrial processes such as autogenous welding with acetylene.

Cycloalkanes and cycloalkenes

These are cyclic variants of aliphatic hydrocarbons, where the carbon atoms are bonded to form one or more closed rings; cycloalkanes have only single bonds within the ring, while cycloalkenes contain double bonds within the ring. These cyclic structures modify the physical properties and reactivity compared to their acyclic counterparts.

Aromatic hydrocarbons: Benzene rings and arenes

These hydrocarbons form a unique class of compounds, historically associated with “pleasant aromas” due to their origin in plant extracts. Their technical definition centers on the presence of one or more benzene rings as their fundamental structural unit, and this subclass is referred to as arenes. 

Benzene C₆H₆, the prototypical aromatic hydrocarbon, is a planar molecule of six carbon atoms in a ring; its particular structure lies in the electronic delocalization pi (π), a phenomenon that gives it thermodynamic stability, differentiating it from other cyclic compounds.

In addition to the classical arenes, the classification of aromatics includes non-benzene aromatic hydrocarbons. Although these compounds do not possess a benzene ring, they do exhibit stability derived from conjugated π-electron systems that adhere to Hückel’s rule (4n+2 π electrons). Examples of arenes include benzene itself and toluene, precursors in the manufacture of solvents, synthetic resins and explosives.

However, despite their usefulness, it is important to note that a large number of these compounds are recognized for their toxicity and carcinogenic potential; this characteristic has prompted the development and implementation of strict regulations on their handling and industrial use.

The hydrocarbon formula: Chemical isomerism 

This formula is represented in several ways: empirical, molecular or structural. The structural formula, particularly relevant in organic chemistry, illustrates the arrangement of atoms and the type of bonds (single, double or triple bonds) between carbon and hydrogen atoms.

Example: Ethane: C₂H₆ (molecular formula) → CH₃-CH₃ (structural formula). These representations allow the inference of properties such as boiling point, polarity, energy density and chemical reactivity. 

The structural formula also facilitates the analysis of isomerism, common in hydrocarbons with more than three carbons. Different structures can share the same molecular formula, but with different behavior.

This leads to structural (different bond order) and geometrical (cis/trans in alkenes) isomers. These structural differences influence the reactivity, boiling points and specific industrial applications of each isomer.

Physicochemical properties of hydrocarbons

These properties of hydrocarbons are a direct consequence of their molecular structure, degree of saturation and carbon chain length. The most relevant ones include:

• Boiling and melting point: This property is related to the number of carbon atoms, attributable to an increase in Van der Waals forces. Low molecular weight hydrocarbons (e.g., methane, ethane, propane) are gases at room temperature; those with medium chains are liquids, and those with long chains tend to be waxy solids.
• Density: Aromatics have a higher density compared to aliphatics of comparable molar mass, due to the structure of their benzene rings.
• Solubility: Due to the non-polar nature, they are insoluble in water, but dissolve readily in apolar organic solvents such as ether, benzene or chloroform.
• Chemical reactivity: Varies with bond type: saturated ones (alkanes) are notably less reactive, while unsaturated ones (alkenes and alkynes) exhibit greater reactivity due to the presence of multiple bonds.
• Flammability: This is a property especially in light hydrocarbons, which is the basis for their use as fossil energy in controlled combustion processes.

These properties are determinant for the optimum conditions for processing, storage, transport and application in chemical synthesis and energy generation.

Environmental effects of the use of hydrocarbons

The massive use of these compounds leads to serious environmental concerns; because the environmental effects of burning hydrocarbons are considerable. 

The combustion of fossil fuels releases large volumes of carbon dioxide (CO₂), the main greenhouse gas, contributing to global warming. Nitrogen oxides (NOx), sulfur oxides (SOx) and particulate matter are also emitted, aggravating atmospheric pollution, contributing to the formation of tropospheric ozone and triggering phenomena such as acid rain.

Additional environmental impacts:

• Acceleration of global warming due to CO₂ and methane emissions.
• Deterioration of air quality with consequent respiratory ailments in the population.
• Oil spills with impacts on soil and marine ecosystems.

In response to these impacts, the industry and the scientific community are continuously driving innovation, implementing strategies such as the development of biofuels, the application of carbon capture and storage (CCS) technologies, and research into low-carbon synthetic fuels, in an effort to mitigate the ecological footprint of these compounds.

Energía fósil y aplicaciones industriales 

Los hidrocarburos, en sus diversas formas de energía fósil (petróleo, gas natural y carbón), constituyen la fuente primaria de energía a nivel global, representando más del 80 % del consumo energético. Sus aplicaciones son múltiples en sectores industriales y cotidianos.

Main energy applications

Electricity generation: In thermal plants that operate with natural gas or fuel oil. 

• Transportation: As fuels for vehicles (gasoline, diesel), aircraft (jet fuel) and ships (LNG – Liquefied Natural Gas). 
• Heating: For industrial and residential use, using LPG (Liquefied Petroleum Gas) or natural gas.

However, its dominant energy function, the transition towards renewable energy sources, requires a progressive reduction in its consumption, promoting alternatives with a lower environmental impact.

Applications in industrial processes

Aliphatic hydrocarbons: Alkanes, alkenes and alkynes, in particular, are the fundamental raw materials for the petrochemical industry. From them are manufactured:

• Liquid and gaseous fuels with high added value (gasoline, LPG, naphtha).
• Lubricants, industrial waxes and a wide variety of organic solvents.
• High-production polymers such as polyethylene and polypropylene.
• Synthetic rubbers, detergents and synthetic textile fibers such as polyester.

Their versatility lies in the ease with which they can be chemically modified by processes such as cracking, catalytic reforming and polymerization.

Aromatic hydrocarbons: Especially arenes such as benzene, toluene and xylenes (BTX) have a highly stable electronic structure that makes them chemical intermediates of strategic value. Their main applications include:

• Manufacture of technical plastics (e.g. polystyrene, polycarbonate).
• Production of high-performance synthetic fibers (e.g. nylon, Kevlar).
• Synthesis of epoxy and phenolic resins.
• Development of dyes, pharmaceuticals, pesticides and explosives.
• Development of highly efficient industrial solvents. 

Although its use is subject to strict regulations due to its toxicity in many countries, its industrial value persists as strategic in sectors as diverse as automotive, electronics, textiles and pharmaceuticals.

Synthesis: Technical classification of hydrocarbons

Type	Structure	Link	Examples
Alkane	Linear, branched	Single (C-C)	Methane, butane
Alkene	Linear, branched	Double (C=C)	Ethene, propene
Alkyne	Linear, branched	Triple (C≡C)	Ethene, butino
Cycloalkane	Cyclic	Single	Cyclopentane, cyclohexane
Cycloalkene	Cyclic	Double (C=C)	Cyclohexene
Aromatic (sands)	Cyclic (Benzene Ring)	Conjugated (delocalized)	Benzene, toluene, xylene
Aromatic (not sands)	Cyclic (Conjugate System)	Conjugated (delocalized)	Azulene, fulvene

Conclusions

Knowing what hydrocarbons are allows us to understand why their classification into aliphatic hydrocarbons and aromatic hydrocarbons is essential to anticipate their behavior in different industrial applications. While the former comprise simple structures such as alkanes, alkenes and alkynes, the latter are defined by conjugated rings with specific electronic properties, which condition both their reactivity and their environmental risks.

Moving towards a more responsible and sustainable use of hydrocarbons represents a priority action in the search for industrial efficiency with environmental responsibility. In this context, the future of organic and energy chemistry will depend, to a large extent, on the ability to redesign processes, optimize the molecular use of carbon and apply sustainability criteria throughout the value chain associated with these compounds.

References

1. https://www.britannica.com/science/hydrocarbon/Cycloalkanes
2. https://es.wikipedia.org/wiki/Hidrocarburo