Hydrocarbons are organic molecules consisting entirely of carbon and hydrogen, such as methane (CH4). The many covalent bonds between the atoms in hydrocarbons store a great amount of energy, which releases when these molecules burn (oxidize).
Methane is the simplest hydrocarbon molecule, with a central carbon atom bonded to four different hydrogen atoms. The shape of its electron orbitals determines the shape of the methane molecule’s geometry, where the atoms reside in three dimensions. The carbons and the four hydrogen atoms form a tetrahedron, with four triangular faces. For this reason, we describe methane as having tetrahedral geometry.
Generalities and classes
As the backbone of the large molecules of living things, hydrocarbons may exist as linear carbon chains, carbon rings, or combinations of both. Furthermore, individual carbon-to-carbon bonds may be single, double, or triple covalent bonds, and each type of bond affects the molecule’s geometry in a specific way. This three-dimensional shape or conformation of the large molecules of life (macromolecules) is critical to how they function.
The tendency of carbon atoms to bind together forming open chains, more or less long, branched or not, or rings with or without side chains, makes possible the existence of many types of hydrocarbons. There are three major classes aliphatic hydrocarbons, which possess an open chain of carbon atoms and for this reason are also called acyclic; they can be saturated and unsaturated (aliphatic, hydrocarbons); alicyclic hydrocarbons, also called, less properly, naphthenic (because many are contained in petroleum), which are cyclic, that is, with a closed chain (alicyclic, hydrocarbons); aromatic hydrocarbons, characterized by the presence of at least one benzene ring (aromatic, hydrocarbons).
The great variety of molecules with different structures and properties, which characterizes hydrocarbons, finds its origin in the formation between hydrogen and carbon atoms of stable covalent bonds and properly oriented in space. The many combinations that can be assumed by hydrocarbon molecules are due to the ability of the carbon atom to form covalent bonds, single, double and triple.
To the wide class of aliphatic hydrocarbons belong alkanes, alkenes and alkynes. The former, also called paraffins, are characterized by having all the simple carbon bonds saturated with hydrogen atoms or other carbon atoms. The simplest alkane is methane, which was discovered in 1776 by Alessandro Volta during a boat trip on Lake Maggiore near Angera, where he observed a hitherto unknown gas bubble rising from the muddy bottom, which he called “inflammable air native to swamps”.
In addition to possessing linear and branched structures, alkanes are also present as cyclic and polycyclic compounds. Alkenes are molecules characterized by the presence of at least one double bond between two carbon atoms. In these compounds, also referred to as olefins, the double bond can be interpreted as the result of a dehydrogenation of the corresponding alkane.
The carbon atoms participating in the double bond are sp2 hybridized and, in addition to the carbon atom with which they share the double bond, are bonded to two other carbon or hydrogen atoms. The three bonds of the unsaturated carbon atom are planar and form 120° bonds between them. Finally, alkynes, the parent of which is acetylene, are unsaturated hydrocarbons in which there is a carbon-carbon triple bond involving sp-hybridized carbon atoms, bonded to another atom, either carbon or hydrogen, and forming 180° angles between them.
The other major class of compounds formed from hydrogen and carbon is the aromatic hydrocarbons. The founder of this class is benzene, discovered by Michael Faraday in 1825 shortly after his appointment as director of the laboratory of the Royal Institution, isolating it from the distillation products of an oil obtained as a by-product of the manufacture of illuminating gas. It is a very stable compound for which Friedrich August Kekulè proposed the typical cyclic structure with three adjacent double bonds that seems to have been inspired by a dream. In-depth study of the characteristics of benzene has given rise to an important field of theoretical organic chemistry.
The main sources of hydrocarbons are currently natural gas, oil, and coal, which are the main energy sources currently available, but they are also used as raw materials in the chemical industry. Their composition depends mainly on the places of extraction. In general it is possible to classify them according to the ratio between hydrogen and carbon contained: therefore we go from H/C ratios included between 3 and 4 for natural gas, to values included between 1.2 and 2.5 for oil and less than unity in coal.
Hydrocarbons are the basic substances from which, in principle, all other organic compounds can be derived by replacing one or more hydrogen atoms with atoms of other elements or with different atomic groups. They can be liquid, solid, gaseous; generally they are colorless but those with many conjugate bonds (such as polydienes and polycondensed aromatic hydrocarbons) can be colored, as, for example, carotene and orange-red rubrene, pyrene and yellow cholanthrene.
They are practically insoluble in water but soluble in organic solvents; in the latter their solubility decreases with increasing molecular weight, so that some of the polycyclic aromatic hydrocarbons and the upper terms of the aliphatics may be considered practically insoluble. The melting and boiling points also increase with molecular weight. The physical properties of hydrocarbons also depend on the molecular structure: e.g., branched hydrocarbons have lower boiling points than linear-chain isomers.
Hydrocarbons are all combustible compounds, which give carbon dioxide and water on complete combustion; in general, their ignition temperature in air, all things being equal, increases with the number of carbon atoms. The chemical properties of hydrocarbons vary with the presence or absence of unsaturated bonds and, in part, with molecular structure. Paraffinic hydrocarbons do not show much chemical activity (kerosene).
Unsaturated aliphatic hydrocarbons, due to the presence of double and triple bonds, give addition reactions with various elements and compounds: they add chlorine, bromine, hydrogen, halogen acids, sulfuric acid, acetic acid, etc. The oxidizing media easily attack them forming different products according to the conditions in which the reaction takes place: so for oxidation of olefins with potassium permanganate in alkaline solution glycols are obtained, with ozone ozonides are obtained which in water split giving aldehydes or ketones.
Aromatic hydrocarbons, compared to other unsaturated ones, give more easily substitution reactions on the ring rather than addition: sulfuric acid, nitric acid and in certain conditions halogens, alkyl and acyl chlorides react with benzene substituting one or more hydrogen atoms. By action of energetic oxidants the aromatic ring can be opened with formation of unsaturated bicarboxylic acids.
Diffusion and use
In nature, hydrocarbons are widely diffused: among the gaseous ones, the most common is methane, often in mixture with higher hydrocarbons (propane, butane, pentane, hexane, heptane). Liquid hydrocarbons make up the various types of crude oil (paraffinic, naphthenic, mixed base, aromatic). Solid and semisolid hydrocarbons constitute gilsonite, ozocerite, elate;rite, hatchettite, fichtelite, simonellite, bombicite, hydralite, succinite, and others less frequently encountered.
Many hydrocarbons can be prepared synthetically from simpler hydrocarbons or by decomposition of complex hydrocarbons (as in the case of distillation and hydrogenation of coal, cracking of medium and heavy fractions of oil, etc.).
Hydrocarbons are used as fuels, solvents, fuels; as starting products for the synthesis of dyes, pharmaceutical products, to manufacture plastics and synthetic rubber, as ingredients of essences and perfumes. Thanks to the application of biotechnological processes, hydrocarbons can also be used in the food industry for the production of proteins used as animal feed supplements. These are the so-called SCP (single cell protein), which can be obtained by inoculating a suitable microorganism in a fermenter containing nutrients (substrates) suitable for the multiplication of the microorganism itself.
Successive bonds between carbon atoms form hydrocarbon chains. These may be branched or unbranched. Furthermore, a molecule’s different geometries of single, double, and triple covalent bonds alter the overall molecule’s geometry. The hydrocarbons ethane, ethene, and ethyne serve as examples of how different carbon-to-carbon bonds affect the molecule’s geometry. The names of all three molecules start with the prefix “eth-,” which is the prefix for two carbon hydrocarbons.
The suffixes “-ane,” “-ene,” and “-yne” refer to the presence of single, double, or triple carbon-carbon bonds, respectively. Thus, propane, propene, and propyne follow the same pattern with three carbon molecules, butane, butene, and butyne for four carbon molecules, and so on. Double and triple bonds change the molecule’s geometry: single bonds allow rotation along the bond’s axis; whereas, double bonds lead to a planar configuration and triple bonds to a linear one. These geometries have a significant impact on the shape a particular molecule can assume.
So far, the hydrocarbons we have discussed have been aliphatic hydrocarbons, which consist of linear chains of carbon atoms. Another type of hydrocarbon, aromatic hydrocarbons, consists of closed rings of carbon atoms with alternating single and double bonds. We find ring structures in aliphatic hydrocarbons, sometimes with the presence of double bonds, which we can see by comparing cyclohexane’s structure to benzene.
Examples of biological molecules that incorporate the benzene ring include some amino acids and cholesterol and its derivatives, including the hormones estrogen and testosterone. We also find the benzene ring in the herbicide 2,4-D. Benzene is a natural component of crude oil and has been classified as a carcinogen. Some hydrocarbons have both aliphatic and aromatic portions. Beta-carotene is an example of such a hydrocarbon.
Role of hydrocarbons in chemistry
All hydrocarbons have an enormous importance in modern economics and geopolitics, both for their role as a primary source of energy and, at the same time, for the vast number of consumer products that are prepared through their transformations. In the last century, the basic intermediate of the chemical industry was acetylene (CH≡CH), which by virtue of its high unsaturation can allow the production of a wide range of intermediate products: for example, through the addition of halogens, or in particular of water, using mercury as a catalyst, acetaldehyde can be produced. Acetylene is still used today in some organic syntheses, even if its use is dangerous and its cost is high.
It was important to discover catalysts able to activate, in a selective and economically advantageous way, the olefinic double bond: for example, palladium chloride is used for liquid phase oxidation of ethylene to acetaldehyde. Acetylene, therefore, has been replaced by ethylene as a staple of organic industrial chemistry, with production exceeding 20 million tons in the 1990s. This substitution has a well-defined logic that can be traced back to the depletion of hydrogen in ethane with the formation of hydrocarbons with higher unsaturation, and therefore functionality, such as ethylene and acetylene respectively.
Next to ethylene, propylene and the C4 hydrocarbon fraction also represent basic compounds in the large organic chemical industry. Their transformations often involve a partial, or selective, oxidation stage.
C-H bond functionalization
C-H bonds are present in most hydrocarbons, including polyaryls consisting of condensed aromatic hydrocarbons, as well as in polymers, pharmaceutical active ingredients, and almost all biological compounds. Therefore, the possibility of functionalizing this bond can contribute to the expansion of synthetic chemistry in numerous areas.
The direct and selective substitution of C-H bonds, which are very stable and therefore not very reactive, with new bonds – such as C-O, C-N, C-C – is, as already noted, an important goal for synthetic organic chemistry. In traditional syntheses, in fact, this substitution must be preceded by the insertion of functional groups (GF), characterized by a certain chemical reactivity, such as hydroxyl, amine and halogen groups.
Due to the widespread presence of C-H bonds, especially in hydrocarbons, the direct and selective substitution of C-H bonds urges new synthetic solutions. The major problem encountered is the difficulty in achieving adequate selectivity with respect to the different C-H bonds present in organic compounds.
The methodologies to operate these innovative chemical syntheses are innumerable and range from the use of complex metal catalysts, to the formation of coordination compounds with heavy metals and the use of intramolecular radical reactions. Although the use of such processes has already demonstrated its effectiveness in a fair number of organic syntheses, the possibility of extending this strategy to the preparation of a large number of compounds still remains an open challenge.
Calibration of an appropriate methodology makes possible the redesign of traditional synthetic processes, as well as the introduction of new synthetic routes to produce already known organic substances or even new chemicals.
Origin of natural hydrocarbons
Natural hydrocarbons are the final product of a series of very complex transformations (naphthogenesis), in which many factors contribute. At present, the origin of hydrocarbons is considered in light of two theories.
Theory of inorganic origin
Based on the fact that some hydrocarbons were synthesized from inorganic substances, the theory of inorganic origin was abandoned and then resumed (1979) assuming the presence within the earth’s crust at considerable depth of hydrocarbons, which would subsequently have migrated upward. This migration would have led them to the surface or to remain trapped within the surface sedimentary rocks.
The presence of these hydrocarbons dates back to the beginning of the Earth, when there had to be in considerable quantities: to support this hypothesis, the supporters of the theory adduce the presence of complex hydrocarbons in some meteorites, while the presence of organic substances within the hydrocarbons would be explained as a result of contamination that they would have with sedimentary rocks containing organic matter.
Organic origin theory
Among the many evidences supporting the origin of hydrocarbons from a fundamental organic component, three constitute the main assumption:
- hydrocarbons are found in sedimentary rocks that contain substantial amounts of organic matter;
- the presence of nitrogen and porphyrin pigments found in hydrocarbons is typical of plant and animal matter, lacking in inorganic matter;
- optical activity, recorded in the components of petroleum, is characteristic of biological synthesis, not having been found in hydrocarbons derived from inorganic matter.
Regarding the starting material from which hydrocarbons originate (animal or plant origin), a mixed origin has been found in most cases. However, among all the organisms that may contribute to the process of naphthogenesis, marine ones play a fundamental role, even if they must be associated, subordinately, with those of continental origin. In particular, an important place is given to plankton, both animal and vegetable, followed by algae and animal organisms (fossil and microfossil) among which a rather important role is played by corals, which, as building organisms, constitute very porous rocky masses, often impregnated with hydrocarbons.
Among the continental organisms, the most important role is that of plants, in the form of both humic and woody substances. The transformations of these substances occur through a source of energy that originates from several factors: pressure and temperature, radioactivity, catalytic action of some metals (nickel, vanadium, molybdenum) and the presence of bacteria that act as triggers for chemical reactions.
To all this must be added the environmental conditions in which the process of naphthogenesis occurs. In this sense, particular importance is linked to the agents of decomposition, from which the organic matter must be removed before it is completely destroyed.
The best preservation conditions are represented, therefore, by those sea basins where organic matter accumulates and is preserved because of either the depth or the poor circulation of the waters which, not being oxygenated, determine anaerobic and reducing conditions. The persistence of these conditions must also be coupled with a continuous and regular subsidence of the bottom, so as to compensate for the contribution of sediment and not cause drastic changes to the environment.
- Biology 2e. OpenStax. Authors: Mary Ann Clark, Matthew Douglas, Jung Choi. https://openstax.org/books/biology-2e/pages/1-introduction