- Nomenclature of alkanes
- Nomenclature of linear saturated hydrocarbons
- Branched saturated hydrocarbon nomenclature
- Nomenclature of cyclic saturated hydrocarbons
- Properties
- Geometric isomerization
- Acidity
- Polarity
- Boiling and melting points
- Examples of alkanes
- Linear alkanes
- Branched alkanes
- Cycloalkanes
- References
The alkanes or saturated hydrocarbons are characterized by single bonds only in the structure of the covalent type. This means that the carbon atoms present in these species are bound to the maximum amount of hydrogen atoms with which they can form bonds, for this reason they are known as saturated.
In the universe of organic chemistry, alkanes, also known as paraffins, are considered to be quite abundant and very important species, belonging to the group of aliphatic hydrocarbons (like unsaturated hydrocarbons).
The simplest saturated hydrocarbon that can be formed is taken as an example: methane, a compound found in the gas phase under standard ambient conditions (25 ° C and atm), whose formula is CH 4.
As can be seen, the only carbon atom present in this molecule has four simple bonds, one with each hydrogen atom.
Alkenes and alkynes have important commercial uses, as in the case of ethylene and propylene; but they are also more reactive compounds than saturated hydrocarbons, making them have a high range of reactions that arise from common alkenes and alkynes.
Nomenclature of alkanes
In order to correctly name alkanes or saturated hydrocarbons, the first thing to keep in mind is that according to the IUPAC (International Union of Pure and Applied Chemistry) the systematic nomenclature for the four simplest alkanes should not be applied.
Nomenclature of linear saturated hydrocarbons
These compounds have the general formula C n H 2n + 2, where the value of n can only be positive integers (n = 1,2,…), and they are named using the prefix corresponding to the number of carbon atoms and are add the suffix –ano.
So, the first four saturated molecules are: methane (CH 4), ethane (C 2 H 6), propane (C 3 H 8) and butane (C 4 H 10).
To begin with the nomenclature of alkanes that have between five and ten carbon atoms, the number of these atoms that are present in the longest chain is counted, as long as it is continuous.
Furthermore, in the event that a hydrogen atom is subtracted from an alkane, it becomes a substituent, that is, a group whose termination -ane is changed to -yl. For example, methane (CH 4) would become methyl (-CH 3) and similarly with the other molecules.
Taking into account what has been stated so far, and adding that the count must always be started with the carbon atom that has the closest substituent, the position of the substituent followed by the name of the alkane is indicated.
Thus, the above compound is called 3-methylpentane.
Branched saturated hydrocarbon nomenclature
Similarly, branched-chain alkanes have the same general formula as linear ones, but with n> 2. So each time one or more atoms or groups of atoms substitute for one or more hydrogen atoms, the location of these substituents must be noted.
If there are several branches of groups of the same alkyl type, the expressions di-, tri- or tetra- are used to indicate the amount of these substituents, preceded by the indication of their positions and ending with the name of the alkane.
In case the substituents are different, they are named according to alphabetical order, and may also have non-carbon substituents, such as chlorine (Cl) or nitro (NO 2).
In all cases, to count the carbon numbers of the main chain, the smallest number is given to the carbon that is linked to the lowest substituent in the alphabetical order, and continues in that direction.
Nomenclature of cyclic saturated hydrocarbons
Cyclic-type saturated hydrocarbons, better known as cycloalkanes, have the general formula C n H 2n, where n = 3,4,…
In these organic molecules, the carbon atoms that make it up are arranged in a closed manner, that is, their structure forms a ring.
To name these species, the guidelines described above for linear and branched alkanes are followed, only adding the prefix cyclo-. Likewise, cyclopropane (C 3 H 6) is considered the simplest cycloalkane.
Similarly, these molecules can contain more than one ring integrated into their main chain, with a minimum of three carbon atoms and even forming highly complex structures.
Properties
Saturated hydrocarbons have the main characteristic of forming simple bonds between their atoms, which makes them a very large group of molecules and gives them quite specific properties as detailed below:
Geometric isomerization
The structure of alkane molecules produces modifications in their physical and chemical properties, due to the conformation of the four bonds that carbon can form.
This means that despite the fact that in these molecules the carbon has an sp 3 type hybridization, the angles between its adjacent atoms can vary depending on the type of atom.
To explain it more precisely, cycloalkanes have torsion angles that give them a unique characteristic called stereochemistry, which can affect the energies of the molecule and other factors inherent to it, such as giving it spectroscopic and optical properties.
Acidity
Saturated hydrocarbons show fairly low reactivity to ionic and other polar species. At the same time, they have practically no interaction with acidic and alkaline substances.
Polarity
Alkanes are considered non-conductive, because they have practically zero polarity in the presence of an electric field. So hydrogen bonds cannot be formed to allow its solubility in polar solvents.
So they are practically soluble in all non-polar solvents, being immiscible with polar solvents such as water.
Boiling and melting points
In saturated hydrocarbons, intermolecular interactions occur due to van der Waals forces, in which stronger interactions translate into higher boiling points.
A similar trend is observed for melting points, but this is due to the packing capacity of the molecule.
Since these interactions are directly related to the molecular weight of the species, the larger the molecule its boiling and melting points will be higher.
Thus, by having a more rigid structure that gives them an intermolecular contact plane, cycloalkanes have higher boiling and melting points than their corresponding linear alkanes.
Examples of alkanes
Linear alkanes
Methane: It is a colorless and odorless gas that occurs abundantly in nature and as a product of certain human activities. Methane is the simplest member of alkanes and is among the most potent of the greenhouse gases (Encyclopædia Britannica, 2017).
Ethane: is a gas that is mainly found in natural gas and is used in mixtures with other gases to produce fuels.
Propane: It is a colorless gas, found in natural gas and used as a fuel in homes and industries. The chemical formula for propane is C 3 H 8 and the extended formula is CH 3 CH 2 CH 2 (Propane Formula, SF).
Butane: or n-butane is one of dozens of gases extracted from crude natural gas and can also be produced from crude oil. N-butane is a colorless multi-purpose gas. Butane can be used for heating, cooling, and lighter fuel.
N-Pentane: is a clear colorless liquid with a petroleum-like odor. Pentane is found in alcoholic beverages and hop oil. This alkane is a component of some fuels and is used as a specialty solvent in the laboratory.
N-Hexane: is a colorless clear liquid with a petroleum-like odor. It is found in citrus fruits and is used to extract edible oils from seeds and vegetables, as a special-purpose solvent, and as a cleaning agent.
N-Heptane: is a colorless transparent liquid with a petroleum-like odor. It is found in cardamom. Less dense than water and insoluble in water. Vapors heavier than air.
N-Octane: It is a colorless liquid with a gasoline odor. Less dense than water and insoluble in water. Therefore it floats on the water. Produces irritating vapor.
Methyl Chloride: Also called chloromethane, it is a colorless gas. It is the simplest haloalkane, used in the manufacture of silicone polymers and in the manufacture of other chemical products.
Chloroform: it is a colorless, odorous and highly volatile liquid that has been widely used for its anesthetic properties. Due to these properties, it has a reputation for being able to stun or hit people, even when consumed in small doses (MoviesDoes Chloroform Really Knock You Out As Quickly As They Show In Movies ?, 2016).
Carbon tetrachloride: also called tetrachloromethane, a colorless, dense, highly toxic, volatile, non-flammable liquid that has a characteristic odor and is used as a solvent.
Chloroethane: is a gas that condenses under slight pressure. Chloroethane is used primarily for local pain relief in sports medicine (National Center for Biotechnology Information., 2017).
Bromoethane: also known as ethyl bromide, it is a colorless volatile liquid, slightly soluble and denser than water. Vapors are heavier than air. It is used to make pharmaceuticals and as a solvent.
Branched alkanes
Isobutane: is a colorless gas with a faint petroleum odor. It is shipped as a liquefied gas under its vapor pressure. Contact with the liquid can cause frostbite. It turns on easily.
Isopentane - Also called 2-methylbutane, it is a colorless aqueous liquid with a gasoline odor. Floats in the water. It produces flammable and irritating vapor (National Center for Biotechnology Information. PubChem Compound Database;, 2017).
2-methylpentane: it is a branched-chain alkane with the molecular formula C 6 H1 4. It is a watery liquid with a gasoline odor that floats on water and produces an irritating vapor.
3,3-Dimethylhexane - Found in herbs and spices. 3, 3-Dimethylhexane is a component of Osmanthus fragrans (sweet osmanthus) and ginseng oil.
2, 3-Dimethylhexane: found in fruits. 2,3-Dimethylhexane is a volatile component of starch.
Neopentane: it is a liquid less dense than water. Insoluble in water but soluble in alcohol (National Center for Biotechnology Information., 2015).
2, 2, 4-trimethylpentane: or isooctane is released into the environment through the manufacture, use and disposal of products associated with the petroleum industry. 2,2,4-trimethylpentane penetrated the skin of a human being and caused necrosis of the skin and tissue in the hand, requiring surgery (National Center for Biotechnology Information., 2017).
Cycloalkanes
Cyclopropane - is a colorless gas with a petroleum-like odor. Contact with the liquid can cause frostbite. It can suffocate due to air displacement and has a narcotic effect in high concentration.
Cyclobutane: Gas that condenses to a liquid at 13 ° C. Insoluble in water. Soluble in alcohol, acetone and ether.
Cyclopentane - is a colorless clear liquid with a petroleum-like odor. Less dense than water and insoluble in it. Vapors are heavier than air.
Cyclohexane: found in kohlrabi. Diluent in color additive mixtures for food use.
Cycloheptane: it is a colorless, insoluble, oily liquid that is less dense than water. Inhalation of high concentrations can have a narcotic effect. It is used to make other chemicals.
Cyclooctane: it is a polycyclic hydrocarbon with nine carbon atoms. Insoluble in water.
Methylcyclohexane - is a colorless clear liquid with a petroleum-like odor. In methylcyclohexane, the chair conformation in which the large methyl group is equatorial is the most stable and therefore the most populated of all possible conformations (Carey, 2011).
Isopropyl Cyclohexane: It is a colorless liquid found in fruits. Isopropyl cyclohexane is found in Carica papaya (papaya).
methylcyclopentane: it is a colorless, insoluble liquid and less dense than water. Vapors can be narcotic and irritating. Methylcyclopentane is isolated from Helianthus annuus (sunflower).
Norborane: it is a bicyclic alkane also called heptane bicycles with the formula C7H12.
References
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