We explain what alkanes are, the uses and properties of these hydrocarbons and their general characteristics.
What are alkanes?
Alkanes make up one of the main families of hydrocarbons (organic compounds made up of carbon and hydrogen atoms).
Alkanes are very important as household and industrial fuels since they are used for the operation of boilers, turbo generators and gas cookers, among others. They are also inputs in various industries such as glass , textiles and plastic .
Types of alkanes
Alkanes can be aliphatic or cycloalkanes.
- Aliphatic alkanes . Also called paraffins, they have a linear structure and obey the general formula C n H 2n + 2 , where n represents the number or quantity of carbon atoms in the compound (they contain more than twice as many hydrogen atoms as carbon atoms )
- Cycloalkanes . They have a structure in the form of a cycle and their general formula is C n H 2n .
Characteristics of alkanes
- They are associated with living beings . Although alkanes are not essential materials for living things , they often appear as intermediate or end products of microbial metabolism. Such is the case, for example, of the methanogenic fermentation carried out by bacteria in the rumen of animals.
- Some alkanes, such as methane (CH 4 ), can be flammable, explosive, or toxic , so they must be handled with extreme caution.
- They are saturated hydrocarbons because the bonds between the carbon atoms are simple and stable. This differentiates them from alkenes and alkynes, which are characterized by having double and triple bonds, respectively, between their carbon atoms . For example:
- They can have substituents . The hydrogen atoms attached to each carbon atom can be replaced by other atoms, such as halogens (fluorine, bromine, chlorine) or various groups or “radicals” (hydroxyl (OH – ) and methyl (CH 3 -) groups are very common. ). In the latter case, the structure becomes more complex, giving rise to branched alkanes. For example:
- They can form closed structures . Carbon atoms bond with each other giving rise to the formation of chains, but the ends of these chains can combine and then form the so-called cyclic alkanes, as in the case of cyclohexane, cyclopentane and cyclobutane.
Physical properties of alkanes
The physical properties of alkanes are largely determined by the number of carbon atoms they contain in their structure . Thus, for example, alkanes that have between 1 and 4 carbon atoms are gases at room temperature; those with 5 to 17 carbon atoms are liquid at the same temperature and those with 18 or more carbons are solid at room temperature.
- Low solubility . In general, alkanes are poorly or not at all soluble in water, and as molecular weight increases, solubility becomes even lower. In organic solvents, however, the solubility is high.
- Low density . Their density is less than that of water, which is why they tend to float. This looks great when oil spills occur at sea , such as oil spills (mixture of many hydrocarbons). On the other hand, the density of alkanes increases as their molecular mass increases, that is, the number of carbon atoms.
- Variable melting and boiling point . The melting and boiling point depend on the number of carbon atoms (the higher the number, the higher the melting point and the boiling point), but also on the structure: linear structures have a higher melting and boiling point than the branched ones. The boiling point of alkanes increases by approximately 30 ° C each time a carbon atom is added to the compound.
- Electrical conductivity . Alkanes generally do not conduct electricity .
Chemical reactions of alkanes
The most common reactions that alkanes can undergo are:
- Oxidation . When combined with oxygen they can form carbon dioxide and water and release energy in the form of heat. This is the typical combustion reaction.
- Halogenation . Chlorine, bromine, fluorine, and iodine can both substitute for hydrogen atoms in the alkane. The reaction products consist of different proportions of different halogenated alkanes. An example is chloroform, which is a halogenated hydrocarbon (trichloromethane).
- Nitration . At high temperatures and in the presence of nitric acid vapors, the substitution of hydrogen for the NO 2 – group can occur , which generates nitroalkanes in different proportions.
- Isomerization . It is the restructuring of the molecule without loss or gain of atoms. Generally, this reaction requires the use of catalysts. For example:
- Pyrolysis . It is the decomposition of alkanes by exposure to a very high temperature and without the presence of air , in this way their combustion is avoided and the decomposition of the molecules is prioritized by breaking their chemical bonds . This reaction is used in industry to obtain short-chain alkenes and alkanes from longer-chain alkanes.
- “Alcano” on Wikipedia .
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