Homologous series. Present in each and every organic substance, and allows defining the physical properties of a series of substances that have the same functional group in common.
Summary
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- 1 History
- 2 Functional group
- 3 Homologous Series
- 4 alkanes
- 1 Properties and uses
- 5 Alkenes
- 1 Properties and uses
- 6 Alkynes
- 1 Properties and uses
- 7 Source
- 8 External Links
History
The adoption of unequivocal and systematic names, as well as simple, for all organic compounds is today one of the biggest concerns of chemists. The number of known organic compounds is very high (more than 2,000,000) and in order to carry out their study a great systematization is necessary when carrying out a careful distribution of such compounds.
It was possible to carry out a classification of organic compounds, introducing the concepts of functional group and homologous series. A functional group can be defined as an atom or a group of atoms whose presence in the molecule gives it a characteristic chemical behavior. Example of functional groups:
Functional group
A set of atoms present in the carbon chain of a compound and that by its reactivity characteristics defines the chemical behavior of the molecule. Each functional group will therefore define a different type of organic compound.
Homologous Series
A homologous series is made up of a group of compounds with the same functional group and such that each term differs from the previous and the subsequent one in that it has a plus and minus —CH 2 – group , respectively.
With these concepts, the classification of organic compounds as derivatives of the unbranched chain homologous series of saturated hydrocarbons (or paraffins) can be introduced, by substituting a corresponding functional group for a hydrogen atom.
Regarding the nomenclature, in general terms, the name of an organic compound consists essentially of two parts: a prefix, which indicates the number of carbons in the chain and a termination, which characterizes the function.
Homologated series
Alkanes
If we examine the molecular formulas of alkanes considered so far, we find that butane contains one carbon and two more hydrogens than propane, which, in turn, has one carbon and two hydrogens more than ethane, and so on. The alkane family forms such a homologous series, with the constant difference between successive members equal to CH 2 . We also appreciate that in each of these alkanes, the number of hydrogens is more than twice twice the number of carbon atoms, so we can write a general formula for their members, which is C n H 2n + 2 . As we will see later, other homologous series have their own characteristic general formulas.
According to this general formula, the next alkane, pentane, corresponds to C 5 H 12 , followed by hexane, C 6 H 14 , heptane, C 7 H 14 , and so on. It is to be expected that, as the number of atoms grows, the number of possible arrangements also increases, which indeed turns out to be the case; the number of successive homolog isomers increases surprisingly: there are 3 isomeric pentanes, 5 hexanes, 9 heptanes and 75 decanes (C10); for the 20-carbon eicosanne there are 366,319 possible isomeric structures.
Properties and uses
- The physical state of the first 4 alkanes: methane, ethane, propane and butane is gaseous. From pentane to hexadecane (16 carbon atoms) they are liquid and from heptadecane (17 carbon atoms) they are solid.
- The melting, boiling point and density increase as the number of carbon atoms increases.
- They are insoluble in water
- They can be used as solvents for low polar substances such as fats, oils and waxes.
- Household gas is a mixture of alkanes, mainly propane.
- The gas from the lighters is butane.
- The main use of alkanes is as fuels due to the large amount of heat that is released in this reaction.
Alkenes
The alkenes are other homologous series with the same increase alkanes: CH 2 . The general formula for this family is C n H 2n . As we move through the alkene series, the number of isomeric structures for each member increases.
Since alkenes are formed with double bonds, there is only room for each carbon atom to join 2 hydrogen atoms , so you can obtain the homologous series with the following formula C n H 2n , remaining as follows:
- ethylene C 2H 4
- propene C 3H 6
- butene C 4H 8
- pentene C 5H 10
- hexene C 6H 12
- heptene C 7H 14
- octene C 8H 16
Properties and uses
- The first three compounds, ethene (ethylene), propene, and butene, are gaseous at room temperature; the following are liquids to those with more than 16 carbonsthat are solid.
- They are relatively poorly soluble in water, but soluble in concentrated sulfuric acid and in nonpolar solvents.
- Its density, melting point and boiling point rise as the molecular weight increases.
- The most important use of alkenes is as a raw material for the production of plastics.
Alkynes
- The alkynesform a homologous series with an increase in the carbon atoms to like alkanes and alkenes.
- Alkynes are unsaturated hydrocarbonsthat contain at least one carbon-carbon triple bond in their structure.
- General formula: C nH 2n-2
- The systemic termination of alkynes is INO.
- The simplest of the alkynes has two carbons and its common name is acetylene, its systemic name ethin.
Properties and uses
- The first three alkynes are gaseous under normal conditions, the fourth to the fifteenth are liquid and those with 16 or more carbonatoms are solid.
- The density of the alkynes increases as the molecular weight increases.
