In chemistry, there are groups of organic substances that differ in similar properties and are united by a general formula that describes the regularity of the structural difference of each subsequent member of the group from the previous one. For example, the homologous series of alkanes, alkenes, alkynes or other groups. This nomenclature is of great importance for research, forecasting or practical application. For organic substances in a group, regular changes in chemical and physical properties are observed, and they all correlate with a change in molecular weight.
No less important are the rules that describe how the properties of substances change during the transition from one group to another. To understand what a homologous series is, specific examples should be considered. For any group of compounds, increasing melting (crystallization), boiling (condensation) and density temperatures with increasing molecular weight and number of carbon atoms in a molecule are characteristic.
Saturated hydrocarbons are called saturated or paraffinic; they are acyclic (no rings) compounds of normal or branched structure, the atoms in the molecules of which are connected by single bonds. The general formula has the form CnH2n + 2 and describes the homologous series of alkanes. The molecule of each subsequent member increases compared with the previous one by one C atom and two H atoms. Saturated hydrocarbons include:
- methane;
- ethane;
- propane and so on.
Saturated hydrocarbons also include cycloparaffins. This is a large group of organic compounds whose molecules are closed by rings. Their homological series has the formula CnH2n, starting with a chemical substance with three carbon atoms. Examples of cycloparaffins:
- cyclopropane;
- cyclobutane;
- cyclopentane and so on
Unsaturated or unsaturated hydrocarbons are also acyclic. These include normal substances and isostructure. The homologous series of alkenes has the general formula CnH2n. These compounds are distinguished by the presence of one double bond between two carbon atoms. If the previous row began with a hydrocarbon with one carbon atom (methane), then this begins with a substance in the molecule of which contains two carbon atoms. Examples of alkenes:
- ethene;
- propene;
- butene and so on.
Hydrocarbons, in the molecule of which two carbon atoms are connected by a triple bond, are even more unsaturated, otherwise they are called acetylene. They are united by a homologous series of alkynes. It is described by the formula CnH2n-2 and begins with acetylene, in the formula of which there are two C atoms. Examples of alkynes:
- ethine;
- propine;
- butin-1 and so on.
Unsaturated acyclic hydrocarbons in the molecule of which there are two double bonds are called diene. They have the general formula CnH2n-2. Their homologous series begins with a hydrocarbon with three carbon atoms in the molecule. Double bonds can be conjugated (separated by one single bond), cumulated (located at adjacent atoms) or isolated (separated by several single bonds). Examples of dienes:
- 1,2-propadiene;
- 1,3-butadiene;
- isoprene and so on
A special group is formed by chemicals of a cyclic structure, in the molecule of which there is a benzene ring. The homologous series of the simplest aromatic hydrocarbons begins with a compound with six carbon atoms - benzene. Homologs of this series are formed when one or several hydrogen atoms attached to a benzene ring are replaced by radicals. Thus, a number of substances are obtained: benzene, toluene, xylene. If there are two or more substituents in a molecule, then they indicate the presence of isomers for these substances. Other homologous series of aromatic compounds are formed from naphthalene, anthracene and other substances.
If there is a functional group in the hydrocarbon molecule, then such chemical compounds also form a homologous series.
- A number of alcohols differ in the presence of a hydroxyl group (—OH) in the molecule. For monohydric alcohols, one hydrogen atom in an acyclic hydrocarbon is replaced by a hydroxyl group; their formula: CnH2n + 1OH. There are also rows of polyhydric alcohols.
- A number of phenols is also characterized by the presence of a hydroxyl group (—OH) in the molecule, but it replaces hydrogen in the benzene ring.
- A number of aldehydes are distinguished by the presence in the molecule of a chemical compound of a carbonyl group (> C = O); general formula of aldehydes: R — CH = O.
- A number of ketones are also distinguished by the presence of a carbonyl group (> C = O), but if in aldehydes it is connected to one radical, then in ketones there are two hydrocarbon radicals. Ketone formula: R1 – CO – R2.
- A number of carboxylic acids are distinguished from other chemicals by a carboxyl group that combines carbonyl and hydroxyl groups. The formula is RCOOH.
For each series, whether it is a homologous series of aldehydes, carboxylic (organic) acids, alcohols or other substances, their properties will largely be determined by the type of functional group and will naturally vary along with an increase in the molecular weight of the substance. Such a classification of an extensive class of chemical compounds helps to understand nature and study their properties.