CLASSES OF ORGANIC COMPOUNDS (BASED ON THE KIND OF ATOMS)
Other than carbon, organic compounds contain atoms like hydrogen, oxygen, nitrogen, etc., bonded to the carbon. Combination of these kinds of atoms with carbon gives different classes of organic compounds. In the following section, let us discuss various classes of organic compounds.
The organic compounds that are composed of only carbon and hydrogen atoms are called hydrocarbons. The carbon atoms join together to form the framework of the compounds. These are regarded as the parent organic compounds and all other compounds are considered to be derived from hydrocarbons by replacing one or more hydrogen atoms with other atoms or group of atoms. Hydrocarbons are, further, sub divided into three classes such as:
(a) Alkanes: These are hydrocarbons, which contain only single bonds. They are represented by the general formula CnH2n + 2 (where n = 1,2, 3, ……). The simplest alkane (for n=1) is methane (CH4). Since, all are single bonds in alkanes, they are saturated compounds.
(b) Alkenes: The hydrocarbons, which contain one or more C=C bonds are called alkenes. These are unsaturated compounds. They are represented by the general formula CnH2n. The simplest alkene contains two carbon atoms (n=2) and is called ethylene (C2H4).
(c) Alkynes: The hydrocarbons containing carbon to carbon triple bond are called alkynes. They are also unsaturated as they contain triple bond between carbon atoms. They have the general formula CnH2n – 2. Acetylene (C2H2) is the simplest alkyne, which contains two carbon atoms. Table 11.1 lists the first five hydrocarbons of each class:
� Lower hydrocarbons are gases at room temperature E.g. methane, ethane are gases.
� They are colourless and odourless.
� The boiling point of hydrocarbons increases with an increase in the number of carbon atoms.
� They undergo combustion reaction with oxygen to form CO2 and water.
� Alkanes are least reactive when compared to other classes of hydrocarbons.
� Alkynes are the most reactive due to the presence of the triple bond.
� Alkanes are saturated whereas alkenes and alkynes are unsaturated.
� They are insoluble in water.
� Take the given sample solution in a test tube.
� Add a few drops of bromine water and observe any characteristic change in colour.
� If the given compound is unsaturated, it will decolourise bromine water.
� Saturated compounds do not decolourise bromine.
The structural frameworks of organic compounds are made of carbon and hydrogen, which are relatively less reactive. But, the presence of some other atoms or group of atoms makes the compounds more reactive and thus determines the chemical properties of the compound. These groups are called functional groups.
A functional group is an atom or group of atoms in a molecule, which gives its characteristic chemical properties.
The chemical properties of an organic compound depend on its functional group whereas its physical properties rely on remaining part of the structure. Carbon to carbon multiple bonds (C=C, CC) also are considered as functional groups as many of the properties are influenced by these bonds. Other functional groups include atoms of halogens, –OH, –CHO, –COOH, etc.
For example, ethane is a hydrocarbon having molecular formula C2H6. If one of its hydrogen is replaced by –OH group, you will get an alcohol. Leaving the functional group, the rest of the structure is represented by ‘R’.
Thus an alcohol is represented by ‘R-OH’
A series of compounds containing the same functional group is called a class of organic compounds. Table 11.2 shows various classes or families of organic compounds and their functional groups: