The organic compounds in which all the chains of atoms are open are called aliphatic compounds or open chain compounds. For example -
CH3
|
CH3 - CH2 - CH - CH3
|
CH3 - CH2 - CH - CH3
iso-pentane
CH3 – CH 2 – CH2 – CH3
n-butane
n-butane
In the beginning of the development of organic chemistry, the compounds obtained from fats were called Aliphatic compounds. With the development of organic chemistry, it became known that they are all open chain compounds. For this reason, in modern times all open chain compounds are called aliphatic compounds whether they are derived from fats or not.
It is not necessary that only one open chain of carbon atoms is present in open chain compounds. Other chains may also be present on the side of the main chains present in the compound. These are called side chains. And they are attached to the main chain through a carbon-carbon bond. For example-
Organic compounds in which at least one cyclic chain of atoms is present are called cyclic compounds or closed chain compounds. In these compounds, some atoms are linked to each other by covalent bonds in such a way that pearls are strung in a rosary.
Those cyclic compounds which are similar to benzene in properties and structure of the ring chain are called Aromatic compounds. The structure of benzene and its short form are as follows-
Comparison of Properties of Aliphatic and Aromatic Compounds
The special properties of aromatic compounds due to which they are studied in a separate group apart from aliphatic compounds are called aromatic properties. Aromatic properties are different from the properties of aliphatic compounds.
The following is a comparative description of the properties of aliphatic and aromatic compounds-
- Aromatic compounds have a characteristic odor called aromatic odor. Aliphatic compounds are odorless or have a distinct odor from them.
- The ratio of the percentages of carbon and hydrogen in aromatic compounds is usually greater than the ratio of the percentages of these elements in aliphatic compounds. For this reason most aromatic compounds burn with a smoky flame when ignited while most aliphatic compounds burn with a non smoky flame.
- The molecular formulas of aromatic compounds show that they contain high degree of unsaturation i.e. they contain many carbon-carbon double bonds. In spite of the presence of carbon-carbon double bonds, aromatic compounds do not readily exhibit addition reactions. Aliphatic compounds readily exhibit addition reactions.
- Aromatic compounds readily undergo substitution reactions such as halogenation, nitrification, sulfonation and Friedel-Crafts reaction. In these reactions, one or more hydrogen atoms are replaced by other atoms or groups and the characteristic properties of aromatic compounds and their characteristic structure remain unaffected. Aliphatic compounds often undergo other reactions more easily than substitution reactions.
- The data on the heat of hydrogenation show that the stability of the structures of aromatic compounds is greater than the stability of structures of aliphatic compounds. Therefore, in most of the reactions of aromatic compounds, their characteristic structure remains unaffected.
- Aromatic hydroxy compounds (phenols) are acidic while aliphatic hydroxy compounds (alcohols) are neutral.
- Aromatic amino compounds are weak aromatics as compared to aliphatic amino compounds.
- Aromatic compounds are cyclic. In aromatic halogen compounds, the halogen atoms present in the cycle cannot be easily replaced by other atoms or groups, whereas in aliphatic compounds, the halogen atoms present in the cycle can easily undergo a number of substitution reactions.
The special properties of aromatic compounds are due to their structural features. The structure of benzene is cyclic. Benzene cycle is present in the derivatives of benzene. Other aromatic compounds either contain the benzene cycle or have a cyclic structure similar to that of benzene.
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