Classification is important for all branches of science, especially for chemical sciences. Grouping of more than ten million known organic compounds into several dozen general families is exclusively helpful for studying organic chemistry. In addition, due to a big variety of organic compounds it is essential to be able to assign a definite name to each compound.
2.1. CLASSIFICATION
Organic compounds are classified according to the following features:
• a structure of molecular framework (sometimes called a molecular skeleton);
• the presence of functional groups in a molecule.
2.1.1. Classification According to the Molecular Framework
Organic compounds are subdivided into the following groups.
Acyclic compounds. They have unbranched or branched carbon chain, but no rings. In the examples below, the first two represent compounds with unbranched carbon chain, whereas the third one is a compound with a branched chain:
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Carbocyclic compounds. They contain a ring (or rings) of carbon atoms only. The ring may contain multiple bonds and may have side carbon chains.
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Heterocyclic compounds. They contain a cyclic skeleton having at least one heteroatom, an atom that is not carbon. The most common heteroatoms are nitrogen, oxygen, or sulfur. More than one heteroatom may be present and these atoms may be identical or different. The structures of some natural heterocyclic compounds are presented below:
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2.1.2. Classification According to Functional Groups
Hydrocarbons are parent compounds in organic chemistry, which, according to their name, consist of only carbon and hydrogen atoms. Most organic molecules involve functional groups, i. e. an atom or a group of atoms of non-hydrocarbon origin that determine chemical properties of a compound. Indeed, chemical changes occur in most reactions at the functional group whereas the molecular framework remains unchanged. Thus, the knowledge of properties of the functional groups will greatly help in the study of organic chemistry.