Homologous Series-Functional Groups and Isomerism, Chemistry tutorial


The word homologous series and functional groups describe a few of the umbrella principles that assist in the understanding of organic compounds in spite of their large number. Isomerism describes the existence of organic compounds having the similar number and type of atoms however showing differing features due to the differences in the manner the atoms are arranged in the molecules.

The Homologous Series:

The organic compounds are abundant in number; however, they are in millions. In dealing with such large number of compounds, a few form of classification is necessary.

It was noticed that several set of compounds illustrate structural, physical and chemical similarities and are stated to comprise a homologous series. A homologous series can be stated as a collection of a small number of series of organic compounds having unique structural pattern and properties; each and every member differs via a constant -CH2- unit. The word homologous is employed to explain a group of compounds with the given features:

a) They can be made by similar processes.

b) They have similar chemical properties.

c) There is a regular gradation of the physical properties (that is, melting or boiling points, solubility).

d) They can be symbolized via a general formula example: CnH2n+2 for alkanes; CnH2n for alkenes, CnH2n-2 for alkynes, CnH2n+1 OH for alkanols and CnH2n+1 COOH for the alkanoic acids.

e) Each and every member distinct in molecular formula from the preceding compound through -CH2- example: in alkanes, CH4, C2H6, C3H8, C4H10 and so on.

f) Each and every member includes at least one functional group example: C=C for Alkene, C≡C for alkynes, -OH for alkanols and -COOH for the alkanoic acid.

Such features are very necessary for the understanding of organic chemistry and will be encountered all through your study of organic chemistry.

Functional Groups:

One of the features of a homologous series is that each and every member of a series includes at least one functional group. However, the functional group finds out the properties of a homologous series. The functional group is a structural feature comprising of atoms or bonds found in the organic compounds which control the categorization and reactivity of the molecules. The study of organic chemistry becomes simplified whenever examined according to their functional groups. One of the major reasons for categorizing compounds via their functional groups is that it as well categorizes their chemical behavior and, to some level, their physical properties. The table below summarizes some of the general functional groups.

Table: Organic Compounds by Functional Groups

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Note: 'R' in the organic chemistry symbolizes an alkyl group or radical that is an alkane less one hydrogen atom.

The number of common functional groups is small in comparison by the total number of organic compounds. The contents of organic chemistry are, thus, arranged altogether if the emphasis is put on the features of individual functional groups. Once these properties example method of preparation, chemical reactivity and so on have been illustrated it is possible, with reasonable accuracy, to predict the properties of any compound having such functional groups. This is what makes the study of organic chemistry simple in spite of the large number of compounds comprised.


This is the word employed to illustrate the existence of two or more organic compounds by the same molecular formula, however different structural formulae. The different forms of the compounds are termed as isomers and have different physical and chemical properties.

C2H6O                   =>          CH3-O-CH3   CH3-CH2-OH

Molecular formula               structural formula of isomers

Stated in the other way, compounds having the similar number and type of atoms however the atoms are arranged differently are termed as isomers. Your left and right hands have the same features however arranged differently; they are isomers of one another.

Structural isomerism:

In structural isomers, the molecules have the same number of each and every type of atom however differs in regard to which atom is linked to which. In structural isomerism, the difference arises simply from the arrangement of atoms in the molecule, resultant in two or more different structural formulae.

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Fig: Structural isomerism

The isomers are all separate, different compounds and even the names might be different. If they have the similar functional group and are members of the similar homologous series example: C4H10. They have numerous chemical properties in common however encompass different physical properties. If they contain different functional groups and belong to different homologous series they differ in both physical and chemical properties, example: C2H6O

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Fig: Structural isomerism with functional groups

Geometric isomerism:

This is a kind of isomerism existing in compounds having double bonds as there is no free rotation of the carbons regarding double bonds so that such bonds lock the two groups they link in the permanent positions.

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Fig: Geometric isomerism

Usually, all alkenes that contain two different groups linked to each carbon atom having the double bond might illustrate geometric isomerism. The two arrangements are possible; an isomer in which similar groups are on the similar side of the double bond (it is termed as cis-isomer from the Latin term 'same') and the other isomer in which the identical groups are on the opposite sides of the double bond (it is known as trans-isomer from the Latin word meaning 'across'). The Geometric isomerism is at times termed as cis-trans isomerism.

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Fig: Examples of geometric isomerism

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