How many isomers of C3H8 are there


For the three alkanes CH4 Methane, C.2H6 Ethane and C3H8 Propane can only a Build up structural formula, as the carbon atoms in these compounds can only be bound in a straight-chain arrangement. In addition to the straight-chain molecule, for the first time in the series of alkanes, butane C4H10 also has a branched molecule.

This phenomenon is known as Isomerism and the two connections as Isomers. From the C4H10 Butane has two isomers: n-butane (straight chain) and i-butane (branched chain).




These examples already show the advantages of the structural formulas over the empirical formulas.

Isomerism is a phenomenon of carbon compounds that is made possible by the linkage of carbon atoms with one another and is an essential cause of the large number of carbon compounds.

Isomeric compounds have the same molecular formula but different structural formulas. Physical and chemical properties are mainly determined by the structure of the molecule. Isomeric molecules therefore give the associated compounds different properties:

Molecular formula

Structural formula


M.p. [° C]

Bp [° C]





- 135

- 0,5

2-methyl propane


- 145

- 10





- 130




- 159


2,2-dimethyl propane


- 20


Elemental analysis cannot distinguish between isomers, since they consist of the same type and number of atoms and therefore provide the same empirical formula when evaluating the analysis results. For the same reason, the molecular weight determination for isomeric compounds gives identical values. Indications for the presence of isomers are given by physical properties, e.g. B. melting point, boiling point, density and others. or different behavior in chemical reactions.

As the number of carbon atoms in the molecule increases, so too do the isomerism possibilities. The number of possible isomers can be determined in advance for a given molecule by calculation. The individual isomers of the simple members of the homologous series of alkanes have been synthesized and their properties are therefore known. However, the difficulty in synthesizing isomers increases considerably as the carbon number increases.

Isomers occur not only with alkanes, but also with all other classes of compounds. There are also different types of isomerism. The isomerism, which is based on the different connection of the carbon atoms in the molecules, is generally referred to as Structural isomerism and the case treated so far as Chain isomerism.