How many isomers are there

Isomeric compounds (gr. isos: equal; meros: Part) have different chemical and physical properties with the same empirical formula.

A simple example is butane, which consists of 4 carbon atoms. If its chain is unbranched, the connection is called n-butane (n stands for normal).

However, if the four carbon atoms are arranged in a branched chain, one speaks of Isobutane.

The prefix Iso- refers to a molecule with one or more branches in the main chain.

It is about a Constitutional isomerism. The term "skeletal isomerism" is also used for them, since only the structure of the actual hydrocarbon chain differs from isomer to isomer. While butane only has the two isomeric forms shown above, there are already five possible constitutional isomers for hexane.


Naming of the molecules
The connections are named after their longest chain. The position of the side chains is indicated by a number, with counting starting at the end of the main chain that is closest to the first branch. The number in front of the name of the side chain describes at which C atom of the main chain the side chain branches off. If a side chain occurs several times in the molecule, you first have to count how often this happens. The frequency of the side chain is then also indicated by Greek prefixes such as di- (two), tri- (three), tetra- (four), etc. All of these rules can be found in the following example.

As the number of carbon atoms in the molecule increases, so does the number of possible isomers.
The following table shows how many constitutional isomers exist with a certain number of carbon atoms in the molecule:

Number of carbon atomsNumber of possible isomers
C.11
C.21
C.31
C.42
C.53
C.65
C.79
C.818
C.935
C.1075
C.11159
C.12355
C.13802
C.141 858
C.154 347
C.1610 359
C.1724 894
C.1860 523
C.19147 284
C.20366 319
C.2536 797 588
C.304 111 846 763
C.4062 491 178 805 831


At this point a small request to the teachers: There is nothing more boring than endlessly drilled rules of nomenclature. A few simple examples are enough to help you understand the basics. In class work and exams, too, it does not seem to make much sense to only ask for names of hydrocarbon molecules.


Further texts on the topic of `` hydrocarbons ''