Isomerism of organic compounds

Prevalence of the phenomenon and exceptions

In inorganic chemistry, the phenomenon of isomerism is rarely encountered by researchers. It is primarily observed when studying coordination compounds. However, in organic chemistry, isomerism is truly widespread, serving as one of the fundamental reasons for the enormous variety of organic substances.

Despite the pervasive nature of this phenomenon, certain organic compounds do NOT have isomers. These include the simplest representatives of various classes:

• Oxygen-containing substances (consisting of C, H, O): methanol ($CH_3OH$), formaldehyde ($CH_2O$), formic acid ($HCOOH$).
• Hydrocarbons (five simplest representatives): methane, ethane, propane, ethylene, and acetylene.

Carbon skeleton isomerism

This type of isomerism directly depends on the order in which carbon atoms are bonded to one another. The longer the carbon chain, the more structural variations can be formed.

The dependence of the number of isomers on chain length demonstrates a cascading, avalanche-like growth:

• Structures containing two or three carbon atoms ($C_2$ and $C_3$) have no skeletal isomers at all.
• For the $C_4$ structure, two ways of atomic bonding are already possible (excluding cyclic structures): a linear chain and a branched chain.
• With skeleton complexity increasing to $C_5$, 3 isomers arise; for $C_6$, there are 5 isomers.
• A structure containing 13 carbon atoms ($C_{13}$) forms already more than 1,000 different isomers.

Theoretical limit and cosmic scale To demonstrate the incredible growth rate of the number of isomers in homologous series, chemists cite an impressive calculation. The total number of nucleons (protons and neutrons) in our Universe is estimated by a constant of approximately $10^{80}$. If we consider a saturated hydrocarbon (alkane) with the formula $C_{167}H_{336}$ (a low-molecular-weight compound with a molecular weight of 2344, which is a higher homolog of methane), it turns out that the number of its theoretically possible isomers exceeds the number of nucleons in the Universe. Thus, the matter of the entire Universe simply would not suffice to form even a single molecule of each existing isomer of this alkane.

Position isomerism

This type of isomerism occurs when functional groups or multiple bonds between carbon atoms occupy different positions along the same carbon skeleton.

Features and examples:

1. Substituent position. In the simplest three-carbon structure ($C_3$), any substituent can occupy either a terminal or a central position. Consequently, exactly two isomers are formed.
2. Branched chains. In complex structures (for example, in a molecule with six carbon atoms and one hydroxyl group —OH), moving the functional group to different carbon atoms leads to the formation of many different molecules. A strict pattern dictates that as molecular complexity increases, the number of isomers differing solely in the position of functional groups steadily grows.
3. Position of multiple bonds. A double or triple bond can change its position only in molecules starting from the $C_4$ structure. In this case, the multiple bond can be located either at the terminal carbon atom or in the middle of the chain.

An important property of substances formed due to positional isomerism of a functional group or a multiple bond is that they all belong to the same class of organic compounds. The chemical and physical properties of such isomers, as a rule, do not show radical differences.

Functional group isomerism

Unlike the previous type, functional group isomerism implies that substances with the same qualitative and quantitative composition contain completely different functional groups. This means that such isomers belong to different classes of chemical substances, and the difference in their properties is particularly pronounced.

A striking example is the structure with the molecular formula $C_2H_4O_2$. Depending on the arrangement of atoms, it forms substances with distinct chemical properties:

• Carboxylic acid (acetic acid): both oxygen atoms are bonded to a single carbon atom, forming a unified carboxyl group ($-\text{COOH}$).
• Hydroxy aldehyde: oxygen atoms are distributed among different carbon atoms. As a result, two different functional groups appear within a single molecule simultaneously — an alcohol group ($-\text{OH}$) and an aldehyde group ($-\text{CHO}$).

Historical significance: The phenomenon of isomerism played a crucial role in the development of organic chemistry, especially in the era before the invention of direct physical methods for analyzing atomic structure. The prominent chemist A.M. Butlerov, while developing his theory of the chemical structure of organic compounds, brilliantly utilized available data on the number of known isomers to draw precise conclusions about the invisible structure of substances.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Cyclic structures and their effect on skeletal isomerism
• Spatial arrangement of atoms
• A.M. Butlerov's theory of chemical structure of organic compounds
• Isomerism of coordination compounds in inorganic chemistry

Other topics in "Chapter 14. Theoretical Foundations of Organic Chemistry"

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