Difference Between Position Isomerism and Metamerism

Main Difference – Position Isomerism vs Metamerism

Isomers are molecules that have the same molecular formula but have a different arrangement of the atoms in space. Therefore, isomerism can be defined as the existence of chemical compounds having the same structural formula but different spatial arrangements. Position isomerism describes the differences in the location of the functional group. Metamerism describes the structure of alkyl groups attached to a functional group. This is the main difference between position isomerism and metamerism.

Key Areas Covered

1. What is Position Isomerism
      – Definition, Explanation with Examples
2. What is Metamerism
      – Definition, Explanation with Examples
3. What is the Difference Between Position Isomerism and Metamerism
      – Comparison of Key Differences

Key Terms: Aldehydes, Alkyl Groups, Carboxylic Acids, Isomerism, Metamerism, Position Isomerism

What is Position Isomerism

Position isomerism is the presence of a similar carbon skeleton and functional groups in two or more organic compounds when the location of the functional groups is different from each other. The number of carbon atoms, molecular formula, the carbon backbone structure, and the number of functional groups are same for the isomers in position isomerism. This type of isomerism is absent in compounds having end groups like carboxylic acids, aldehydes, etc. since these groups cannot be positioned in the middle of a carbon chain.

Let us consider an example to understand this type of isomerism. The alcoholic compounds for the chemical formula C₅H₁₂O, can be written in three main ways depending on the position of the –OH group. Here, the –OH group can be positioned in the terminal of the molecule, in the middle of the molecule or on the 2^nd carbon atom from one terminal.

Figure 1: Position Isomers of C₅H₁₂O

Position isomerism can be observed in alkene and alkynes as well. Here, the position of the double bond or the triple bond is different from one molecule to another. But in carboxylic acids, amides, and aldehydes, position isomerism is absent because these functional groups are essentially located only in the terminals of the molecule.

What is Metamerism

Metamerism is the presence of same molecular formula with different alkyl groups present on either side of a functional group. These molecules have the same chemical formula, but the structural formulas are different.

Metamerism belongs to the same homologous series; this means that the number of carbon atoms can be increased gradually to get different isomers. Therefore, the structures differ only in the number of CH₂ groups in the main carbon chain.

But alkyl groups are always attached to sides of a divalent atom like oxygen or sulfide. Or else, alkyl groups may be attached to a divalent functional group such as -NH-. Metamerism is rarely found due to these limitations.  Therefore, most compounds found in metamerism are ethers and amines.

Figure 2: Metamerism of Diethyl ether and Methoxypropane

There are some other conditions as well. The functional group should be polyvalent; this means, the functional group should have more than one bond connected to it. The nature of the functional groups should not be changed. If the functional group is changed, isomerism also changes.

Difference Between Position Isomerism and Metamerism

Definition

Position Isomerism: Position isomerism is the presence of a similar carbon skeleton and functional groups in two or more organic compounds, but the location of the functional groups is different from each other.

Metamerism: Metamerism is the presence of same molecular formula with different alkyl groups present on either side of a functional group.

Functional Groups

Position Isomerism: Position isomerism can be observed in any organic compound, except in carboxylic acids, amides, and aldehydes.

Metamerism: Metamerism can be observed in organic compounds with polyvalent functional groups.

Carbon Skeleton

Position Isomerism: The carbon skeleton is similar in compounds with position isomerism.

Metamerism: The carbon skeleton is different from each other in compounds having metamerism.

Series

Position Isomerism: Position isomerism belongs to a non-homologous series.

Metamerism: Metamerism belongs to a homologous series.

Conclusion

Position isomerism and metamerism are two categories of isomerism. These are different from each other due to several factors as discussed above in this article. The main difference between position isomerism and metamerism is that position isomerism describes the different locations of a functional group whereas metamerism describes different alkyl groups attached to the same functional group.

References:

1. “Positional isomersm.” OChemPal, Available here.
2. “Basic principles in organic chemistry: Structural isomerism.” Open Teaching Project, Available here.
3. “Metamerism.” Www.jeechemistry.com, Available here.

Image Courtesy:

1. “Pentan-3-ol-2D-skeletal” (Public Domain) via Commons Wikimedia, “Pentan-2-ol-2D-skeletal” (Public Domain) via Commons Wikimedia and “1-Pentanol”By Edgar181 – Own work (Public Domain) via Commons Wikimedia (Combined)
2. “Diethyl-ether-2D-skeletal” By Ben Mills – Own work (Public Domain) via Commons Wikimedia  and “Methoxypropane” By Edgar181 – Own work (Public Domain) via Commons Wikimedia (Combined)