Key Difference – Position Isomerism vs Metamerism
Isomerism can be defined as the existence of chemical compounds having the same structural formula but different spatial arrangements. This means, isomers have the same number of atoms in each element, but their arrangement is different. Isomers are mainly categorized into two groups named structural isomers and stereoisomers. Structural isomers are again divided into three groups as chain isomers, position isomers, and functional group isomers. Metamers are also a type of structural isomers, but they are not commonly found. The main difference between position isomerism and metamerism is that, in position isomerism, the functional group is attached to different positions whereas, in metamerism, different alkyl groups are attached to the same functional group.
CONTENTS
1. Overview and Key Difference
2. What is Position Isomerism
3. What is Metamerism
4. Side by Side Comparison – Position Isomerism vs Metamerism
5. Summary
What is Position Isomerism?
The position isomerism can be defined as the “movement” of the functional group in the molecule. This means, only the position of the functional group is changed in this type of isomerism. 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. But 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.
For example, propyl bromide and isopropyl bromide are position isomers. In propyl bromide, the functional group is -Br and it is attached to the end of the carbon chain whereas in isopropyl bromide, -Br group is attached to the middle carbon atom of the carbon chain.
Figure 01: Position Isomerism in o-dichlorobenzene and p-dichlorobenzene
What is Metamerism?
In the case of metamerism, the types of alkyl groups on the sides of the functional groups will differ from each other. It is an unequal distribution of carbon atoms. Metamerism belongs to the same homologous series, which means, that the number of carbon atoms can be increased gradually to get different isomers. Therefore, the structures differ only by the number of CH2 groups in the main carbon chain.
Alkyl groups are always attached to sides of a divalent atom like oxygen or sulfide, or alkyl groups may be attached to a divalent group such as -NH-. Metamerism is rarely found due to these limitations. Therefore, most of the compounds found in metamerism are ethers and amines.
For example, diethyl ether and methyl propyl ether are metamers. Here, the functional group is ether and the divalent atom is oxygen atom. Diethyl ether has two ethyl groups whereas methyl propyl ether has a methyl and a propyl group on the sides of the oxygen atom.
Figure 02: Metamerism in Methyl propyl ether and Diethyl ether
What is the difference between Position Isomerism and Metamerism?
Position Isomerism vs Metamerism |
|
| In position isomerism, the position of the functional group differs. | In metamerism, the type of alkyl group that is attached to the functional group differs. |
| Number of Isomers | |
| Position isomerism shows a number of isomers that differ only by the position of the functional group | Metamerism has a limited number of isomers due to its limitations such as the alkyl groups being attached only to divalent atoms or groups. |
| Specific Functional Groups | |
| Position isomerism cannot be seen in compounds having only aldehyde, carboxylic like end groups. | Metamerism can be seen only in ethers or other compounds containing divalent atoms. |
| Alkyl Groups | |
| Same alkyl groups are attached to the functional groups in isomers of position isomerism. | Different alkyl groups are attached to the functional group in metamerism. |
| Series | |
| This belongs to a non-homologous series. | This belong to a homologous series |
Summary – Position Isomerism vs Metamerism
The main difference between position isomerism and metamerism is that, in position isomerism, the location of the functional group is changed whereas, in metamerism, the type of alkyl groups in the sides of the functional group is changed.
Reference:
1. Carey, F. A. & Sundberg, R. J., 2007. Advanced organic chemistry. 5th ed. Charlottesville: Springer.
2. “Basic principles in organic chemistry: Structural isomerism.” Open Teaching Project. N.p., n.d. Web. 25 May 2017.< http://padakshep.org/otp/subjects/chemistry/organic-chemistry/structural-isomerism/>
3. Vardhan, V. Aditya. “ISOMERISM IN ORGANIC COMPOUNDS.” AidChemistry. N.p., n.d. Web. 25 May 2017. <http://www.adichemistry.com/organic/basics/isomerism/structural/structural-isomerism.html (24.50.2017)>.
Image Courtesy:
1. “1,2-Dichlorobenzene” By The original uploader was Sbrools at English Wikipedia – (Public Domain) via Commons Wikimeida
2. “O-Dichlorobenzene” Public Domain) via Commons Wikimedia
3. “Diethylether” By Gaussianer – Own work, Public Domain) via Commons Wikimedia
4. “1-methoxypropane 200” By Emeldir (talk) – Own work (Public Domain) via Commons Wikimedia