Structural isomer

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Structural isomerism, or constitutional isomerism, is a form of isomerism in which molecules with the same molecular formula have atoms bonded together in different orders, as opposed to stereoisomerism ^[1].

Three categories of constitutional isomers are skeletal, positional, and functional isomers.

Contents 1 Skeletal isomerism 2 Position isomerism 3 Functional group isomerism 4 Metamerism 5 Isomer counting 6 References

[edit] Skeletal isomerism

In skeletal isomerism, or chain isomerism, components of the (usually carbon) skeleton are distinctly re-ordered to create different structures. For example 3-methylpentane is a chain isomer of 2-methylpentane. Pentane exists as three isomers: n-pentane, isopentane and neopentane.

Example of skeletal isomerism
n-pentane	Isopentane	Neopentane

[edit] Position isomerism

In position isomerism a functional group changes position on the chain. In the table below, the hydroxyl group can occupy three different positions on an n-pentane chain forming three different compounds.

Example of position isomerism
Pentan-1-ol	Pentan-2-ol	Pentan-3-ol

Many aromatic isomers exist because substituents can be positioned on different parts of the benzene ring. Only one isomer of phenol or hydroxybenzene exists but cresol or methylphenol has three isomers where the additional methyl group can be placed on three different positions on the ring. Xylenol has one hydroxyl group and two methyl groups and a total of 6 isomers exist.

Xylenol isomers
2,3-Xylenol	2,4-Xylenol	2,5-Xylenol
2,6-Xylenol	3,4-Xylenol	3,5-xylenol

[edit] Functional group isomerism

In functional group isomerism a functional group splits up and becomes a different group. Here is an example of functional group isomerism: take cyclohexane, C₆H₁₂ and hex-1-ene, also C₆H₁₂. These two are considered functional group isomers because cyclohexane is a cycloalkane and hex-1-ene is an alkene. Both must have the same molecular formula.

Example of functional group isomerism
Cyclohexane	1-hexene

[edit] Metamerism

This type of isomerism is due to the unequal distribution of carbon atoms on either side of a functional group. Metamers belong to the same homologous series, for example diethyl ether and methyl propyl ether.

Example of metamerism
methyl propyl ether	diethyl ether

[edit] Isomer counting

As an example of isomer counting, 12 isomers exist with molecular formula C₄H₁₀O₂ each with different bond connectivities and air-stable at ambient temperature ^[2].

Chemical compound	Molecular structure	Melting point (°C)	Boiling point (°C)
Acetaldehyde dimethylacetal		-113.2	64.5
1,2-Butanediol		-	190.5
1,3-Butanediol		-	207.5
1,4-Butanediol		20.1	235
2,3-Butanediol rac		7.6	182.5
2-Methyl-1,2-propanediol		-	176.8
Tert-Butyl hydroperoxide		6	89 (d)
Ethyl diethylperoxide		- 70	65
1,2-Dimethoxyethane		- 58	83
2-Ethoxyethanol		- 70	135
2-Methoxy-1-propanol		-	130
1-Methoxy-2-propanol		-	118

[edit] References