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Structural Isomerism in Organic Compounds

An isomer is a molecule with the same molecular formula, but with different arrangements of atoms in space.

Structural isomerism

An isomer is a molecule with the same molecular formula, but with different arrangements of atoms in space. A molecule's rotation around a specific bond, or rotation as a whole, will not affect the arrangement. Let's consider the following two molecules as examples. It is not an isomer. They are both butanes.

It can also be twisted in different directions with no influence. Every carbon-carbon bond is completely free to rotate. When you have a model of a molecule in your hands, you will need to disassemble and rebuild it to produce an isomer. A molecule that appears to be different by rotating only one bond is the same. Structure isomerism refers to the fact that each atom of a molecule is arranged differently. Here are some examples to illustrate this. In the following section, structural isomers are discussed in more detail. It probably doesn't matter as much what the different names for structural isomerism are, but you must be aware of them when drawing isomers.

Chain isomerism

Carbon chains have the potential to branch, leading to these isomers. Butane, for example, is composed of two isomers, C4H10C4H10. A straight-chain of carbon atoms are found in one of the molecules, whereas a branched-chain is found in the other.

Don't draw twisted versions of the original molecules if you are drawing "false" isomers. In this compound, carbon-carbon bonds are rotated around the central carbon-carbon bond, giving the compound its structure.

Position isomerism

Position isomerism involves moving important groups around on a carbon skeleton but leaving the basic carbon structure unchanged. Alcohols, such as C4H9OH, provide an example of this.

As long as you keep the chain at four carbons, these are the only two options. In any case, keeping the chain at four carbons is unnecessary. Combinations of chain isomerism and position isomerism are possible - they are not mutually exclusive.

Two more butanol isomers are:

Position isomers can also be obtained on benzene rings. C7H7ClC7H7Cl is the molecular formula. The chlorine atom could be placed in four different positions to come up with four different isomers. One of the possible positions is attached to the group's carbon atom, and then the other three are opposite the CH3CH3 group, next-to-but-not next-to the CH3CH3 group, and opposite the CH3CH3 group.

Functional isomerism

This type of structural isomerism involves isomers that have a variety of functional groups, i.e., belong to a variety of families of compounds (different homologous series). Molecular formulas similar to this can also contain other possibilities including double bonds (an alkene) and OH groups (alcohol).

An example of this is C3H6O2C3H6O2. Among the structural isomers of the molecule, propanoic acid (a carboxylic acid) and methyl ethanoate (an ester) both exist.

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Ankur Choudhary is India's first professional pharmaceutical blogger, author and founder of, a widely-read pharmaceutical blog since 2008. Sign-up for the free email updates for your daily dose of pharmaceutical tips.
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