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Structural Isomers: The Shapeshifters

Updated: Jul 22, 2023

In chemistry, a structural isomer (constitutional isomer) is a type of isomer in which molecules with the same molecular formula have a different arrangement of atoms. This means that the atoms are bonded together in a different order, resulting in structures with different physical and chemical properties.

There are several ways that structural isomers can differ from one another:


Chain Isomerism - branching of the carbon chains




Skeletal formulae for the five structural isomers of hexane. They all have the same molecular formula, C6H14






Functional Group Isomers

Functional group isomers have the same carbon skeleton, but they have different functional groups


Functional isomers include acetic acid and methyl formate. C2H4O2 is the same chemical formula for both. Methyl formate is an ester with a -COOCH3 group. Acetic acid is a carboxylic acid with a -COOH group.


Positional Isomerism - the position of functional groups on the molecule.

For example, 1-propanol and 2-propanol are structural isomers of each other because they have the same molecular formula (C3H8O) but the position of the hydroxyl (-OH) group is different.

1-propanol has the hydroxyl group bonded to the first carbon atom, while 2-propanol has the hydroxyl group bonded to the second carbon atom.


Geometry around double bonds

Alkenes, which contain at least one double bond, can show cis-trans isomerism.


Cis-trans isomers (geometric isomers) have different shapes (geometry) due to the arrangement of groups around the double bond. These isomers can only occur in molecules that have two or more different groups attached to a double bond. The term "cis" refers to the arrangement of the groups around the double bond where the groups are on the same side of the bond. The term "trans" refers to an arrangement where the groups are on opposite sides of the double bond.


2-butene has cis-2-butene and trans-2-butene isomers.



In cis-2-butene, the two methyl groups (CH3) are on the same side of the double bond. In trans-2-butene, the two methyl groups are on opposite sides of the double bond.






In addition to cis-trans isomers, there are other types of geometric isomers, such as enantiomers, which have different properties and can be difficult to distinguish from one another.


Enantiomers

Enantiomers are a type of geometric isomer that are mirror images of each other and are not superimposable*. They are also known as optical isomers because they rotate plane-polarized light in opposite directions.


Enantiomers occur when a molecule has a chiral centre - a carbon atom bonded to four different groups.


An example of a molecule with a chiral centre is 2-butanol:


Summary

  • Structural isomers are compounds with the same molecular formula but a different arrangement of atoms in the molecule

  • Types of structural isomers include chain isomers, positional isomers, and functional group isomers

  • Chain isomers have the same number of carbon atoms, but they are arranged in a different order

  • Positional isomers have the same functional group, but it is attached to a different carbon in the chain

  • Functional group isomers have the same carbon skeleton, but they have different functional groups



*If you place your left hand over your right hand, the thumb will be on opposite sides and the index finger of your left hand will lie over the little finger of your right - not superimposable.






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