What are Constitutional Isomers?
Constitutional isomers, also known as structural isomers, are specific types of isomers that share the same molecular formula but have different bonding atomic organization and bonding patterns. The three prominent types of constitutional isomers are:
- Skeletal isomers (commonly referred to as chain isomers)
- Positional isomers (also known as regioisomers)
- Functional isomers (sometimes referred to as functional group isomers)
A detailed explanation of each of these types of constitutional isomers are provided with examples in this article.
Chain Isomers or Skeletal Isomers
Chain isomers or skeletal isomers are the constitutional isomers in which components of the skeleton of the molecule are ordered in different ways to create different skeletal structures. This type of isomerism commonly arises in organic compounds containing a long carbon chain. For example, the carbon chain in pentane can be rearranged in three different ways, resulting in three different chain isomers:
- N-pentane, the skeletal isomer of pentane in which there is only one, five-membered carbon chain with no branching.
- Isopentane, the skeletal isomer of pentane which consists of a four-membered parent chain of carbon atoms, which branches in the second position.
- Neopentane, the skeletal isomer of pentane in which a three-membered parent chain of carbon atoms branches twice from the second position.
An illustration detailing the structures of the three skeletal isomers of pentane is provided below.
It can be noted that in the illustration provided above, the leftmost molecule is n-pentane and the rightmost molecule is neopentane. The molecule placed in the middle is isopentane.
Positional Isomers or Regioisomers
Positional isomers or regioisomers differ from each other based on the position at which the functional group is attached to the molecule. For example, in the straight chain structure of n-pentane, a hydroxyl functional group can be added in one of three different positions. This results in three possible positional isomers:
- Pentan-1-ol, where the OH group is attached to either end of the parent carbon chain.
- Pentan-2-ol, where the OH group is attached either at the second or fourth position of the parent carbon chain.
- Pentan-3-ol, where the OH group is attached to the third position of the parent carbon chain.
Functional isomers are the constitutional isomers which share the same molecular formula but vary in the manner in which the atoms are connected to each other. An important example of functional isomerism can be observed in 1-hexene and cyclohexane. The former has a straight chain structure with one carbon-carbon double bond whereas the latter has a cyclic structure with no carbon-carbon double bonds.
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