The compounds having the same molecular formula but different bond connectivities are called constitutional isomers. They have different structural formulas that’s why they are also known as structural isomers. They are distinguishable by their two-dimensional structure. Also, they have different physical and chemical properties.
Constitutional isomerism is due to different arrangements of atoms within molecules. The same atoms can be connected in two or more different ways. Smaller alkanes like methane, ethane, and propane, do not show isomerism. They only exist in one structural form. The phenomenon of isomerism is mainly seen in compounds having more than three carbons.
The number of isomers increases with the increase in the carbon chain. i.e, butane has 2 isomers, pentane has three isomers, and decane has seventy-five isomers, whereas compounds with more than thirty carbon have millions to trillions. Tetracontane (C40H82) has 625 trillion different structures.
Constitutional isomers have different names because of the different attachments of bonds. Due to which they may have different colors, solubilities, melting, and boiling points. Similarly, they also have a different rate of reaction with same regents.
Types of constitutional isomerism
Chain isomerism (skeletal isomers)
Chain isomers are also called skeletal isomers. They differ in the nature of the carbon chain. Branched and straight-chain molecules are examples of chain isomers. For example:
Butane
Butane has two isomers. One is n-butane and the other is isobutane which can also be named as 2-Methylpropane. Both have the same molecular formula but different structural arrangements.
Propene
Propene show chain isomerism. Both of the below molecules have the same molecular formula but they differ in the arrangement of the carbon chain. This type of isomerism is also known as ring chain isomerism.
Positional isomerism (regioisomers)
In this type of isomerism, molecules differ only in the position of the same functional group on the carbon chain. Positional isomers are also known as regioisomers. For example:
Pentene
Pentene shows position isomerism. There is a difference in the position of the double bond in these two isomers.
Another example of positional isomerism is:
Functional group isomerism
Compounds having the same molecular formula but different functional groups are called functional group isomers. Certain groups showing function group isomerism are,
An example of functional group isomerism is propanol:
Propyne
Metamerism
In this type of isomerism, two compounds have unequal distribution of carbon atoms on either side of functional groups. Such molecules have the same functional groups.
Example of metamerism
Ethers
Tautomerism
Tautomerism is intramolecular hydrogen shifting. Here two molecular structures are readily interconvertible. The two structures obtained after H shifting are called tautomers. Tautomers are only different in the position of the hydrogen atoms.
Tautomerism is the 1,3 migration of hydrogen atoms within a molecule. Keto enol tautomerism is one of the common types of tautomerism.
Example of tautomerism
Another example of tautomerism is:
Linkage isomers
Linkage isomerism is also known as ambidentate isomerism. Here ligands are differently attached to a central metal atom. The composition of complex ion is the same but the point of attachment of ligand is different. These ligands can be bonded to a central metal ion by using any of two electron-donating groups. For example:
There are different ligands that can show linkage isomerism. For example, thiocyanate (SCN–), nitrite ion (NO2–) isothiocyanate (NCS–), sulfite (SO32-), selenocynate (SeCN–), etc. All these ligands have two sites of attachment. They can be bonded to central metal ions through either site.
For example, thiocyanate can attach by using sulfur lone pairs or nitrogen lone pairs. Similarly, nitrite ions can also use in nitrogen lone pairs or oxygen lone pairs for bonding.
Coordination isomers
Coordination isomers have different compositions of complex ions. In coordination isomerism, there is the interchange of ligands from the cation to the anion and vice versa. There is a change in the attachment of the ligand to metal ions. For example:
In the first case, SO4-2 is directly attached to the metal atom whereas bromide ( Br–) is a counter ion. In the second case, SO4-2 is a counter ion and Br– is directly bonded to a central metal atom. So, there is the exchange in a position of ligands.
Concepts berg
What is the other name of constitutional isomers?
Constitutional isomers are also known as structural isomers.
What are regioisomers?
Positional isomers are called regioisomers. They have different positions of the same functional group on the carbon chain.
Do constitutional isomers have different properties?
Yes, constitutional isomers have different physical and chemical properties.
What are isomers?
Different compounds having the same molecular formula are called isomers.
What are the types of isomerism?
Isomerism is classified into two types:
- Constitutional Isomerism
- Stereoisomerism
Name all possible isomers of hexane.
There are 5 possible isomers of hexane. i.e,
- n-hexane
- 3-methyl pentane
- 2-methyl pentane
- 2,2-dimethyl butane
- 2,3-dimethyl butane
How do constitutional isomers differ from one another?
Constitutional isomer is mainly structurally different from one another. They have different connectivities. Similarly, they have different physical and chemical properties.
How to find constitutional isomers?
If two compounds have the same number of carbon atoms and have the same hydrogen deficiency index (HDI), then these compounds are constitutional isomers of each other. In other words, quickly count the number of carbons and the degree of unsaturation.
References
- Tenth edition of Chemistry by Zumdahl and DeCoste.
- Second edition of Organic Chemistry Principles and Mechanisms by Joel Karty
