Constitutional Isomers and Stereoisomers
Isomerism is one of the most important topics covered in organic chemistry. In most organic reactions, the products are formed based on the concept of isomerism. Students may land up predicting the wrong product of a given reaction if they do not have a clear idea of the concept of isomerism. By the virtue of isomerism, compounds have the same molecular formula but differ in physical and chemical properties. Such compounds are referred to as Isomers. The classification of isomers is based on connectivity. Diastereomers, and enantiomers are also stereoisomers. Isomers are broadly classified as Constitutional Isomers and Stereoisomers.
What Are Constitutional Isomers?
These are also known as structural isomers. The formula of these isomers are the same but the arrangement of atoms in the compounds are different.
Types of Constitutional Isomers
Constitutional isomers are classified into the following types.
Position Isomers
These isomers have a different position of the functional groups in them and the carbon skeleton chain remains the same. These isomers are also known as regioisomers.
For example 1-propanol and 2-propanol, ortho dichlorobenzene, and meta-dichlorobenzene. This phenomenon is Position isomerism.
Functional Isomers
Compounds that have the same molecular formula but different functional groups are said to be functional isomers. For example alkene and cycloalkene. This phenomenon is the functional group isomerism.
For example, C3H8O can be presented as:
CH3OCH2CH3
Or
CH3CH2CH2OH
Chain Isomers
When compounds have the same molecular formula but the carbon chain of their structure is different then they are called chain isomers. These isomers are also known as skeletal isomers. This phenomenon is also known as chain or nuclear isomerism.
As seen in the figure the carbon chain is different but the number of carbon atoms is the same.
Metamers
These isomers are formed when different groups are attached to the polyvalent atom of the functional group and thus formula remains the same but the groups attached to the polyvalent atom become different. This phenomenon is known as Metamerism. It is exhibited by ketones and ethers.
Tautomers
These are the constitutional isomers that undergo rapid interconversion with each other. Tautomers cannot exist independently of each other. Tautomers exist in dynamic equilibrium.
Stereoisomerism
Compounds that have the same chemical formula, as well as the same structure but different spatial arrangement or configuration, are known as Stereoisomers. This phenomenon is known as Stereoisomerism.
Types of Stereoisomers
Geometrical isomers
Optical isomers
Geometrical isomers
These isomers are also known as cis, trans-isomers. This phenomenon is geometrical isomerism and is exhibited by alkenes. In alkenes, there is a restricted rotation of groups about the double bonds between the two carbon atoms. Geometrical isomerism can also be seen in compounds having groups C=N or N=N bond-like in aldoximes. When similar groups are present on the same side of the double bond then it is called the cis isomer. When similar groups are on the opposite side of the double bond then it is called the trans isomer.
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cis and trans stereoisomers can be interconverted under extreme conditions like very high temperatures.
Optical Isomerism
Only those compounds with a chiral carbon can exhibit optical isomerism. Chiral carbon is that carbon that is attached to all the four different functional groups. The compound with a chiral carbon is said to have chirality. If chirality is present then the compound becomes optically active. A compound must be asymmetric to exhibit optical isomerism.
FAQs on Constitutional Isomers
1. What exactly are constitutional isomers in simple terms?
Constitutional isomers are compounds that share the exact same molecular formula but have their atoms connected in a different order. Think of them as being made from the same set of building blocks (atoms), but assembled into different structures. Because their structures are different, they are unique compounds with their own distinct properties.
2. What are the main types of constitutional isomerism?
Constitutional isomerism, also known as structural isomerism, is mainly classified into several types based on how the structures differ:
- Chain Isomerism: The isomers have different arrangements of the carbon chain (e.g., a straight chain vs. a branched chain).
- Position Isomerism: The isomers have the same carbon chain but differ in the position of a functional group or substituent.
- Functional Isomerism: The isomers have different functional groups, leading to entirely different chemical properties (e.g., an alcohol and an ether).
- Metamerism: This arises from different alkyl chains attached to the same functional group.
3. How are constitutional isomers different from stereoisomers?
The key difference lies in their atomic connections. Constitutional isomers have a different connectivity of atoms. In contrast, stereoisomers have the exact same connectivity but differ in how their atoms are arranged in three-dimensional space. Enantiomers and diastereomers are common examples of stereoisomers.
4. Can you provide a clear example of constitutional isomers?
A classic example is the formula C₄H₁₀. This formula can represent two different molecules:
- Butane: A straight chain of four carbon atoms.
- Isobutane (or 2-methylpropane): A branched chain where three carbons are in a line and the fourth is attached to the middle carbon.
Both have 4 carbons and 10 hydrogens, but their atomic connections are different, making them constitutional isomers.
5. Are constitutional isomers and conformational isomers the same thing?
No, they are very different. Constitutional isomers are distinct and separate compounds. To change one into another, you would need to break and re-form chemical bonds. Conformational isomers (or conformers) are just different spatial arrangements of the same molecule that can be interconverted simply by rotating around single bonds, without breaking any bonds.
6. Is there another name for constitutional isomers?
Yes, constitutional isomers are very commonly referred to as structural isomers. This alternative name is often used because it directly highlights that the difference between the isomers lies in their fundamental structure or connectivity.
7. Why is it important to distinguish between different constitutional isomers?
It's crucial because even with the same molecular formula, constitutional isomers are completely different chemical compounds. This means they have unique physical properties (like boiling points, melting points, and density) and chemical properties (like reactivity). For example, ethanol (an alcohol) and dimethyl ether are functional isomers (C₂H₆O), but one is a liquid at room temperature and the other is a gas.
8. How can I quickly tell if two structures are identical or constitutional isomers?
The easiest way is to try and name both structures using IUPAC nomenclature. If they have the same molecular formula but different IUPAC names, they are constitutional isomers. If they have the same molecular formula and the same IUPAC name (even if they look like they are drawn differently on paper), they are just identical molecules.
