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Posted on Jul 4, 2023 in Chemistry

Isomerism is the phenomenon where two or more compounds have the same molecular formula, but different arrangements of their atoms. These compounds are called isomers.

Isomers can differ in their physical and chemical properties, such as boiling and melting points, reactivity, and solubility. There are two main types of isomerism: structural isomerism and stereoisomerism.

Structural isomerism occurs when the atoms in the molecules are arranged in different ways. This can include differences in the bonding pattern, such as whether the atoms are connected in a chain or a ring structure, or differences in the position of functional groups.

Stereoisomerism, on the other hand, occurs when the atoms are arranged in the same order, but differ in their spatial arrangement. This can include differences in the orientation of substituent groups around a double bond or a chiral center.

Isomerism is an important concept in chemistry because it affects the properties and behavior of molecules. By understanding isomerism, chemists can predict and manipulate the behavior of compounds, which is useful in fields such as drug discovery and materials science.

describe its type

There are two main types of isomerism: structural isomerism and stereoisomerism.

1. Structural Isomerism: This occurs when the atoms in the molecules are arranged in different ways. This can include differences in the bonding pattern, such as whether the atoms are connected in a chain or a ring structure, or differences in the position of functional groups. Structural isomers have the same molecular formula, but different structural formulae.

For example, butane and isobutane are structural isomers. Both have the same molecular formula, C4H10, but butane has a linear chain structure, while isobutane has a branched structure.

2. Stereoisomerism: This occurs when the atoms are arranged in the same order, but differ in their spatial arrangement. This can include differences in the orientation of substituent groups around a double bond or a chiral center. Stereoisomers have the same structural formula, but different spatial arrangements.

There are two types of stereoisomers: enantiomers and diastereomers.

• Enantiomers are mirror images of each other and have the same physical and chemical properties, except for their interaction with polarized light. Enantiomers have a chiral center, which is a carbon atom bonded to four different substituent groups. Enantiomers rotate plane-polarized light in opposite directions, which is known as optical activity.
• Diastereomers are stereoisomers that are not mirror images of each other and have different physical and chemical properties. Diastereomers have more than one chiral center, and the spatial arrangement of substituent groups around each chiral center is different.

For example, cis- and trans-isomers of 1,2-dichloroethene are diastereomers. They have the same molecular formula and the same structural formula, but the spatial arrangement of the chlorine atoms around the double bond is different.

describe types of structural isomerism

There are three main types of structural isomerism:

1. Chain Isomerism: In chain isomerism, the isomers differ in the arrangement of the carbon chain. This can involve straight, branched, or cyclic chains. For example, pentane and 2-methylbutane are chain isomers. Both have the same molecular formula, C5H12, but pentane has a straight chain, while 2-methylbutane has a branched chain.
2. Position Isomerism: In position isomerism, the isomers differ in the position of a functional group or substituent on the carbon chain. For example, in dichlorobenzene, the chlorine atoms can be positioned in three different ways: ortho, meta, and para, giving rise to three different position isomers.
3. Functional Group Isomerism: In functional group isomerism, the isomers have the same molecular formula, but differ in the functional group or substituent. For example, ethanol and dimethyl ether are functional group isomers. Both have the molecular formula C2H6O, but ethanol has a hydroxyl (-OH) functional group, while dimethyl ether has an ether (-O-) functional group.

Structural isomerism is an important concept in organic chemistry, and it affects the physical and chemical properties of molecules. By understanding structural isomerism, chemists can predict and manipulate the behavior of compounds, which is useful in fields such as drug discovery and materials science.

There is another type of structural isomerism called Tautomerism, where the isomers exist in dynamic equilibrium and rapidly interconvert due to the movement of a proton. In tautomerism, the isomers differ in the position of a proton and the double bond. For example, keto-enol tautomerism in which a carbonyl group can shift to form an enol (an alcohol with a double bond), and vice versa. Tautomers have different physical and chemical properties, and their interconversion plays an important role in biochemical processes such as DNA replication and protein synthesis.

classification of sterioisomerism

Stereoisomers are molecules that have the same molecular formula and the same connectivity of atoms, but different spatial arrangements of their atoms. There are two main types of stereoisomers: enantiomers and diastereomers.

1. Enantiomers: Enantiomers are stereoisomers that are non-superimposable mirror images of each other. They have the same physical and chemical properties except for their interaction with plane-polarized light (optical activity). Enantiomers rotate plane-polarized light in equal and opposite directions and are labeled as (+) or (-) depending on the direction of rotation. Enantiomers have opposite configurations at every chiral center in the molecule.
2. Diastereomers: Diastereomers are stereoisomers that are not mirror images of each other. They have different physical and chemical properties and can have different reactivities. Diastereomers differ in their configuration at one or more, but not all, chiral centers in the molecule.

There are several subcategories of diastereomers:

a. Geometric isomers: Geometric isomers have different spatial arrangements of substituents around a double bond or a ring. They are also known as cis-trans isomers.

b. Conformational isomers: Conformational isomers are stereoisomers that can be interconverted by rotation about single bonds. They have different spatial arrangements due to the rotation of one or more atoms around a single bond.

c. Epimers: Epimers are diastereomers that differ in configuration at one chiral center.

d. Anomers: Anomers are epimers that differ in configuration at the anomeric carbon, which is the carbon that is attached to two oxygen atoms in a cyclic sugar molecule.

e. Cis-trans isomers: Cis-trans isomers have different spatial arrangements of substituents around a double bond or a ring. They are also known as geometric isomers.

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