Investigate The Variety Of Organic Compounds - Session 2

Isomerism is a phenomenon that describes the occurrence of two or more molecules with the same molecular formula, but different structural arrangements. Molecules that have the same molecular formula but different structural formula are called isomers. For example, butane and isobutene have the same molecular formula C₄H₁₀. Their structural formula is different. Butane has a linear chain structure. Isobutane has a branched structure.

In the same way propanal and propanone have same molecular formula C₂H₆O. Their structural formula is different. Propanal has the aldehyde functional group. Propanone has the ketone functional group. But the number and type of atoms are same in both of them. Because of the difference in functional group, their properties are also different.

Structural Isomerism is a type of isomerism. Structural isomerism occurs when two or more molecules have the same molecular formula but differ in their structural arrangement. This difference in structure can result in differences in physical and chemical properties. For example, pentane and 2-methylbutane have the same molecular formula but different structural arrangements. Their molecular formula is C₅H₁₂ .

Structural isomerism is further categorized into chain isomerism, position isomerism and functional group isomerism. Chain isomerism occurs in organic molecules where the carbon skeleton of the molecule differs between isomers. Chain isomerism is commonly observed in alkanes. Alkanes with four or more carbon atoms can exhibit chain isomerism. For example, the molecule pentane can exist in three isomeric forms. These are n-pentane, isopentane and neopentane. All of these isomers of pentane have molecular formula C₅H₁₂.

Position isomerism occurs when functional groups or substituents in a molecule take up different positions on the carbon chain. For example, the molecule butanol C₄H₁₀O has two isomers. These isomers are 1-butanol and 2-butanol. These two isomers differ in the position of the hydroxyl group -OH on the carbon chain.

Functional group isomerism occurs when molecules have the same molecular formula but different functional groups. One example of functional group isomerism is seen in butanal and 2-butanone . The molecular formula of both molecules is C₄H₈O . Butanal has an aldehyde functional group -CHO. While 2-butanone has a ketone functional group -CO-.

Stereoisomerism is a type of isomerism that occurs when molecules have the same molecular formula and structural formula but differ in the three-dimensional arrangement of atoms in space. The difference in three-dimensional arrangement of atoms in space can result in different physical and chemical properties. A general example of stereoisomerism is seen in 2-butene. The two stereoisomers of 2-butene are cis-2-butene and trans-2-butene.

Both stereoisomers have same molecular formula and structural formula. The only difference between both of them is the three-dimensional arrangement of atoms in space. In cis-2-butene two methyl groups attached to double bonded carbon atoms are on one side of carbon carbon double bond plane. While in tran-2-butene the same two methyl groups are on opposite sides.

Optically active compounds are those compounds that can rotate the plane polarized light either in clockwise direction or anticlockwise direction. Optically active compounds are also called chiral compounds. If four different atoms or groups are attached to carbon atom then molecule is called chiral molecule and that carbon is called asymmetric carbon or chiral carbon. For example, 2-hydroxypropanoic acid is optically active. It is a chiral molecule. Four different atoms or group are attached to the carbon atom.

Stereoisomers are further classified into enantiomers and diastereomers. Enantiomers are a type of stereoisomers that are non super-imposable mirror images of each other. Enantiomers are optically active compounds. For example, enantiomers of 2-hydroxypropanoic acid are non super-imposable mirror images of each other.

Diastereomers are stereoisomers that are not mirror images of each other and are therefore not enantiomers. They arise when a molecule has two or more chiral centers. Diastereomers have different physical and chemical properties, such as melting point, boiling point, and solubility. One way to identify diastereomers is to examine their molecular formulas and three-dimensional structures. If two molecules have the same molecular formula but different arrangements of groups around stereocenters, they are likely to be diastereomers.

For example, D-Glucose and D-Altrose are not mirror images of each other. They have same molecular formula and structural formula. Arrangement of atoms in three-dimensional space is different for both of them. They are diastereomers.

Another example of diastereomers is cis-2-butene and trans-2-butene. There is a double bond between two carbon atoms in 2-butene. As we can see, two methyl groups are on one side of carbon carbon double bond in cis-2-butene. Can you see that these methyl groups are on opposite side in trans-2-butene? However, they are not mirror images of each other.