isomerism

Isomerism is a fundamental concept in chemistry where two or more compounds possess the same molecular formula but exhibit different physical and chemical properties due to a difference in the arrangement of atoms within the molecule. These compounds are referred to as isomers.

1. Structural Isomerism

Structural isomers, also known as constitutional isomers, differ in the order in which their atoms are connected. This category is further divided into several types:

  • Chain Isomerism: These isomers differ in the structure of the carbon skeleton (straight vs. branched). For example, butane (C{4}H{10}) has two isomers: n-butane and isobutane (2-methylpropane).

  • Positional Isomerism: These isomers have the same carbon skeleton and the same functional groups, but the functional groups are attached at different positions. An example is the difference between 1-chloropropane and 2-chloropropane (C{3}H{7}Cl).

  • Functional Group Isomerism: These isomers share the same molecular formula but contain different functional groups. For instance, ethanol (CH{3}CH{2}OH) and dimethyl ether (CH{3}OCH{3}) both have the formula C{2}H{6}O.

  • Metamerism: This arises due to unequal distribution of alkyl groups on either side of a polyvalent functional group. For example, diethyl ether and methyl propyl ether (C{4}H{10}O).

2. Stereoisomerism

Stereoisomers have the same molecular formula and the same connectivity of atoms, but they differ in the spatial orientation of those atoms.

  • Geometrical Isomerism: Often found in alkenes due to the restricted rotation around the carbon-carbon double bond (C=C).

    • Cis-isomer: The identical groups are on the same side of the double bond.

    • Trans-isomer: The identical groups are on opposite sides of the double bond.

  • Optical Isomerism: This occurs in molecules that are non-superimposable mirror images of each other. Such molecules are called enantiomers.

    • A molecule must typically have a chiral center—a carbon atom bonded to four different groups—to exhibit optical isomerism.

    • These isomers are distinguished by their ability to rotate plane-polarized light in different directions (d- for dextrorotatory and l- for levorotatory).