Question

For the following formulas, what types of isomerism could be exhibited? For each formula, give an example that illustrates the specific type of isomerism. The types of isomerism are structural, geometric, and optical. a. \(C_{6} \mathrm{H}_{12}\) b. \(\mathrm{C}_{5} \mathrm{H}_{12} \mathrm{O}\) c. \(\mathrm{C}_{6} \mathrm{H}_{4} \mathrm{Br}_{2}\)

Short answer

a. C6H12 can exhibit structural and geometric isomerism. Examples: Structural - Hex-1-ene and Hex-2-ene; Geometric - cis-Hex-2-ene and trans-Hex-2-ene. b. C5H12O can exhibit structural and optical isomerism. Examples: Structural - Pentan-1-ol and Pentan-2-ol; Optical - (R)-Pentan-2-ol and (S)-Pentan-2-ol. c. C6H4Br2 can only exhibit structural isomerism. Examples: Structural - Ortho-dibromobenzene, Meta-dibromobenzene, and Para-dibromobenzene. No examples for geometric or optical isomerism.

Step-by-step solution

Probable Isomerism for C6H12 (a)

In case of C6H12, it possible that structural isomerism, and geometric isomerism can be exhibited.

Example for each Isomerism of C6H12 (a)

1. Structural Isomerism: Hex-1-ene and Hex-2-ene are examples of structural isomers, as they have the same molecular formula \(C_{6}H_{12}\) but different arrangements of the double bond. 2. Geometric Isomerism: cis-Hex-2-ene and trans-Hex-2-ene are examples of geometric isomers, as they have the same molecular formula (C6H12) and the same arrangement of the double bond but different arrangements of substituents.

Probable Isomerism for C5H12O (b)

In case of C5H12O, it's possible that structural isomerism and optical isomerism can be exhibited.

Example for each Isomerism of C5H12O (b)

1. Structural Isomerism: Pentan-1-ol and Pentan-2-ol are examples of structural isomers, as they have the same molecular formula \(C_{5}H_{12}O\) but different positions of the hydroxyl group (OH). 2. Optical Isomerism: (R)-Pentan-2-ol and (S)-Pentan-2-ol are examples of optical isomers, as they have the same molecular formula (C5H12O), the same position of the hydroxyl group, and a chiral carbon center but are non-superimposable mirror images.

Probable Isomerism for C6H4Br2 (c)

In case of C6H4Br2, only structural isomerism and no other types of isomerism can be exhibited.

Example for each Isomerism of C6H4Br2 (c)

1. Structural Isomerism: Ortho-dibromobenzene, Meta-dibromobenzene, and Para-dibromobenzene are examples of structural isomers, as they have the same molecular formula \(C_{6}H_{4}Br_{2}\) but different positions of the bromine atoms on the benzene ring. 2. There are no chiral carbons or double bonds in dibromobenzenes, so there are no examples of geometric or optical isomerism exhibited by this molecular formula.

Key concepts

Structural Isomerism

Structural isomerism occurs when compounds have the same molecular formula but different structural arrangements of atoms. This is like having the same building materials but constructing different houses. For instance, the molecular formulas for hex-1-ene and hex-2-ene are both \(C_6H_{12}\), but they differ in the position of the double bond within the carbon chain. Similarly, for \(C_5H_{12}O\), pentan-1-ol and pentan-2-ol illustrate structural isomerism by their differing locations of the hydroxyl group (OH). Another example is dibromobenzenes like ortho-, meta-, and para-dibromobenzene, which have the same \(C_6H_4Br_2\) formula but differ in the arrangement of bromine on the benzene ring.
To identify structural isomers, examine how the atoms are connected. Different connectivity leads to different structures, and therefore, unique isomers.

Geometric Isomerism

Geometric isomerism, also known as cis-trans isomerism, arises from restricted rotation around a double bond or within a cyclic structure. This form of isomerism is akin to couples sitting on opposite sides of a bench, facing different directions. In the case of \(C_6H_{12}\), cis-hex-2-ene and trans-hex-2-ene exhibit geometric isomerism. Both molecules retain the same molecular assembly but display different spatial orientations of certain atoms or groups around the fixed double bond. The term 'cis' refers to similar groups on the same side, and 'trans' indicates they are on opposite sides.
Geometric isomers exhibit different physical properties, like boiling points and solubility, which helps chemists distinguish them in practice.

Optical Isomerism

Optical isomerism is exhibited by molecules that have chiral centers. These molecules are non-superimposable mirror images, much like your left and right hands. For example, \(C_5H_{12}O\) includes optical isomers like (R)-pentan-2-ol and (S)-pentan-2-ol. Both forms share the same molecular formula and hydroxyl group positioning but differ in the 3D spatial arrangement around their chiral carbon.
Molecules exhibiting optical isomerism can rotate plane-polarized light in different directions. This behavior is not only fun to observe but also critically important in fields like pharmacology, where one optical isomer of a drug might be effective while its mirror image isn’t.