Question

Draw all the possible structural isomers for the molecule having the formula \(\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{Cl}\). All isomers contain one benzene ring.

Short answer

There are three structural isomers: ortho-, meta-, and para-chlorotoluene.

Step-by-step solution

Understanding Isomers

Isomers are molecules with the same molecular formula but different structural arrangements. In this case, they share the formula \(\mathrm{C}_{7} \mathrm{H}_{7} \mathrm{Cl}\), implying different placements of the \(\mathrm{Cl}\) atom on the benzene ring.

Examination of Substitution Patterns

A benzene ring has six carbon atoms, which allows various placement possibilities for the \(\mathrm{Cl}\) atom, considering symmetrical and asymmetrical positions. This means the \(\mathrm{Cl}\) atom could be in different positions relative to the \(\mathrm{H}\) atoms.

Para-Isomer

The para-isomer is where the \(\mathrm{Cl}\) atom is positioned opposite to the substituent group on the benzene ring. In this structural isomer, the \(\mathrm{Cl}\) atom is on the 4th carbon atom relative to another substitution point.

Meta-Isomer

The meta-isomer has the \(\mathrm{Cl}\) atom positioned two carbons away from another substitution point on the benzene ring. It is placed on the 3rd carbon relative to another substitution point.

Ortho-Isomer

The ortho-isomer occurs when the \(\mathrm{Cl}\) atom is adjacent to another substituent group, placed on the 2nd carbon atom relative to that group.

Key concepts

Benzene Ring

A benzene ring is a fundamental structure in organic chemistry, known for its aromatic properties. It consists of six carbon atoms arranged in a hexagonal shape. Each carbon atom is bonded to one hydrogen atom and linked to its neighbors by alternating single and double bonds. This creates a conjugated system where electrons are shared across the ring, giving benzene its stability and unique properties.
Benzene rings are a common feature in many organic compounds, contributing to their chemical reactivity and aromaticity. Additionally, the benzene ring's symmetry allows for different substitution patterns, crucial for understanding isomers.

Molecular Formula

The molecular formula is a way to represent the exact number and types of atoms in a molecule. For the compound \(C_7H_7Cl\), the formula shows there are 7 carbon atoms, 7 hydrogen atoms, and 1 chlorine atom. This formula forms the basis for identifying possible structural isomers, as the arrangement of these atoms can vary without changing the total count.
While the molecular formula provides a snapshot of the molecule's composition, it doesn't depict connectivity or structure, which is why we also examine structural arrangements to differentiate isomers.

Substitution Patterns

Substitution patterns describe how atoms or groups of atoms replace hydrogen atoms on a benzene ring. These patterns are determined by the position of the substituent groups (in this case, chlorine) relative to other atoms or groups.
The three principal substitution patterns on a benzene ring include ortho, meta, and para positions. These positions dictate how substituents are spatially arranged, affecting chemical reactivity and physical properties. Understanding these patterns is vital for drawing structural isomers correctly.

Para-Isomer

The para-isomer is one of the possible structural arrangements where a substituent is located at the fourth carbon relative to another substituent on the benzene ring.
Imagine placing the chlorine atom directly opposite another substitution group across the benzene ring. This arrangement is symmetric and often contributes to specific interaction patterns in chemical reactions. The para position is denoted as the 1,4 positioning on the benzene structure, demonstrating distinctive properties due to its symmetry.

Meta-Isomer

In a meta-isomer, the chlorine atom is positioned on the third carbon atom relative to the other substituent on the benzene ring.
This means there is a single carbon separating the chlorine and the reference substituent group, referred to as the 1,3 positioning. Unlike the para position, the meta-isomer exhibits asymmetry, influencing its interaction with other molecules and contributing to its unique chemical profile.

Ortho-Isomer

The ortho-isomer features a substituent at the second position, adjacent to the reference substituent on the benzene ring.
This proximity can lead to interactions between the two substituents that are not possible in meta or para isomers, often resulting in different chemical and physical properties. The ortho configuration, represented as 1,2 positioning, is essential for understanding how subtle changes in atom placement affect a compound's behavior and reactivity.