Question

Give structural formulas and systematic names for the three structural isomers of trimethylbenzene, \(\mathrm{C}_{6} \mathrm{H}_{3}\left(\mathrm{CH}_{3}\right)_{3}\)

Short answer

The structural isomers are 1,2,3-Trimethylbenzene, 1,2,4-Trimethylbenzene, and 1,3,5-Trimethylbenzene.

Step-by-step solution

Understanding Trimethylbenzene

Trimethylbenzene is an aromatic compound made of a benzene ring with three methyl (CH₃) groups attached to it. Its molecular formula is C₉H₁₂. Since there are three positions available (ortho, meta, and para) for these methyl groups to attach to the benzene ring, they can form three different isomer structures.

Identifying Possible Positions for Methyl Groups

A benzene ring has six carbon atoms. Numbering these as positions 1 to 6, we need to place three CH₃ groups on these positions. The benzene ring is symmetrical, so the methyl groups can be placed in different relative positions to create the structural isomers.

Creating the First Isomer - 1,2,3-Trimethylbenzene

Here, the methyl groups are attached to adjacent positions 1, 2, and 3 on the benzene ring. This structure is also known as hemimellitene. The structural formula is represented as follows: \[ \text{1,2,3-Trimethylbenzene:} \quad \begin{array}{c}\text{CH}_3 \quad \text{CH}_3 \\begin{array}{c}\quad \text{C}_6\text{H}_3\end{array}\quad \text{CH}_3\end{array} \]

Creating the Second Isomer - 1,2,4-Trimethylbenzene

In this isomer, the methyl groups are attached to positions 1, 2, and 4. This arrangement avoids adjacent methyl groups but still attaches them close together. This isomer is also known as pseudocumene. The structural formula is:\[ \text{1,2,4-Trimethylbenzene:} \quad \begin{array}{c}\text{CH}_3 \quad \\begin{array}{c}\quad \text{C}_6\text{H}_3\end{array}\quad \text{CH}_3\end{array} \quad \text{CH}_3 \]

Creating the Third Isomer - 1,3,5-Trimethylbenzene

For this isomer, the methyl groups can be placed on positions 1, 3, and 5, which are alternating positions on the benzene ring. This is known as mesitylene. The structural formula can be represented as follows:\[ \text{1,3,5-Trimethylbenzene:} \quad \begin{array}{c}\text{CH}_3 \text{C}_6\text{H}_3 \quad \\quad \text{CH}_3 \end{array} \text{CH}_3 \]

Confirming Unique Structural Isomers

By organizing methyl groups on the benzene ring differently, each of the three structures is unique. None of these arrangements can be transformed into another by rotating or flipping without breaking and reforming bonds.

Key concepts

Aromatic Compounds

Aromatic compounds are a fascinating group of organic chemicals known for their exceptional stability and distinctive smell, often referred to as "aromaticity." The most iconic example of such a compound is benzene, a simple structure consisting of a six-carbon ring with alternating double bonds. This particular arrangement of electrons creates a very stable molecule due to the continuous overlap of p-orbitals, creating a conjugated system that greatly increases the molecule's stability.
Aromatic compounds, like benzene, are often found in many important materials, including products in the petrochemical industry and various dyes and pharmaceuticals. Their unique electronic properties make them a cornerstone for organic chemistry.
Trimethylbenzene is an aromatic compound because it features a benzene ring with three added methyl groups. These groups introduce slight variations in structure but do not disrupt the basic aromatic character of the benzene ring itself, keeping its stability and chemical nature intact.

Chemical Structure

The chemical structure of a compound refers to the specific arrangement of atoms within a molecule. In organic chemistry, understanding a compound's structure is crucial for predicting its reactivity and physical properties.
In the case of trimethylbenzene, the chemical structure is based on a benzene ring which is a hexagonal arrangement of six carbon atoms, each bonded to one hydrogen atom. What distinguishes trimethylbenzene is the substitution of three hydrogen atoms with three methyl ( (CH₃) ) groups.
These methyl groups can attach in different positions relative to each other on the benzene ring, creating different structural isomers. The isomers of trimethylbenzene include:

• 1,2,3-Trimethylbenzene (hemimellitene), where the methyl groups are adjacent.
• 1,2,4-Trimethylbenzene (pseudocumene), with methyl groups in a non-adjacent setup.
• 1,3,5-Trimethylbenzene (mesitylene), with the methyl groups at alternating positions around the ring.

Each arrangement alters the physical and chemical characteristics of the molecule without changing the overall molecular formula of C₉H₁₂ .

Systematic Naming

Systematic naming in chemistry, also known as IUPAC naming, is a method used to provide a unique and clear name to chemical compounds based on their structure.
For trimethylbenzene, the systematic naming involves two main parts: identifying the main carbon framework, which is benzene in this case, and showing how many and where the methyl groups are attached.
To correctly name the isomers of trimethylbenzene, the benzene ring is usually numbered. For example:

• 1,2,3-Trimethylbenzene implies that the methyl groups are attached to the first, second, and third positions on the benzene ring in a contiguous manner.
• 1,2,4-Trimethylbenzene indicates a non-contiguous attachment of the methyl groups, equidistant around the ring.
• 1,3,5-Trimethylbenzene denotes that the methyl groups are placed in alternating positions on the benzene structure.

The systematic names make it efficient for chemists to convey the structure of complex organic molecules without confusion, making communication and research more streamlined.