Question

There are three compounds with the formula \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Cl}_{3}\) in which three of the hydrogen atoms of the benzene molecule have been replaced by chlorine atoms. Draw structures for these compounds.

Short answer

Answer: The three isomers of the compound C6H3Cl3 are: 1. 1,2,3-trichlorobenzene (also known as ortho-isomer) 2. 1,2,4-trichlorobenzene (also known as meta-isomer) 3. 1,3,5-trichlorobenzene (also known as para-isomer)

Step-by-step solution

Knowing the benzene structure

In order to draw the isomers of the compound, we must first understand the structure of the benzene molecule. Benzene has a hexagonal ring structure with each carbon atom bonded to two other carbon atoms and one hydrogen atom. All carbon-carbon bonds in the ring are equivalent with a bond order between a single bond and a double bond, known as resonance.

Replacing hydrogen atoms with chlorine atoms

In our compound, three hydrogen atoms are replaced by chlorine atoms. We'll create different isomers based on the position of the chlorine atoms with respect to each other.

Creating the first isomer (1,2,3-Trichlorobenzene)

In the first isomer, we replace three consecutive hydrogen atoms with chlorine atoms. The isomer is called 1,2,3-trichlorobenzene, also known as the ortho-isomer, and its structure is: ``` Cl Cl \ / C === C // \ C === C--Cl \ / H H ```

Creating the second isomer (1,2,4-Trichlorobenzene)

In the second isomer, we replace hydrogen atoms at positions 1, 2, and 4 with chlorine atoms. The resulting isomer is 1,2,4-trichlorobenzene or meta-isomer, and its structure is: ``` Cl Cl \ / C === C // \ C--Cl === C \ / H H ```

Creating the third isomer (1,3,5-Trichlorobenzene)

In the third isomer, we replace hydrogen atoms at positions 1, 3, and 5 with chlorine atoms. This isomer is called 1,3,5-trichlorobenzene or para-isomer, and its structure is: ``` Cl H \ / C === C // \ C--Cl === C--Cl \ / H H ``` So, the three possible isomers corresponding to \(\mathrm{C}_{6} \mathrm{H}_{3} \mathrm{Cl}_{3}\) are 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, and 1,3,5-trichlorobenzene.

Key concepts

Benzene Structure

Benzene is a fundamental organic molecule with the formula \(\mathrm{C}_6\mathrm{H}_6\). Its structure is iconic, depicted as a hexagonal ring. This shape represents six carbon atoms connected to one another, forming a continuous loop. Each carbon atom bonds with two adjacent carbon atoms and also with one hydrogen atom. This creates a simple hexagonal form.

The magic in benzene occurs because of its unique bonding, known as resonance. Resonance implies that the electrons in the bonds are delocalized, or spread evenly across all six bonds in the ring. This means all carbon-carbon bonds exhibit a bond length between that of a single and a double bond.

This equal distribution of electrons gives benzene extra stability, as it is not simple alternating single and double bonds but a more robust system shared throughout the ring. This will help us understand how benzene reacts with other chemicals, like chlorine, to form various isomers.

Chlorination

Chlorination is a chemical reaction where chlorine atoms substitute other atoms within a molecule, in this case, hydrogen atoms in benzene. When chlorine gas is bubbled through benzene, under certain conditions, a substitution reaction occurs.

In our example, three hydrogen atoms in benzene can be replaced by chlorine atoms, resulting in a trichlorobenzene molecule with the formula \(\mathrm{C}_6\mathrm{H}_3\mathrm{Cl}_3\).

Chlorine is quite reactive, and this reaction involves breaking the strong carbon-hydrogen bonds and forming new carbon-chlorine bonds. This substitution is selective and can occur in different positions around the benzene ring, leading to the formation of different structural isomers.

Chemical Isomerism

Chemical isomerism refers to the phenomenon where molecules have the same molecular formula but differ in the arrangement of their atoms. In the case of trichlorobenzene, there are different ways the chlorine atoms can be arranged around the benzene ring, leading to structural isomers.

There are three main types of isomers for \(\mathrm{C}_6\mathrm{H}_3\mathrm{Cl}_3\):

• Ortho-isomer (1,2,3-trichlorobenzene): Here, the chlorine atoms are positioned on three consecutive carbon atoms in the benzene ring, creating what is known as the ortho-position.
• Meta-isomer (1,2,4-trichlorobenzene): This configuration places the chlorine atoms at positions 1, 2, and 4, skipping every other carbon atom, known as the meta-position.
• Para-isomer (1,3,5-trichlorobenzene): In this case, chlorine atoms are placed at the 1, 3, and 5 positions, opposite each other in the ring, forming the para-position.

Understanding isomerism is essential in chemistry because different isomers have unique physical and chemical properties, despite having the same molecular formula. This affects how they might be used or how they interact in chemical reactions.