Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Assign the IUPAC name for the following compound. (a) 3,4 -dichloroazobenzene (b) (4-chlorophenyl) (3-chlorophenyl) diazene (c) 3,4 -bis(chlorophenyl) diazene (d) (3-chlorophenyl) (4-chlorophenyl) diazene

Step-by-Step Solution

Answer

Option (d) (3-chlorophenyl) (4-chlorophenyl) diazene is the correct IUPAC name.

Step 1: Analyze the Structure

Examine the name given in the option. Option (b) is stated as (4-chlorophenyl) (3-chlorophenyl) diazene, indicating two phenyl groups, each containing a single chlorine atom at different positions, joined by a diazene linkage (-N=N-). The first phenyl ring has the chlorine at position 4, and the second phenyl ring has the chlorine at position 3.

Step 2: Understand Substituent Positions

The IUPAC naming convention assigns substituents' positions based on attachment to the parent hydrocarbon. In option (b), (4-chlorophenyl) indicates that the chlorine is on the 4th position of the first benzene ring, while (3-chlorophenyl) indicates that the chlorine is on the 3rd position of the second benzene ring.

Step 3: Determine Main Functional Group

The diazene group (-N=N-) is the main linking functional group between the two phenyl rings. In diazenes, the naming typically focuses on the substituents on the phenyl rings.

Step 4: Assign Correct IUPAC Name

By IUPAC rules, we list out the substituents with their respective position numbers, and then name the compound according to alphabetical order of substituents and symmetry. Here, option (d) is correctly ordered: (3-chlorophenyl) (4-chlorophenyl) diazene indicates correct positions and respects IUPAC naming conventions by prioritizing the chlorophenyl groups.

Key Concepts

Substituent Positions

The placement of substituents is crucial when naming chemical compounds using the IUPAC system. In our example, substituents refer to the chlorine atoms attached to the phenyl rings. Position numbers are assigned to these substituents based on their location relative to a reference point in the ring, often the position of a specified group or a point of attachment to a chain.
For a mono-substituted benzene ring, the substituent receives the lowest possible position number, typically starting from one of the carbons adjacent to any functional group present.
In the compound (4-chlorophenyl) (3-chlorophenyl) diazene, the numbers 3 and 4 specify where each chlorine atom is attached on its respective phenyl ring. This approach permits chemists to accurately describe where on the molecule each substituent is located, a critical requirement for communication and understanding in chemistry.

Phenyl Rings

Phenyl rings are a fundamental component in organic chemistry, representing the benzene ring minus one hydrogen atom, usually denoted as -C₆H₅. Phenyl groups are quite common due to their planar structure and unique electronic properties, which contribute to stability through resonance.
When phenyl groups appear in a compound, they can be either substituted or unsubstituted by various other groups. This can greatly affect the chemical and physical properties of the overall compound.
In the context of the given problem, each phenyl ring has a chlorine substituent, creating a substituted phenyl group. These substitutions are key to naming the compound as they impact both the structure and its corresponding IUPAC name.

Diazene Linkage

The diazene linkage, expressed as (-N=N-), is a functional group that connects two phenyl rings in the compound. Known as an azo linkage as well, this connection involves two nitrogen atoms double-bonded to each other.
Diazene linkages are prominent in azo compounds, often manifesting in vivid colors and widely used in dyes and pigments. The presence of this linkage between the phenyl groups is the principal feature for determining the compound's name.
In IUPAC naming, the linkage's presence does not alter the substituent positions on the phenyl rings but significantly contributes to the compound's functionality and categorization. It's vital to ensure correct positioning of substituents relative to the diazene linkage for accurate chemical identification.