AIPMT PRELIMS 2004 Chemistry Isomerism MCQ Question

To solve the problem of determining how many structural isomers are possible for diphenyl methane (C₁₃H₁₂) when one of the hydrogens is replaced by a chlorine atom, we start by analyzing the structure and the implications of substituting one hydrogen atom.

Step 1: Understanding the Structure of Diphenyl Methane

Diphenyl methane has the following structure:

• It consists of a central carbon (the methylene group, -CH₂-) that is bonded to two phenyl groups (C₆H₅).

The molecular formula for diphenyl methane is $C_{13}H_{12}$, which indicates it has:

• 13 carbon atoms
• 12 hydrogen atoms

Step 2: Substituting Hydrogen with Chlorine

When one of the hydrogen atoms is replaced by a chlorine atom (Cl), the new molecular formula becomes $C_{13}H_{11}Cl$. This substitution leads to the formation of different structural isomers based on the position of the chlorine atom.

Step 3: Identifying Possible Isomers

The two phenyl rings in diphenyl methane can affect the position of the chlorine atom. The possible positions for the chlorine substitution are:

1. Chlorine on the carbon adjacent to one phenyl ring.
2. Chlorine on the carbon adjacent to the other phenyl ring (which is essentially the same as the first due to symmetry).
3. Since the substitution affects the overall structure, we need to consider both the ortho (adjacent) and para (opposite) positions relative to the methylene group.

However, in a symmetrical molecule like diphenyl methane:

• The substitution at the methylene group itself (which is the only carbon that can have a chlorine atom) does not lead to different isomers since both phenyls are identical.

Step 4: Counting Isomers

The following are the distinct positions where chlorine can be placed:

1. Ortho positions (both phenyl groups are identical, hence this position is unique).
2. Para position (again, identical due to symmetry).

Considering the symmetry of the molecule:

• Only two unique structural isomers can be formed by substituting a hydrogen atom with chlorine:
  • Isomer 1: Chlorine in the ortho position.
  • Isomer 2: Chlorine in the para position.

Conclusion

Thus, the total number of structural isomers when one of the hydrogens in diphenyl methane is replaced by chlorine is 2.

Clarification of Other Options

• Option A (4): Incorrect, as the symmetry of diphenyl methane limits the number of unique positions.
• Option C (3): Incorrect, similar reasoning as above; the unique structures do not add up to three.
• Option D (6): Incorrect, as the maximum unique structures are governed by the symmetry, leading to only two distinct isomers.

Therefore, the correct answer is B) 2.

By understanding the symmetry and structure of diphenyl methane, we can confidently conclude that only two structural isomers are possible with the substitution of a hydrogen atom by chlorine.