2004 AIPMT Organic Chemistry - Some Basic Principles and TechniquesStructural Isomerism Medium Question 79 of 88 in Organic Chemistry - Some Basic Principles and Techniques

The molecular formula of diphenyl methane, , is C₁₃H₁₂. How many structural isomers are possible when one of the hydrogens…

The molecular formula of diphenyl methane, , is $C_{13}H_{12}$. How many structural isomers are possible when one of the hydrogens is replaced by a chlorine atom :-
A 4
B 8
C 7
D 6

The answer is A.

Solution

Diphenylmethane is symmetrical: the two phenyl rings are equivalent, and each ring is symmetrical about the axis through the $CH_2$ carbon and the para carbon.
So the distinct types of hydrogen are:
1. The two H atoms of the $CH_2$ group: replacing one gives $C_6H_5-CHCl-C_6H_5$.
2. Ortho H of a ring (2 per ring, all equivalent): gives o-chlorodiphenylmethane.
3. Meta H of a ring: gives m-chlorodiphenylmethane.
4. Para H of a ring: gives p-chlorodiphenylmethane.
(Substituting in the second ring gives the same compounds, because both rings are equivalent.)
So 4 structural isomers are possible.

Concept Counting monochloro isomers by symmetry

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