IMAT 2011 Q62 [Benzene Rings]

An aromatic compound consists of two benzene rings joined together.

Which of the following could be its formula?

  1. {C_{10}H_8}
  2. {C_{10}H_{10}}
  3. {C_{10}H_{12}}
  4. {C_{12}H_{10}}
  5. {C_{12}H_{12}}

A. 1 and 4
B. 2 and 4
C. 3 and 4
D. 2 and 5
E. 3 and 5

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The first possibility when two benzene rings join is that they share a side, connecting to each other with a pair of carbons.

This forms what is actually called Naphthalene, the simplest polycyclic aromatic hydrocarbon. Meaning option 1 must be correct and the only answer possible is A.

For the sake of chemistry, we could also explore different possibilities!

Option 4 is the organic compound Biphenyl. Biphenyl forms when the two benzene rings attach to each other by a single carbon bond. It ends up looking like so:

Options 2, 3 and 5 are simply not possible and therefore incorrect.


hi can you explain why option 5 is not possible?
we can attach c2 h5 to first compound.

Hi Paria,

You cannot use a side chain (ethyl group) in this example otherwise you have not obeyed the properties described in the first line of the case text.

As the text points out, you have to have:

  • two aromatic rings, meaning two benzene rings.
  • and they must be connected

If you use an ethyl group as a side chain, then you loose the aromatic property.