Question

A hydrocarbon of unknown structure has the formula $C_8{H}_{10}$. On catalytic hydrogenation over the Lindlar catalyst, 1 equivalent of $H_2$ is absorbed. On hydrogenation over a palladium catalyst, 3 equivalents of $H_2$ are absorbed. Suggest several structures:

• A. The number of structures < 2
• B. The number of structures < 3
• C. The number of structures < 4
• D. None of the above

Answer 1

The correct option is D None of the above

Start by calculating the degree of unsaturation or the Double bond equivalents:

Calculate DBE = { C + 1 - (H/2) + (N/2) }
= 8 + 1 - 5
= 4



There has to be exactly one triple bond (which takes of 2 out of 3 $\pi$ bonds).
Total DBE = 4; And there are exactly 3 $\pi$ bonds of which 2 has been accounted for.

So definitely, there has to be a ring. Some of the possible structures are: