Count the isomers that follow. In each, the other ring positions hold hydrogen atoms…
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Count the isomers that follow. In each, the other ring positions hold hydrogen atoms.
- Dibromobenzene, .
- Tribromobenzene, .
- Bromochloroiodobenzene, .
- The aromatic isomers of , counting ethylbenzene and the dimethylbenzenes.
Need a reminder? The key ideaShow
A compound is aromatic when its molecule is cyclic, planar, fully conjugated and has pi electrons. Benzene is about more stable than three separate double bonds, which we find from the heats of hydrogenation. Disubstituted benzenes have three isomers: ortho, meta and para.
The worked solution
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Work it out on paper first. It is fine to get stuck or make a mistake: that is how we learn. When you are ready, go through the solution one step at a time, and check each step against your own.
Step 1. (a) Ortho (1,2), meta (1,3) and para (1,4): .
Step 2. (b) The 1,2,3-, 1,2,4- and 1,3,5-isomers: .
Step 3. (c) Three different groups can sit in three kinds of pattern. In the 1,2,3-pattern the middle group can be any of the three: isomers. In the 1,2,4-pattern all orders are different: isomers. In the 1,3,5-pattern, going clockwise or anticlockwise gives the same molecule once the ring is flipped: isomer. That is .
Step 4. (d) Ethylbenzene is one. The dimethylbenzenes are ortho, meta and para, which is three. The total is .
Answer: , , and .
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