Five species are listed. For each, decide whether it is aromatic by Hückel's rule…
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Five species are listed. For each, decide whether it is aromatic by Hückel's rule. Assume that a ring is flat if all its atoms can be hybridised, and that it is not flat if one of them is .
- The cyclopropenyl cation, a ring of three carbon atoms with one double bond and a positive charge on the third carbon.
- Cyclopenta-1,3-diene, a ring of five carbon atoms with two double bonds and one group.
- The cycloheptatrienyl anion, a ring of seven carbon atoms with three double bonds and a negative charge on the seventh carbon.
- Pyrrole, a ring of four carbon atoms and one NH group, with two double bonds.
- The cyclooctatetraene dianion, a flat ring of eight carbon atoms with a charge of and four double bonds.
How many of the five are aromatic?
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.
(a) The ring has 3 carbon atoms, all . The double bond gives 2 pi electrons, and the positive carbon has an empty orbital. Total with . Aromatic.
(b) The group is , so the ring is not fully conjugated. Not aromatic.
(c) Three double bonds give 6 electrons, and the lone pair of the negative carbon adds 2. Total . Antiaromatic if flat, so it does not count.
(d) Two double bonds give 4 electrons, and the lone pair of nitrogen joins the cloud with 2. Total . Aromatic.
(e) Neutral cyclooctatetraene has 8 electrons. The charge of adds 2: total with . The ring is flat, so it is aromatic.
Answer: . The aromatic ones are (a), (d) and (e).
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