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Classification of organic compounds have open and closed chain compounds. Closed chain further have homocyclic and heterocyclic componds. In homocyclic compounds there are two types: alicyclic and aromatic compounds.

In alicyclic compounds, only singles bond are allowed, whereas in the aromatic category the ring of carbon atoms has alternate single and double bonds (i.e. conjugated double bonds). So what happens to the compounds which have a lone double or triple bond?

For example, cyclohexane is classified under alicyclic, but where would cyclohexene or cyclohexyne be classified?

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Setting aside the severe steric strain that cyclohexyne would require, an aromatic all-carbon ring must have a full cycle of unsaturated atoms. Cyclohexene and (if you can make it) cyclohexyne have saturated carbons in the ring and thus must be alicyclic instead of aromatic.

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  • $\begingroup$ fyi, cycloalkynes do exist, though most are only transient/unstable intermediates. Wiki has it covered, here $\endgroup$ – NotEvans. Jul 2 '17 at 21:14
  • $\begingroup$ Adding to @NotEvans comment, the smallest stable cycloalkyne is cyclooctyne. $\endgroup$ – Pritt says Reinstate Monica Jul 3 '17 at 4:07
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If you look on the wikipedia page for alicyclic compound you will find cyclohexene as example for an alicyclic compound there. Cyclohexyne doesn't exist but it would be an alicyclic compound as well.

The IUPAC gold book defines alicyclic compound as:

Aliphatic compounds having a carbocyclic ring structure which may be saturated or unsaturated, but may not be a benzenoid or other aromatic system.

which means double or triple bonds are fine, just no aromatic system.

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