What test should be conducted to test for the presence of Aromatic Functional Groups?

To give some background on the different tests I have went over: The Iron (III) Chlorine Test tests for the presence of phenols. However, phenols contain both the alcohol group and the aromatic group and not the aromatic group individually. The Bromine Water Test also tests for double bonds and single bonds, however, due to the presence of double and single bonds in the compound it will react to create another compound. If the test, when reacted with the unknown compound, is colorless then there is a presence of an alkene. However, if is no change in the color of the solution it would indicate the presence of alkanes. If there is both the presence of alkanes and alkenes within the unknown compound, it does not mean that there is the presence of aromatic compounds since the unknown compound could also be a conjugated chain.




If I understood your question correctly, you are asking how can you distinguish aromatic hydrocarbons from aliphatic hydrocarbons (alkane, alkene, alkyne) and you said that test for unsaturation (bromine water test) will fail because alkane and benzene will both show negative results. You are correct and in that case, you can use the ignition test. Place a small amount of compound on a spatula and apply flame. For highly unsaturated compounds like benzene, it will burn with a yellow, sooty flame. For a chemical test, you can use Friedel Craft reaction. Strong Lewis acid like aluminum chloride will react with benzene to give a bright orange-red complex. Alkenes and alkynes also reacts with $\ce{AlCl3}$ but it will give off a different color (usually more yellowish). Alkanes do not react with $\ce{AlCl3}$.


There is one more selective test for aromatic compounds - [spot test using 2,4,7-Trinitrofluorenone](https://pubs.acs.org/doi/10.1021/ac60146a046) but I don't think this is a readily available laboratory reagent.


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The lowest aromatic is benzene so everything is up from there. Usually physical constants give a clue. With no heteroatoms and only aliphatic side chains aromatic rings sulfonate readily with concentrated sulfuric acid. They react with CHCl3 and anhydrous AlCl3 to give a wild mix of phenylmethanes that supposedly have bizarre colors. Simple alkyl benzenes don't react with Br2 in CCl4 or cold, neutral permanganate; more complicated ones the bets are off. The methods of choice are a combination of ultraviolet, infrared and nmr spectroscopy. As a test look up the physical constants of benzene and cyclohexane. Then figure a way or ways to definitely tell them apart.


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