MediumChemistryHigh School (11–12)Organic Compounds with Functional Groups › Aldehydes, Ketones and Carboxylic AcidsNumericalConceptual
Four hydrocarbons are each heated with alkaline potassium permanganate, and the mixture is then…
Contributed by StemPy LabTry it on paper first. A hint, the key idea and the worked solution are here when you need them.
Four hydrocarbons are each heated with alkaline potassium permanganate, and the mixture is then acidified: toluene, ethylbenzene, tert-butylbenzene and 1,4-dimethylbenzene. How many of them give a carboxylic acid? Name the acid from 1,4-dimethylbenzene, and say how many carboxyl groups it has. Explain why tert-butylbenzene does not react.
Need a reminder? The key ideaShow
The carboxyl carbon is carbon 1. Acids come from oxidation of primary alcohols, aldehydes and alkylbenzenes (a benzylic hydrogen is needed), from hydrolysis of nitriles, amides and esters, and from RMgX with CO, which adds one carbon. Two hydrogen bonds join acid molecules in a dimer, so the molar mass found in benzene is near double.
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. Toluene has a methyl group with three benzylic hydrogens. It gives benzoic acid.
Step 2. Ethylbenzene has a CH next to the ring. The whole ethyl group becomes COOH, and benzoic acid forms.
Step 3. tert-Butylbenzene has no hydrogen on the carbon next to the ring. It is not attacked.
Step 4. 1,4-Dimethylbenzene has two methyl groups, so both become COOH. The product is benzene-1,4-dicarboxylic acid, called terephthalic acid, with 2 carboxyl groups.
Answer: 3 hydrocarbons give an acid, and terephthalic acid has 2 carboxyl groups.
Check. The permanganate attacks the benzylic C-H bond. With no such bond, there is no start to the reaction.
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