Video: EC17-18-S1-Q02B

Explain why the colour of acidified potassium permanganate does not disappear when it is added to 2-methyl-2-propanol.

02:45

Video Transcript

Explain why the colour of acidified potassium permanganate does not disappear when it is added to 2-methyl-2-propanol.

2-methyl-2-propanol is also known as 2-methylpropan-2-OL. That’s propane, three carbon atoms in the backbone, two methyl; that’s a methyl group on the second carbon atom, and two OL; that’s an OH group on the second carbon atom as well. And we get the full structure after saturating the carbon atoms with hydrogens.

Potassium permanganate has the formula KMnO₄ and the acid normally used to acidify it is sulfuric acid. Acidified potassium permanganate is a potent oxidizer and has a strong purple colour. Once it reacts, it turns a pale pink. Since 2-methylpropan-2-OL is an alcohol, let’s review the oxidation of alcohols.

Starting with primary alcohols like ethanol, oxidation, for instance, with potassium permanganate produces the aldehyde, in this case ethanol. Further oxidation would produce the carboxylic acid. Looking back at the primary alcohol, we can see that oxidation removes one hydrogen from the carbon bounds to the hydroxyl group. Secondary alcohols like propan-2-OL are oxidized to ketones like propanone. Once again, the carbon atom bound to the hydroxyl group loses one hydrogen atom as a result of the oxidation.

Now, what happens with tertiary alcohols, like 2-methyl-2-propanol? In this case, the carbon atom bound to the hydroxyl group has no remaining hydrogen atoms. It cannot lose any and therefore cannot undergo oxidation. So the colour of acidified potassium permanganate which would disappear in the presence of a primary or secondary alcohol does not decolorize in the presence of a tertiary alcohol because it cannot react.

Now, we can put all this into full sentences. The colour of acidified potassium permanganate does not disappear when added to 2-methyl-2-propanol because 2-methyl-2-propanol is a tertiary alcohol. The hydroxyl carbon has no extra hydrogens attached to it. So it cannot be oxidized by acidified potassium permanganate.

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