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