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Answer» Tertiary alcohols are more stable because of the THREE alkyl groups.
First of all, the three alkyl groups prevent the tertiary alcohol from being oxidised because there's no HYDROGEN bonded to the carbon atom with hydroxyl GROUP, which means that no hydrogen will be lost from the alcohol. An EXPERIMENTAL evidence is that tertiary alcohols do not change the orange-looking potassium dichromate (VI) solution into a green solution on heating, but primary alcohols do.
Secondly, nucleophilic substitutions on tertiary alcohols are very slow compared to that on primary alcohols, SINCE the three alkyl groups sterically prevent the attacking nucleophile from being bonded to the carbon atom. The experiment for this is that tertiary alcohols react with PCl5 or PI3 more slowly under reflux, compared with when primary alcohols react with these reagents.
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