Ether Cleavage
Ethers resist nucleophilic attack because the possible leaving group, an alkoxide, is very basic.
However, the two lone electron pairs of the ether oxygen make it basic. Using a strong enough acid, an oxonium ion can be formed.
By heating the reaction mixture, one of the two C-O bonds is broken. But, which one of the two C-O bonds would break first?
This suggests that one can employ a tert-butyl ether as a blocking group in order to 'protect' an alcohol, since acid treatment of the ether with a strong acid allows us to recover the alcohol.
If we wanted to obtain 3-butyn-1-ol from 2-bromoethanol, we could not make it react directly with sodium acetylide, because the alcohol, acidic enough, would destroy the nucleophile/base.
In a case like this one, it is mandatory to previously 'protect' the alcohol, by forming the tert-butyl ether, for example.
The ether function is totally inert to the acetylide and does not interfere in the halogen replacement.
The alcohol group is finally recovered by treatment of the ether with HI.
This is an example of a common technique in Organic Chemistry: the protection of functional groups.