Question. Propose a mechanism for the following reaction.

When tetrahydrofuran is treated with excess HBr, two steps are followed.One step is the abstraction of a proton and ring opening in tetrahydrofuran occurs. The next step is the again reaction with HBr and forms 1,4-dibromobutane.

Oxygen in tetrahydrofuran abstracts the acidic proton from hydrogen bromide molecule which results in formation of positive charge on oxygen. As positive charge on electronegative atom is unstable, thus, ring tends to open up to neutralize the charge and bromide ion acts as a nucleophile and attacks on the carbocation which generates after opening of ring.

Ring opening in tetrahydrofuran

Oxygen abstracts acidic proton from hydrogen bromide which results in formation of good leaving group, that is, water. Bromide ion acts as a nucleophile and attacks at the carbocation which is generated after the removal of water as a leaving group and formation of 1,4-dibromobutane occurs.

Formation of 1,4-dibromobutane