How to prepare 2-bromobutane from 1-butanol?
The hydroxyl fragment isn't a good leaving group, unless the medium is acidic. Using just HBr, you can convert the hydroxyl group into an oxonium ion (the H2O fragment would be a good leaving group). Upon heterolysis of the C-O bond, a hydride shift occurs simultaneously to create a secondary carbocation. The exogenous bromide can act as a nucleophile towards the secondary carbocation to form 2-bromobutane.