3-Bromo-2-butanol, neighboring group

Reactions of 1,2-Glycol Derivatives. One of the most interesting examples of the participation of neighboring groups in displacement reactions is the conversion of the 3-bromo-2-butanols to the corresponding dihalides by the action of concentrated hydrogen bromide. The erythro and the threo bromohydrins are converted into the meso and the dl isomers, respectively. Thus no apparent loss of the stereochemical configuration occurs in either reaction.33... 

When the 3-bromo-2-butanol with the stereochemical structure A is treated with concentrated HBr, it yields mestf-2,3-dibromobutane a similar reaction of the 3-bromo-2-butanol B yields (+ )-2,3-dibromobutane. This classic experiment performed in 1939 by S. Winstein and H. J. Lucas was the starting point for a series of investigations of what are called neighboring group efficts. Propose mechanisms that will account for the stereochemistry of these reactions. 

Since the pioneering work on the 3-bromo-2-butanol system similar observations have been made, not only with neighboring ether, ester, and thioether groups, but also with neighboring carbon. 

---