Optically pure (S)-(+)-2-butanol was converted to its methanesulfonate according to the re...

Optically pure (S)-(+)-2-butanol was converted to its methanesulfonate according to the reaction shown.

(a)       Write the Fischer projection of the sec-butyl methanesulfonate formed in this reaction.

(b)      The sec-butyl methanesulfonate in part (a) was treated with NaSCH2CH3 to give a product having an optical rotation αD of –25°. Write the Fischer projection of this product. By what mechanism is it formed? What is its absolute configuration (R or S)?

(c)       When treated with PBr3, optically pure (S)-(+)-2-butanol gave 2-bromobutane having an optical rotation αD = –38°. This bromide was then allowed to react with NaSCH2CH3 to give a product having an optical rotation αD of +23°. Write the Fischer projection for (–)-2-bromobutane and specify its configuration as R or S. Does the reaction of 2-butanol with PBr3 proceed with predominant inversion or retention of configuration?

(d) What is the optical rotation of optically pure 2-bromobutane?