1. Given that (S)-2-bromobutane has a specific rotation of +23.1^o and (R)-2-bromobutane has a specific rotation of -23.1^o, what is the optical purity and % composition of a mixture whose specific rotation was found to be +18.4^o?

The positive sign indicates that the (S)-isomer is in excess.

Optical purity, %  = 100 [a]_mixture / [a]_{pure sample}
                            = 100 (+18.4)  /  +23.1^o
                            = 80%  this indicates a 80% excess of S over R!

The 20% leftover, which is optically inactive, must be equal amounts of both (R)- and (S)-bromobutane.  The excess 90% is all S so there is a total of 10% (R) and 90% (S).

Answers to Absolute and Relative Configurations:
 

Qu 1:
	(a) Propene will react to give 1,2-dibromopropane which contains 1 stereocenter at C2, therefore the product will be a racemic mixture of (R)- and (S)-1,2-dibromopropane.
	(b) E-2-butene will react to give 2,3-dibromobutane which has 2 stereocenters at C2 and C3. However, the stereospecific nature of the addition in an anti fashion will give meso-2,3- dibromobutane
	(c) Z-2-butene, as above, will react to give 2,3-dibromobutane which has 2 stereocenters at C2 and C3. Now the product 1 2 a racemic mixture of (2R, 3R) - 2, 3 - dibromobutane and (2S, 3S) -2, 3 - dibromobutane.
	(d) cyclohexene will react to give trans - 1,2-dibromocylcohexane