Chapter 7 : Stereochemistry

1. Given that (S)-bromobutane has a specific rotation of +23.1^o and (R)-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).

© Dr. Ian Hunt, Department of Chemistry