Question

→ XCIO DE • CH3-H 1,3 diaxial interaction • HH eclipsed • H-CH3 eclipsed • CH3 - CH3 eclipsed • CH3 - CH3 gauche 3.8 kJ/mol 4.0 kJ/mol 6.0 kJ/mol 11 kJ/mol 3.8 kJ/mol Question 11 8.33 pts The specific rotation of enantiomerically pure (+) sucrose is +66º. A particular mixture of () sucrose and sucrose exhibits a specific rotation of (+) 13°. What are the relative percentages of (+) and (-) sucrose in this sample? 80% (+) sucrose: 20% (-) sucrose 40% (+) sucrose; 60% (-) sucrose 50.1% (+) sucrose: 49.9% (-) sucrose 20% (+) sucrose; 80% (-) sucrose 60% (+) sucrose: 40% (-) sucrose 69% (+) sucrose:31% ) sucrose Next « Previous

Answer 1

enantiomeric excess(ee) = observed rotation of mixture / optical rotation of pure S-isomer * 100

   optical rotation of pure (+) sucrose = +66.0 degrees

observed rotation of mixture = 13 degrees

                       = (13/66)*100

                        = 19.7 $\approx$ 20 %

racemic mixture = 100-20 = 80 %

racemic mixture contain 1:1 ratio of (+) and (-) sucrose isomers

so that,

excess enantiomer = (+) sucrose isomer

percentage of (+) - sucrose = 80/2 = 40 %

Therefore, percentage of (+)- sucrose = 40 +20 = 60 %

percentage of (-)- sucrose = 40 %