2. During the course ofyour career as a natural products chemist, you isolate a

Question

2. During the course ofyour career as a natural products chemist, you isolate a newcompound from a plant in South America that has excellent cytotoxicactivity against tumor cells. Because you have isolated thecompound as a pure enantiomer, you wish to report its opticalactivity. A 0.50-g sample of the compound is placed in 10 mL ofethanol in a sample tube and placed in the polarimeter. Theobserved optical rotation (a) is measured to be +0.53 degrees. Ifthe length of the sample tube (l ) is 10 cm, what is thecalculated value for the specific rotation of this newcompound?
A. (+)1.06
B. (+)10.6
C. (+)100.6
D. (+)0.106
E. (+)0.53

Explanation / Answer

We know that specific rotation ,[] = / c*l Where [] is the specific rotation               is the observed rotation               cis the concentration in g/mL               l- length of the tube in decimeters In our problem given that           Observed rotation   ,    = +0.53o            c- concentration = 0.50g / 10 mL = 0.05g/mL            l - length of the tube = 10cm = 1dm Therefore specific rotation , []  = 0.53 / 1*0.05                                                    = 10.6o Thus answer is B) Thus answer is B)

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