Question

Imagine that you were given a 4.00 mL of a 1.50 M solution of an enantiomerically pure sample of (R)-a-methylbenzylamine. The observed optical rotation for this compound has been determined to be +5.45° in a 10 cm sample container. c is in g/mL and l is in dm

a.       What is the molar mass of (R)-a-methylbenzylamine?

b.      What is the specific rotation of (R)-a-methylbenzylamine?

c.  What is the observed rotation if this solution is mixed with an equal volume of a 1.50 M solution of enantiomerically pure (S)-a-methylbenzylamine?

d.      Why do you observe this rotation?

e.      What is the observed rotation if the original 1.50 M solution of (R)-a-methylbenzylamine is diluted with an equal volume of solvent?

f.        What is the specific rotation of (R)-a-methylbenzylamine after the dilution described in pt (e)?

g.       What is the specific rotation for a sample of pure (S)-a-methylbenzylamine?

h. Given the specific rotations of (R)-a-methylbenzylamine and (S)-a-methylbenzylamine (determined above), what is the optical purity and % composition of a mixture whose observed rotation was found to be -15.00^o?

i.What is the observed optical rotation of a 10 mL solution that contains 0.04 mole of (R)-a-methylbenzylamine and 0.1 mole of (S)-a-methylbenzylamine (assume a 10 cm path length)?

j. Explain how you could possibly isolate the other enantiomer (R-enantiomer) of a-phenethylamine from the mother liquor (which contains the more soluble diastereomer) following the fractional crystallization step in an experiment.

Answer 1

a) As the formula for the compound is C₆H₅CH(CH₃)NH_2. The molar mass can be calculated as fallows:

C) 12.01 x 8 = 96.08 g/mol

N) 14 x 1 = 14

H) 1.01 x 11 = 11.11 g/mol

   _____________

   121.19 g/mol

4 ml of a 1.5 M solution to g/ml = 4x10^-3 L x 1.5 mol/L x 121.19 g/mol = 0.7271 g

concentration in g/mol = 0.7271 g / 4 ml = 0.1818 g/ml

b) We have to use the formula    with l = pathlenght c= concentration in g/l and alpha = rotation

= +5.45/(10cm x 0.1818 g/ml) = 2.998 ^o

c) The rotation will be zero because a racemic mixture is formed.

d) Because chiral substances diverts the plane of the polarized light .

e) As the concentration is diluted in a half the rotation reduces in a half too. (5.45/2) = 2.73 ^o

f) = +2.73/(10cm x (0.1818/2) g/ml) = 2.998 ^o ( it is the same)

g) it is the same as 2.998 ^o, because the only change observed for the S isomer is the sense of the polarized plane.