Question

To determine which enantiomer of the minor component is present, calculate the expected rotation for each possible scenario. Use the published specific rotation values for the pure enantiomers of limonene and carvone, along with the mole % of limonene and carvone determined:

limonene mole % = 62.38%   Carvone mole % =37.62% specific rotation of a mixture= -82.79 levorotatory the major component Limonene must be levorotatory

Example: the specific rotation of a mixture of A and B was found to be [α]D20 = - 5.7.

GC analysis revealed the mixture composition to be 75 mol % A and 25 mol % B. Because the mixture is levorotatory, the major component (A) must be levorotatory.

Let’s say that the specific rotations of A and B are as follows:

(-) A: [α]D20 = - 10       (-)-B: [α]D20 = - 6       (+)-B: [α]D20 = + 6

Calculated specific rotation of 75 mol % (-)-A and 25 mol % (+)-B (Scenario 1):

[α]D20 (mixture) = (0.75)(-10) + (0.25)(+6) = - 6

Calculated specific rotation of 75 mol % (-)-A and 25 mol % (+)-B (Scenario 2):

[α]D20 (mixture) = (0.75)(-10) + (0.25)(-6) = - 9

The calculated specific rotation of the mixture is closest to the measured value (-5.7). This is strong experimental evidence that the mixture contains the (+) enantiomer of B.

Answer 1

Specific rotation of (-)-limonene = - 94^o

Specific rotation of (-)-carvone = - 61^o

Specific rotation of (+)-carvone = + 61^o

Limonene mole % = 62.38%

Carvone mole % = 37.62%

Thus, the major component here is limonene.

Because the mixture is levorotatory, the major component i.e. limonene must be levorotatory.

Calculated specific rotation of 62.38 mol % (-)-limonene and 37.62 mol % (+)-carvone (Scenario 1):

Specififc rotation = (- 94)(0.6238) + (+61)(0.3762)

                            = - 58.64 + 22.95

                            = - 35.69^o

Calculated specific rotation of 62.38 mol % (-)-limonene and 37.62 mol % (-)-carvone (Scenario 2):

Specififc rotation = (- 94)(0.6238) + (-61)(0.3762)

                            = - 58.64 - 22.95

                            = - 81.59^o

The calculated specific rotation of the mixture in scenario 2 is closest to the measured value (-82.79).

Therefore, the minor component is the (-)-enantiomer of carvone i.e. (-)-carvone.