Polarimetry: Separation of racemic mixture phenylethylamine – Lab Report Example

Polarimetry: Separation of racemic mixture phenylethylamine Introduction: Enantiometers are the chiral molecules with same physical properties except the rotational direction of the plane polarized light. One enantiomer will rotate the polarized light in the clockwise direction (called as dextrorotatory, “d” or (+) ) and the another enantiomer will rotate the polarized light in the anti-clockwise direction( called as levorotatory, “I” , or (-)). The optical rotation of the molecule is measured using the polarimeter. The rotation of the molecule depends on several factors such as structure of the molecule, temperature of the sample, concentration and identity of the solvent ,length of the sample cell and the wavelength of light used. Since enantiomers have the same physical properties such as boiling point, melting point, solubility, and refractive indices, the need for the separation of the molecules is important.
Enantiomers differ only in the rotation of plane polarized light. To resolve a racemic mixture of ( ± ) phenylamine, tartaric acid was used. Tartaric acid reacts with (±) phenylamine to form (+) amine (+) tartrate and (-) amine (+) tartrate. These salts have different solubility levels in methanol. (-) amine (+) tartrate forms crystal and (+) amine(+) tartrate remains in solution. These enantiomers are then extracted using aqueous NaOH to produce free amines.
Results:
Table 1: Results for SUCROSE concentration experiment:
Literature specific rotation [α]20
Measured Light Path Length [dm]
Measured optical rotation α
Sucrose
+66.37
1 dm
1. 30
2. 30
3. 30
4. 31
5. 30
Average = (30+30+30+31+30) /5
= 30.2
Calculated sucrose concentration [g /mL] = 0.1

Table 2: Results for (-) Amine resolution experiment:
(-) Phenylethyl amine
Calculation
a.
Weight of (-) amine used to make the salt (g)
(4 * 0.94) / 2
1.88 g
b.
The weight of the crystals isolated in Step 5, (g)
3.075g
c.
Theoretical weight of the crystals
1.88 + 4.505 g
6.385 g
d.
% Recovery of crystals = b/c x 100%
(3.075/ 6.385) x 100
48.19 %
e.
The weight of free amine determined in step 12
5.606 g
f.
The weight of free amine divided by the number of students in your group ( after removing the solvent on the Rotavap)
1.1212 g
g.
% Recovery of free amine = f/a x 100%
(1.1212/ 1.88) x 100
59.64%
h.
Observed rotation, α, by each student ( #1, #2, #3, #4 etc, )
Average α
-31.2°
i
Measured Path length, l [ decimeters, dm]
1.0
J
Calculated Specific rotation [α]
-12.48
k.
Calculated % Optical purity, ee
( 12.48/ 31.2) x 100
41.05
l.
Actual composition of the resolved (-) Amine in %
70.525 %
Discussion:
To a 125ml separating funnel with a stopper, 35ml of ethyl acetate was added. The solution containing the (-) amine (+) tartrate was transferred to the separating funnel and the beaker was further rinsed with 2 ml of ethyl acetate solution. Vigorous shaking of the funnel helps to extract the amine from the aqueous layer onto the top organis layer. Venting approximately every 30 seconds enabled the total extraction in 2 minutes. The organic layer containing the pure (-) amine had some water. This water was removed by the addition of sodium sulfate to the amine solution, after the formation of the lumps, little amount of sodium sulfate was added and the flask was swirled. The occurrence of freely moving crystals in the flask was the end point. Pure amine was separated from the solution by evaporation. Using a Rotavap, evaporation was performed. The pure solution was transferred to a vial and the weight of the pure amine was determined.
Pure (-) amine crystals were obtained from the (-) amine (+) tartaric acid salt using the separation and evaporation techniques. The percentage recovery of the crystals was 48.16%. Since the recovery of the crystals was very little, error could have been in the crystallization part or there might be very little salt sticking to the Buchner funnel or Erlenmeyer flask. These may be the reasons for low percentage yield of the salt. The addition of aqueous NaOH, separated the sodium tartrate from the (-) phenyl amine. The free amine was separated using the separating funnel. The percentage yield of (-) free amine was determined to be 59.64%. This percentage yield from the salt mixture is quite higher. The observed rotation was -31.2. The negative sign indicates that (-) amine was extracted and the specific rotation was -12.48. The theoretical specific rotation is -40.4 (Fessenden, Fessenden & Feist, P, 2001). These values indicate that the results are less than that expected. Impurities are present in the sample. The rotation value of the (-) amine is -31.2 when compared to the theoretical value of -40.4. Optical purity of the sample is only 41.05% and this indicates that out of the 1.1212g of crystals extracted, impurities are more than half of the weight.
Reference:
Fessenden, R. J., Fessenden, J. S. & Feist, P. (2001). Organic Laboratory Techniques.
Brooks/Cole.