Patent US4334083
,
Example 1:A round-bottomed flask is used which is equipped with a stirrer, a Vigreaux-type condenser, a thermometer, a dropping funnel and liquid nitrogen traps arranged downstream of the condenser.
Methyl lactate (832 g, 8 mols), [a]
D20 = +7.48° (without a solvent), is placed in the dropping funnel and thionyl chloride (1060 g, 8.9 mols) and pyridine (4.15 g, i.e. 0.49% by weight of the amount of methyl lactate to be converted) are placed in the flask.
The methyl lactate contains about 1.03% of ethyl lactate as an impurity and 0.61% of other impurities including methyl lactyl-lactate.
The temperature of the flask is raised to 60°C, whilst stirring, and the methyl lactate is run in over a period of 4 hours, whilst keeping this temperature constant. When the introduction is complete, the temperature is raised to 75°C. and this temperature is maintained for 1 hour. Analysis by gas phase chromatography, carried out at this moment, shows that there is no more methyl lactate in the reaction mixture. Heating is continued for 20 minutes and the mixture is then left to cool for 20 minutes, under a partial vacuum of about 150 mmHg, to a temperature of about 40-45°C.
A crude product (992.5g 884 ml), comprising crude methyl 2-chloropropionate, SO
2 (19.2 g), SOCl
2 (1.6g) and HCl (1.8 g), is obtained.
Analysis, by gas phase chromatography, of the crude methyl 2-chloropropionate thus obtained gives the following composition by weight (excluding SO
2, SOCl
2, HCl and pyridine):
97.7% - Methyl 2-chloropropionate
1.22% - Ethyl chloropropionate
0.04% - Methyl acetyl-lactate
0.08% - Methyl acetyl-lactyl-lactate
0.67% - Chloropropionate of methyl lactate
0.01% - Chloropropionate of ethyl lactate
0.03% - Lactide (cyclised methyl lactate)
0.11% - Chloropropionate of methyl lactyl-lactate
0.14% - Unidentified impurities
The formation of ethyl chloropropionate, methyl acetyl-lactate and methyl acetyl-lactyl-lactate results from the impurities present in the starting methyl lactate (yield of methyl 2-chloropropionate: 98%).
The crude product obtained above (900 g) is distilled, under a pressure of 20 mmHg, in a 40 cm high packed column. When distillation is complete, the vacuum is increased to 3 mm Hg. At the outlet of the packed column, the uncondensed vapours pass into a liquid nitrogen trap.
Distillation makes it possible to obtain the following three fractions:
Methyl 2-chloropropionate (95 g) of optical rotation [a]
D20 = -25.37° (without a solvent)
Methyl 2-chloropropionate (675 g) of optical rotation [a]
D20 = -25.26° (without a solvent)
Methyl 2-chloropropionate (71 g) of optical rotation [a]
D20 = -24.56° (without a solvent)
At the bottom of the column, a residue (13 g) consisting of heavy products is recovered. In the liquid nitrogen trap, methyl 2-chloropropionate (17 g) containing traces of HCl, SO
2 and SOCl
2 is recovered.
After distillation, and taking account of the 2-chloropropionate recovered in the nitrogen trap, the yield is 97.2%.