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Synthesis of Chloroacetic Acid


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Method A

Prepare a mixture of 13g red phosphorus and 143mL of glacial acetic acid in a 250mL RB flask. Connect a Claisen adapter to the flask; attach a reflux condenser to the side arm, and a thermometer adapter to the straight arm. Then place a gas inlet tube into the thermometer adapter.

When you have your setup placed on a sunlight start heating the flask on a boiling water bath and pass the current of dry chlorine into the acetic acid. [Note: Chlorine gas prepared from potassium permanganate and HCl can be dried by passing it through conc. H2SO4]. After ~12 hours of bubbling the chlorine gas take a small sample into a test tube and cool it in an ice water bath. It should solidify after rubbing the walls of a test tube with a glass stirring rod. If that happens the reaction is complete and you can set up your flask for a simple distillation. Collect the portion distilling between 160 and 190 C. Cool the distillate in a salt-ice bath and rub the walls with a glass stirring rod. The cristals that form are filtered with suction. The filtrate is then redistilled, this time collecting the portion between 175 and 190 C. The crystals are obtained and filtered as before. The combined portions of crystals are distilled to obtain pure chloroacetic acid. Yield ~100g.


Method B [1]

Prepare a mixture of 12 g of red phosphorus and 143 mL of glacial acetic acid in a 250-mL Florence flask. Connect a Clasien adapter to the flask; attach a reflux condenser to the angled arm, and a thermometer adapter to the straight arm. Instead of using a thermometer in the adapter, place a piece of glass tubing that extends to the bottom of the liquid. This is an addition tube for chlorine gas, using a bubbler on the end of the tube can improve the reaction.

Locate the apparatus in a location that it can receive as much sunlight as possible, even to the point of positioning mirrors to get more sunlight. The sunlight is very important as the light provides the photochemical energy necessary for this reaction to succeed. Ordinary lamp light will not work, nor will this reaction be very effective during the winter months. The best time is midday during summer. With adequate sunlight the reaction will require as little as 12 hours (essentially all day while there is light) and in winter it will require two or more days (stopping for the night). The longer it takes, the more chlorine that will be wasted.

While heating the flask on a vigorously boiling water bath, pass a current of dry chlorine gas into the acetic acid. The completion of the reaction can be determined by taking a small sample into a test tube and cooling it in an ice-water bath. If the sample solidifies after rubbing the walls of the test tube with a glass stirring rod, it is done. After the reaction is complete, set the flask up for simple distillation. Distill the contents, collecting the portion that distill over from 150 to 200 C in a beaker. Cool the beaker in a salt-ice bath, rub the walls with a glass stirring rod. The portion that solidifies, consisting of pure chloroacetic acid, is rapidly suction filtered, the loose crystals are to be pressed together with a spatula or spoon to squeeze out excess liquid. The suction must not be continued too long, because the chloroacetic acid gradually becomes liquid in warm air. The filtrate is again distilled, this time the portion distilling over from 170 to 200 C is collected in a beaker. A second portion of chloroacetic acid is obtained by cooling and filtering as before. The two crystalline portions are combined, and then distilled to obtain perfectly pure chloroacetic acid; yield can vary from 80-125 g.

Although this reaction primarily synthesizes chloroacetic acid, some amounts of dichloroacetic acid, and trichloroacetic acid will also be made. These can be obtained from the filtrate and what does not boil over during the distillations. By continuing the reaction beyond what is necessary to make chloroacetic acid, you will eventually end up with mostly trichloroacetic acid. This will take several extra days though. The rate of the reaction can be facilitated by the addition of a small quantity of iodine to the acetic acid and phosphorus. This will cause some amount of contamination (iodoacetic and chloroiodoacetic acids), but a greater yield will be achieved in less time.

It is possible to substitute sulfur for red phosphorus in this reaction, which is much more readily available, it is not as efficient as phosphorus though. It is also possible to conduct this reaction using bromine instead of chlorine; bromoacetic acid is thus obtained. Getting iodine products is only possible by treating the corresponding bromo or chloro compounds with potassium iodide. Furthermore, other carboxylic acids can be used instead of acetic acid, as long as it has an alpha hydrogen (a hydrogen atom on the carbon that is bonded to the carboxylic functional group).


2-Chloroacetic acid: mp 61-63C, bp 189C d 1.580 g/cm2

References

[1] The practical methods of organic chemistry, Ludwig Gattermann (1903)