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Lavoslav Ruzicka
Postova: 28

« na: Srpanj 07, 2013, 08:54:24 »

Da li netko zna kako bih mogao dobiti acetanhidrid??
Ludi znanstvenik
Postova: 839

« Odgovor #1 na: Kolovoz 11, 2013, 08:58:45 »

Nažalost, acetanhidrid je obično jednostavnije i jeftinije nabaviti nego sintetizirati. Industrijski se dobiva karbonilacijom metilnog acetata, sa relativno skupim katalizatorima i u bezvodnim uvjetima. Druga metoda se temelji na uvođenju etenona u octenu kiselinu pri smanjenom tlaku, što također nije baš jednostavno, a ni bezopasno.

Ipak, našao sam ovdje jednu provjerenu sintezu i s nešto lakše nabavljivim početnim tvarima (sumpor, brom i bezvodni natrijev acetat):

A. Equipment Set-Up
Secure an angled 3-neck 500 mL round bottomed flask (RBF) to a ringstand. Attach a pressure-equalizing addition funnel to a side-neck. Install a reflux column equipped with a CaCl2 tube in the other side-neck. If available, install a mechanical stirrer in the center neck. If not, install a plug. All glassware is to have ground-glass fittings, coated with a thin film of silicone grease, and clamped.

Provide cooling water for the reflux column.

B. Reagent Preparation
Weigh out 66.5g (0.81 moles) of anhydrous sodium acetate and set aside in a covered beaker. Weigh out 3.5g (0.11 moles) of sulfur in a beaker and set aside. Have available 20 mL (0.39 moles) of dried Br2. (Note 1)

Note that bromine is the limiting reagent. The sulfur, except that released as SO2, is recycled in situ.

C. Ac2O Generation
Temporarily remove the center plug of the 500 mL RBF and pour in the anhydrous sodium acetate using a powder funnel. Reinstall the plug. Turn on the cooling water.

Pour the 20 mL of dried bromine into a 100 mL beaker. Slowly mix in the sulfur with the bromine using a stirring rod. When the resultant S2 Br2 + Br2 is homogeneous, pour it into the pressure-equalizing addition funnel (valve closed!) and install the plug in the fill port.

Slowly dribble in the S2 Br2 + Br2 onto the sodium acetate powder while stirring continuously mechanically, or intermittently by use of a stirring rod inserted though the center neck. Replace the plug after any manual stirring. Achieving a homogeneous mix may be somewhat difficult. (See Discussion below.) Keep stirring after all of the S2 Br2 has been added. If necessary, remove the pressure-equalizing funnel and manually stir via the side port. Close the port with a plug when not in use. When the reaction finally takes off the products will turn into a creamy slurry. Considerable heat will have been generated, bringing the slurry temperature as high as 80°C. Continue to stir for about 20 minutes. The light yellow color of the slurry is due to the presence of the elemental sulfur byproduct and indicates that the Ac2O formation is nearing completion. Continue stirring as the slurry cools to room temperature.

D. Vacuum Distillation
Set up for vacuum distillation of the Ac2O using the 3-neck 500mL RBF. Place an insulating blanket over the RBF. With a vacuum of 23”Hg (absolute pressure = 178 mmHg) the Ac2O comes over at about 72°C. Yield of the crude Ac2O will be ~30 mL.

E. Simple Distillation
Set up for simple distillation of the crude Ac2O at atmospheric pressure. Use a 50 mL RBF as pot. Collect 2 cuts. The first cut (~15 mL) will come over at about 128-135°C. The 2nd cut (~10 mL) will come over at 135-143°C. Literature value for the bp of Ac2O is 140.0 °C. Yield, based on the 10mL of cut 2 and the amount of Br2 charged, is ~27%.


Scientia potentia est.

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