Safety in preparation of preparation of trimethyl, Chemistry

Safety in Preparation of Preparation of trimethyl:

1 Use a round-bottomed flask equipped with a Claisen head (lagged with glass-wool), Quickfit thermometer (very expensive!), air condenser, receiver adapter and collecting flasks. See Diagram 1. Reject a fairly generous fraction of ‘fore-run', and prepare more of the pure benzaldehyde than you expect to use, so that you can be sure of having enough. Store your purified product in a Quickfit conical flask, under an atmosphere of nitrogen. (If you are unfamiliar with the use of gas cylinders consult the Safety Training Manual issued to you in your first year.) (Samples of aldehydes which have been exposed to air often contain significant amounts of a contaminant. What contaminant?)

2 Ensure that the heavy stirrer motor is supported directly above the retort stand's base-plate, and that the motor shaft, flask and stirrer rod are truly vertical, and in line. In setting up this arrangement, it is usually best to (a) mount the motor on a stout retort stand (b) mount the flask/guide/rod assembly separately on the same retort stand (c) adjust the position of the flask so that the rod fits smoothly into the rubber sleeve on the motor shaft (d) use a piece of copper wire, and pliers, to prevent the rod working itself out of the sleeve.

It does not matter which of the various types of stirrer you use : it could be, eg, a paddle type or a glass-link stirrer.

In using a stirrer motor, remember that good practice requires that you adjust the stirrer speed to the minimum setting before you turn the power off. This will prevent any unpleasant surprises when you turn it back on!

3 ‘Back-washing' with water (as it is often called) is used here to remove the bulk of the ethanol from the ether layer, since ethanol is very soluble in water (as well as in ether). If the ethanol is not removed from the ether, the ether layer will be very much more polar and will dissolve substantial amounts of water. This high water content will cause problems in the drying stage which follows next, since large amounts of drying agent will be required to remove the water.

Washing, being a form of liquid-liquid extraction, is carried out in a separatory funnel like other such extractions.

4 Remember the ‘rule of thumb' which enables you to decide whether you have added enough drying agent : you have added enough if, on addition of a little more desiccant, the added (amorphous) desiccant stays powdery and neither sticks to the sides of the flask nor becomes crystalline.

5 In using a rotary evaporator it is best not to heat the liquid at first, especially if a volatile solvent such as ether is to be removed. Once some of the solvent has been removed, the remaining liquid will have cooled (due to loss of ‘latent heat of evaporation') : at that stage it is permissible to cautiously apply heat via the water-bath. When most of the ether has distilled you can increase the bath temperature until the water is at, or is close to, boiling point. In removing ether, never plunge the flask into a hot water-bath at the beginning, as much of the ether is likely to violently flash boil, with the result that everything will ‘bump' over into the receiver. (Remember that the ‘vacuum' will reduce the bp of ether to well below the atmospheric pressure boiling point of 360.)

Posted Date: 2/11/2013 6:45:25 AM | Location : United States







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