Continuous-still collecting head

A typical design for the continuous still collecting head is outlined in Fig. 5.2, and can be simply constructed from a round-bottom flask, ground glass cone, 2-way tap, and 3-way tap. The 2-way tap allows the solvent to be withdrawn via syringe which is particularly convenient for anhydrous solvents. The 3-way tap allows the solvent to be collected, drawn off, or poured back into the distillation pot. Obviously the size of the still depends upon the quantity of solvent required, however the still head should always be smaller in capacity than the still pot, so as to avoid the possibility of the still boiling dry. 

Solvent stills (with continuous still collecting head)... 

There are two main types of solvent still that are commonly employed in organic research laboratories. One is the classical distillation set-up consisting of distillation pot, still-head, thermometer, condenser, receiver-adaptor, and collection vessel. This arrangement is described in more detail in Chapter 11, and is used for the distillation of solvents that are either required infrequently, or that can be stored without deterioration for long periods of time. The other is a continuous still set-up that consists of a distillation pot, collecting head, and condenser (see Fig. 5.1). 

However, the synthesis of Medicinal Compounds usually makes use of a continuous still set-up which is essentially comprised of a distillation vessel, collecting head, and a condenser as shown in Fig. 3.1. 

The continuous still set-up is designed in such a manner that the distilling solvent gets condensed and collected in a collecting head,... 

Whenever, the collecting head is full the solvent simply goes back into the still pot through an overflow, thereby allowing distillation to take place continuously without any remote possibility of the still boiling absolutely dry, and... 

Conversion of maleic acid into maleic anhydride. CAUTION All operations must be conducted in an efficient fume cupboard, owing to the highly toxic nature of the solvent. Mix 100 g of maleic acid with 1,1,2,2-tetrachloroethane (100 ml) in a distillation flask fitted with a Claisen still-head, a thermometer and a condenser set for downward distillation. Heat the mixture on an air bath when the temperature reaches 150°C, 75 ml of 1,1,2,2-tetrachloroethane and between 15 and 15.5 ml of water are present in the receiver. Continue the distillation using an air condenser and change the receiver flask when the temperature reaches 190°C. Collect the maleic anhydride at 195-197°C. Recrystallise the crude anhydride from chloroform. The yield of pure maleic anhydride, m.p. 54 °C, is 70 g (83%). 

ALL THIS TIME, through the crimson party and beyond, the distillation had continued the water in the bain-marie simmered and a cloud of condensation gathered on the still head, trickled down the delivery spout, and dripped into the receiver. When there had been a period of four hours or more and no moisture was collected, we poured the cloudy distillate from the receiver back into the flask containing the crusty alchemical residue, and began again. 

Kaiser (28) repeated the experiment using a hybrid tea rose. Headspace samples were collected from two flowers of identical maturity. One rose was left attached to the bush the second was cut and placed into water. The headspace compositions of the two flowers were essentially identical. Kaiser continued to take headspace samples throughout the day and found no differences in the concentration of cis-3-hexenyl acetate for the flower attached to the bush and the picked flowers. Kaiser s study concluded that if cut flowers are used for head-space studies, the sample must be collected from properly nourished cut flowers. If this is done, then the results are similar to those obtained from flowers still attached to the plant. 

---