Crystal cold finger

When everything has settled down, slowly begin to heat the bottom of the sublimator, if necessary. You might see vapors coming off the solid. Eventually, you ll see crystals of purified solid form on the cold finger. Since you ll work with different substances, different methods of heating will have to be used. Ask your instructor. 

A short path distillation apparatus with a cold finger but no condenser should be used since the product may crystallize. The distillation is carried out under nitrogen or argon (balloon at capillary). 

Aromatic aldehydes (10 mmol) and trimethylorthoformate (20 mmol) was added to a mixture of sulfonamide (10 mmol), finely powdered calcium carbonate (9 g) and K-10 clay (2 g). The solid homogenized mixture was placed in a modified reaction tube which was connected to a removable cold finger and sample collector to trap the ensuing methanol and methyl formate. The reaction tube is inserted into Maxidigest MX 350 (Prolabo) microwave reactor equipped with a rotational mixing system. After irradiation for a specified period, the contents were cooled to room temperature and mixed thoroughly with ethyl acetate (2 x 20 mL). The solid inorganic material was filtered off and solvent was evaporated to afford tlie residue which was crystallized from the mixture of hexane and ethyl acetate. 

Figure 2.5. Schematic of three common methods used to grow single crystals. Shown is (a) diffusion where vapors from a volatile nonsolvent meet the crystallization solvent (b) interfacial where the nonsolvent is layered on top of the crystalhzation solvent (c) sublinration where the solid mixture is heated and the vapors form crystalhtes on the surface of a cold finger. For this latter technique, the crystallization flask may be opened to vacuum throughout the sublimation process (dynamic vacuum), or closed after maintaining initial vacuum to allow slower crystal growth (static vacuum).

A sublimation apparatus (with a water-cooled probe) is charged with 4.0 g of carbonyltetrakis(trimethylphosphine)iridium(I) chloride, evacuated, and heated to 130°. At first a white film appears on the cold finger, followed by growth of bright yellow crystals. After nearly all the solid has sublimed, the product is scraped from the cold finger (this may be done in air, but exposure should be kept to a minimum) and resublimed once or twice until the sublimate consists of pure yellow crystals with no visible white contaminant. The yield is 2.4 g (87%).+ Anal. Calcd. for C7H,8ClIrOP2 C, 20.6 H, 4.46. Found C, 21.10 H, 4.55. 

In case the hexaphenylbenzene is contaminated with insoluble material, crystallization from a filtered solution can be accomplished as follows Place 10 mL of diphenyl ether in a 25 x 150-mm test tube and pack the sample of hexaphenylbenzene into a 10-mm extraction thimble and suspend this in the test tube with two nichrome wires, as shown in Fig. 1. Insert a cold finger condenser supported by a filter adapter and adjust the condenser and the wires so that condensing liquid will drop into the thimble. Let the diphenyl ether reflux until the hexaphenylbenzene in the thimble is dissolved, and then let the product crystallize, add toluene, collect the product, and wash with toluene as described previously. 

Figure 115 shows three forms of sublimation apparatus. Note all the similarities. Cold water goes in and down into a cold finger on which the vapors from the crystals condense. The differences are that one is larger and has a ground glass joint. The sidearm test tube with cold-finger condenser is much smaller. To use them,... 

To a suspension of tungsten(VI) hexachloride (12 g, 30 mmol) in diethyl ether cooled at -20 °C is added dropwise methyllithium (90 mmol) during a period of 30 min. The resulting greenish brown solution turns to dark brown on raising the temperature to room temperature. The solvent is removed by evaporation under vacuum at about 0 °C and the residue is extracted with petroleum ether. The solvent is evaporated off from the deep red extract under vacuum at -20 °C and the red crystals of W(CH3)6 are vacuum sublimed onto a cold finger kept at -20 °C. Yield 40%. 

For simplicity, we have chosen long Schlenk tubes (Figure 3), without the conventional cold finger condenser. Deposition of crystals takes place on the cold walls of the sublimation flask 1-2 cm above the conical reflector. 

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