Off Melting Point Tubes

Sometimes you may get a supply of tubes that are open on both ends You don t just use these as is. Light a burner, and close off one end, before you start. Otherwise your sample will fall out of the tube (see Closing Off Melting Point Tubes, following). 

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If you have melting point tubes that are open at both ends and you try to take a melting point with one, it should come as no surprise when your compound falls out of the tube. You ll have to close off one end, to keep your sample from falling out (Fig. 32). So light a burner and get a stiff small blue flame. SLOWLY touch the end of the tube to the side of the flame, and hold it there. You should get a yellow sodium flame, and the tube will close up. There is no need to rotate the tube. And remember, touch— just touch—the edge of the flame, and hold the tube there. Don t feel you have to push the tube way into the flame. 

After this sample has melted, lift the thermometer and attached sample tube carefully (it may be HOT) by the thermometer up at the clamp, until they are just out of the oil. This way the thermometer and sample can cool, and the hot oil can drain off. Wait for the thermometer to cool to about room temperature before you remove it entirely from the tube. Wipe off some of the oil, reload a melting point tube (never remelt melted samples), and try again. And heat at 2°C per minute this time. 

The heating liquid is pure concentrated sulphuric acid, with which the bulb of the flask is three-quarters filled. The substance, in powder form, is introduced into a small, thin-walled capillary tube. Such tubes are made as follows from test tubes (preferably from damaged tubes which must, however, be clean and dry ). The tubes are rotated in the flame of the blow-pipe till soft and then drawn out rapidly already after short practice the student can strike the correct diameter, which should be 1 0-1 -5 mm. internally. Suitable portions of the drawn-out material are cut off with scissors. It is convenient to cut double lengths (about 12 cm.), so that by sealing each length in the middle (micro-burner) two melting-point tubes are obtained ready for use. 

Make the melting-point tubes as described on p. 40 from a clean test tube the internal diameter should be about 4-5 mm. Cut off pieces about 5 cm. long. Close the lower ends by sealing as uniformly as possible, also keeping the walls as thin as possible. By drawing the softened glass away sideways avoid reducing the bore more than a little. 

Heat the middle of an open-ended melting point tube at the tip of the hot flame of a microburner until it begins to sag. If the melting point tube is sealed at one or both ends, carefully break off the sealed end(s) wearing gloves for protection. 

The capillary tubes used for the melting-point determinations should be both longer and wider than those normally used they should be at least lo cm. long (to decrease the chance of slipping off the thermometer during the determination) and 2 -3 mm. in diameter (in order to have sufficient semi molten material in the tube to observe accurately the process of melting). 

Into a 3-necked spherical flask of 1 L provided with a dropping funnel, a condenser surmounted by a calcium chloride tube and a mechanical stirrer, is introduced a suspension of 17.6 g (0.2 mol) of ethyl urea in 150 ml of anhydrous benzene. There is added through the dropping funnel in 20 minutes a solution of 18.9 g (0.1 mol) a-chlorophenyl-acetyl chloride in 300 ml of benzene. The mixture is left at ambient temperature for 15 minutes and is then heated under reflux on the water bath with stirring for 5 hours. The benzene solution is decanted at elevated temperature in order to separate it from an oil deposited on the bottom of the flask, the benzene is driven off on the water bath, the last traces removed in vacuum, the crystalline residue is triturated in a mortar in about 200 ml of water, and the crystalline solid is separated off and is with water and dried in vacuum over phosphorus pentoxide. There are thus recovered 19.6 g (yield 82%) of l-(2-chloro-2-phenyl-acetyl)-3-ethyl-urea, which when recrystallized from 80 ml of benzene, takes the form of white crystals soluble in benzene but insoluble in water. The product has melting point 146°C. 

Hot start PCRs can also greatly enhance PCR specificity.,0>n In this procedure, one of the critical components for amplification (commonly Taq polymerase) is added to the tube only after the reaction temperature reaches above the Tm (melting point) of the primers. This allows for primers that annealed nonspecifically at the lower temperatures to be melted off prior to addition of Taq polymerase. As a consequence, unique amplification products can often be obtained from low copy number targets that are present in a vast background of genomic DNA. 

To provide an internal reference, the solvent is sealed in a capillary, which is placed at the bottom of the NMR tube used for the sample solution. For this purpose, a melting-point capillary is closed at one end and a syringe is used to add the reference solution. For aqueous studies, a 2% solution of t-butyl alcohol in water can be used as reference and as solvent for the paramagnetic solute. The shift with respect to the methyl resonance of the r-butyl group is then monitored. With organic ligands such as acetylacetonate (acac) groups, complexes such as Cr(acac)3, Fe(acac)3, and Co(acac)2 are soluble in benzene and the proton resonance of the solvent is a convenient reference. The capillary is filled one-third full, the lower end of the capillary is cooled in ice, and the upper part is sealed off with a small hot flame. Alternatively, the reference solution can be placed in one compartment of a coaxial pair of cyhndrical NMR tubes, which are available commercially. In either case the spectral display should be expanded to permit an accurate measurement of the frequency shift. 

Some care must be exercised, throughout the application of the rod, to avoid any excess thickness of melted glass. Should thick spots appear they must be locally heated and pulled upwards with a rod or tweezers. The resulting thin point is then melted off, near the tube wall, remelted and blown till the shape and wall-thickness of the tube is recovered. 

Mix 10 grammes of benzaldehyde with 20 grammes of alcohol and treat the mixture with a solution of 2 grammes of potassium cyanide and 5 c.c. of water. Boil on the water-bath for one hour (reflux condenser). The hot solution is poured into a beaker and allowed to cool slowly the crystals separating out are filtered off, washed with alcohol, and dried on the water-bath. For conversion into benzil (see next preparation), they need not be recrystallised. In order to obtain perfectly pure benzoin, a small portion of the crude product is recrystallised from a little alcohol in a test-tube. Melting-point, 1340- Yield, about 90% of the theory. 

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