Solids Recrystallization and Melting Points

Figure 3.2. Differential calorimetric curves for the molecular glasses (a) Spiro-sexiphenyl (second heating curve) and (b) Spiro-PBD (first and second heating curve). The glass transition is indicated by a characteristic step, the melting point by an endothermic peak. In (a) recrystallization occurs above Tg, which can be seen by an exothermic peak. The material in (b) forms a stable amorphous glass without recrystallization. The melting point from the first heating curve of a crystalline sample (dotted line) disappears in the second heating cycle (solid line). Only the glass transition is visible.

The nitrogen heteroatoms in imidazole and some closely related heterocycles can stabilize a carbene center at the 2-position (97AG(E)2162). Thus, 1,3-disubstituted imidazole-2-ylidenes (163)-(170), l,3-dimesitylimidazoline-2-ylidene (171), 1,3,4-triphenyl-1H-1,2,4-triazole-5-ylidene (172), and their silylene (173) and germylene (174) analogues are stable (in the absence of oxygen and moisture) solids with definite melting points, which can be recrystallized from appropriate hydrocarbon solvents. The exception is carbene (163) which is an unstable liquid however, it is stable in solution. 

A mixture of 15.6 g 3-bromo-4-methoxy-5-ethoxybenzaldehyde and 10 mL cyclohexylamine was heated with an open flame until it appeared free of H20. The residue was put under a vacuum (0.5 mm/Hg) and distilled at 148-155 °C yielding 19.2 g 3-bromo-N-cyclohexyl-4-methoxy-5-ethoxybenzylidenimine as an off-white crystalline solid with a melting point 66-68.5 °C. Recrystallization from 100 mL boiling MeOH gave a mp of 67-68.5 °C. The C=N stretch in the infra-red was at 1640 cm1. Anal. (C16H22BrN02) C,H. 

N-Heterocyclic carbenes (NHCs) - These are five-membered heterocyles in which a carbene function is stabilized by adjacent nitrogen (or sulfur) atoms on both sides of the carbene. They are often stable solids with sharp melting points and can be recrystallized from hydrocarbon solvents <2000CRV39>. For examples see Sections 24.3.1 and 244.2.5. 

The situation is still more complex, when like in commercial polymers of TXN, the solid phase contains two types of units (most frequently ethylene oxide units from ethylene oxide or 1,3-dioxolane comonomers). Comparison of comonomer contents in the crystalline region of as-polymerized and recrystallized samples shows that segregation occurs in recrystallized samples, leading to a higher fraction of comonomer in the amorphous than in the crystalline phase 78>. The density and melting points decrease with increasing content of the comonomer units in both as-polymerized and recrystallized samples as shown in Fig. 7.8. 

If you started with aniline, transfer the solid to a 10-mL Erlenmeyer flask and recrystallize the product from 95% ethanol (see Technique 11, Section 11.3, and Figure 11.4). Filter the crystals on a Hirsch funnel and dry them for several minutes with suction. The brominated products from either acetanilide or anisole should be crystallized using a Craig tube (Technique 11, Section 11.4 and Figure 11.6). Use 95% ethanol to crystallize the acetanilide product and hexane to crystallize the brominated anisole compound. Allow the crystals to air-dry and determine the weight and melting point. 

If the active reverts to a solid while confined in the pores (e.g., after cooling), its physical properties are likely to be determined by the nanostructure - the freezing and melting points of pore-confined solids, for example, have been shown to be dependent on pore size and geometry (Chamaya et al. 2011 Jackson and McKenna 1990), and, depending on pore size, the active may remain amorphous or recrystallize. 

Trans dihydrohumuiinic acid (0.2 g 7.46 x 10 mol) is added to diphosphorus pentoxide (4 g 2.82 x 10 mol) in phosphoric acid (6 ml) and the mixture is heated at 130°C during 10 min. After cooling and dilution with water, the mixture is extracted with ether. Distillation of the residue at 90-100°C and 3 x 10 mm Hg affords compound 139 (0.08 g 43%). Compound 138 is isolated by recrystallization of the residue from ether pentane (0.04 g from 0.1 g dihydrohumuiinic acid 43%). The copper(ll) salt is a yellow-green solid with a melting point of 145-147°C. 

To 2.1 parts of benzil (0.01 mole) dissolved in 50 parts of glacial acetic acid containing 6 parts of ammonium acetate (0.078 mole) was added 1.4 parts of o-chlorobenz-aldehyde (0.01 mole), and the solution was refluxed for 2 hours. Ibe solution was then drowned in 200 parts of cold water whereupon 3.1 parts of reaction product precipitated. The product was isolated by filtration and purified by recrystallizing twice from ethanol. The product, 2-(o-chloro-phenyl)-4,5-diphenylimidazole, is a white crystalline solid having a melting point of 196° to 197° C. and the following analysis ... 

In a 500-ml. round-bottomed flask fitted with a reflux condenser are placed 16.2 g. (0.08 mole) of dry a-naphthylthiourea (Note 1) and 180 ml. of redistilled chlorobenzene. The flask is heated at the reflux temperature by means of an electric heating mantle. Evolution of ammonia begins almost at once, and all of the solid dissolves after 30-45 minutes. The solution is maintained at reflux for 8 hours (Note 2) and then evaporated on a steam bath at water-pump pressure to remove all of the chlorobenzene. The residue crystallizes on cooling and is extracted with four 30-ml. portions of boUing hexane (Note 3). Removal of solvent from the combined hexane extracts affords pale yellow crystals of naphthyl isothiocyanate, m.p. 58-59°. The yield is 12.7-13.0 g. (86-88%). Recrystallization from hexane (9 ml. of hexane for 1 g. of solute) gives colorless needles, melting point unchanged (Note 4). 

Chloro-2-hydrazino-4-phenylquinoline A stirred mixture of 2,6-dichloro-4-phenylquino-line 12.7 g,0.01 mol) and hydrazine hydrate (6.8 g) was refluxed under nitrogen for 1 hour and concentrated in vacuo. The residue was suspended in warm water, and the solid was collected by filtration,dried and recrystallized from ethyl acetate-Skelly B hexanes to give 1.81 g (67% yield) of 6[Pg.46]

About 3 grams of 0-methylcholine chloride are stirred at room temperature with an excess of phosgene dissolved in 50 grams of chloroform, for about 2 hours. Excess phosgene and hydrochloric acid are removed by distillation under vacuo. Additional chloroform is added to the Syrup and the mixture is poured into excess ammonia dissolved in chloroform and cooled in solid carbon dioxide-acetone. The solid is filtered and extracted with hot absolute alcohol. The solid in the alcohol is precipitated with ether, filtered, and recrystallized from isopropanol. The carbaminoyl-0-methylcholine chloride obtained has a melting point of about 220°C. 

B.Bg (0.15 mol) of 4-phenylbenzophenoneare dissolved in 200 ml of ethanol and 3 g (0.075 mol) of sodium borohydride are added. After heating for 15 hours under reflux, and allowing to cool, the reaction mixture is hydrolyzed with water containing a little hydrochloric acid. The solid thereby produced is purified by recrystallization from ethanol, 36 g (B9% of theory) of (biphenyl-4-yl)-phenyl-carbinol [alternatively named as diphenyl-phenyl carblnol or a-(biphenyl-4-yl)benzylalcohol] of melting point 72°-73°C are obtained. 

After the addition of 750 parts water, a crystalline product is precipitated. The mixture is further stirred overnight. The solid is then filtered off and recrystallized from a mixture of ethanol and water (1 1 by volume) to yield dl-1 -(1 -phenylethyl)-2-mercapto-5-(ethoxycar-bony Dimidazole its melting point is 129.8°C to 130.8°C. 

A mixture of 7.3 parts l-chloro-4,4-di-(4-fluorophenyl)-butane, 5.1 parts l-phenyl-4-oxo-l,3,8-triaza-spiro(4,5)-decane, 4 parts sodium carbonate, a few crystals of potassium iodide in 200 parts 4-methyl-2-pentanone is stirred and refluxed for 60 hours. After cooling the reaction mixture is treated with water. The organic layer is separated, dried, filtered and evaporated. The solid residue is recrystallized from 80 parts 4-methyl-2-pentanone, yielding l-phenyl-4-oxo-8-[4,4-di-(4-fluorophenyl)-] butyl-1,3,8-triaza-spiro(4,5)decane, melting point 187.5° to 190°C. 

The product, 1 -[2,4-dichloro-/3-(2,4-dichlorobenzyloxy)phenethyl] imidazole, is extracted with benzene. The extract is washed with water, dried, filtered and evaporated in vacuo. From the residual oily free base, the nitrate salt is prepared in the usual manner in 2-propanol by treatment with concentrated nitric acid, yielding, after recrystallization of the crude solid salt from a mixture of 2-propanol, methanol and diisopropyl ether, 1-(2,4-dichloro-/3-di-chlorobenzyloxy )phenethyl] imidazole nitrate melting point 170.5°C. 

This is separated, washed with petroleum ether, and dried in vacuo. After some time it hardens into a whitish solid, which if it was prepared from a 1-bromo-pentane which had some of its isomer 3-bromo-pentane copresent with it has a melting point of about 80° to 83°C. However, by using a pure 2-bromo-pentane, and/or by recrystallizing a number of times from dilute alcohol, the melting point may be raised to 98° to 100°C, corrected. 

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