P-form

This form melts at 290 K, and boils at 318 K. The P form, obtained when the a form is allowed to stand for a long time at a temperature below 298 K. exists as asbestos-like, silky, felted needles and has a structure consisting ofSO tetrahedra linked together in long chains. 

Acetates. Complete acetylation of all the hydroxyl groups is desirable in order to avoid mixtures. In some cases, the completely acetylated sugars may be obtained in the a- and p-forms depending upon the catalyst, e.g., zinc chloride or sodium acetate, that is employed in the acetylation. The experimental details for acetylation may be easily adapted from those already given for a- and p-glucose penta-acetates (Section 111,137). 

C,H,CH,- —C-NH,j The compound separates in either, sometimes as both, of two dimorphic forms, m.p. 150° and 175° respectively. The former may be converted into the higher m.p. form by dissolving It in alcohol and seeding with crystals of the form, m.p. 175° the low m.p. form when warmed to 175° gives, after sohdification, a m.p. of 175°. Both dimorphic forms give identical derivatives with carboxyUc acids and sulphonic acids (see Sections 111,85 and IV,33). 

In spite of their easy interconversion in solution a and p forms of carbohydrates are capable of independent existence and many have been isolated m pure form as crys talline solids When crystallized from ethanol d glucose yields a d glucopyranose mp 146°C [a]o +112 2° Crystallization from a water-ethanol mixture produces p d glucopyranose mp 148-155°C [aj +18 7° In the solid state the two forms do not mterconvert and are stable indefinitely Their structures have been unambiguously con firmed by X ray crystallography... 

Absorption of x-rays by a powdered sample of soHd fat has been a useful method for determination of polymorphic character as discussed eadier. The a, and P forms may be distinguished however, interpretation is made more difficult because subsets of the P and P forms have often been encountered. Also, a fat may contain mixtures of polymorphic forms and properties may therefore be difficult to relate to the spectra. 

Aluminum trifluoride trihydrate [15098-87-0], AIF. -3H20, appears to exist in a soluble metastable a-form as well as a less soluble P-form (3). The a-form can be obtained only when the heat of the reaction between alumina and hydrofluoric acid is controlled and the temperature of the reaction is kept below 25°C. Upon warming the a-form changes into a irreversible P-form which is insoluble in water and is much more stable. The P-form is commercially available. 

The third process involves careflil addition of aluminum hydroxide to fluorosiUcic acid (6) which is generated by fertilizer and phosphoric acid-producing plants. The addition of Al(OH)2 is critical. It must be added gradually and slowly so that the siUca produced as by-product remains filterable and the AIF. -3H20 formed is in the soluble a-form. If the addition of Al(OH)2 3H20 is too slow, the a-form after some time changes into the insoluble P-form. Then separation of siUca from insoluble P-AIF. -3H20 becomes difficult. 

Value is for a form. A p form also exists having mp = 62.09°C. 

Tables 1 and 2, respectively, Hst the properties of manganese and its aHotropic forms. The a- and P-forms are brittle. The ductile y-form is unstable and quickly reverses to the a-form unless it is kept at low temperature. This form when quenched shows tensile strength 500 MPa (72,500 psi), yield strength 250 MPa (34,800 psi), elongation 40%, hardness 35 Rockwell C (see Hardness). The y-phase may be stabilized usiag small amounts of copper and nickel. Additional compilations of properties and phase diagrams are given ia References 1 and 2.

Properties. o-Nitiotoluene [88-72-2] is a clear yeUow liquid. The solid is dimorphous and the melting points of the a- and P-forms ate —9.55 and —3.85 C, respectively. o-Nitrotoluene is infinitely soluble in benzene, diethyl ether, and ethanol. It is soluble in most organic solvents and only slightly soluble in water (0.065 g in 100 g of water at 30°C). The physical properties of o-nitrotoluene are hsted in Table 9. 

Anhydrous Oxalic Acid. The anhydrous form of oxaUc acid is odorless and colorless. It exists in two crystal forms, ie, the rhombic or a-form and the monoclinic or P-form (3). The rhombic crystal is thermodynamically stable at room temperature, but the monoclinic form is metastable or slightly stable. The main difference between the rhombic and monoclinic forms exists in the melting points which are 189.5 and 182°C, respectively (Table 1)-... 

Other phosphoms aHotropes can be made from the a-form. At temperatures below —76.9°C a hexagonal P-white phosphoms having a higher (1.88 g/cm ) density can be produced reversibly. The difference between the a- and P-forms has been shown by P nmt to be that the P tetrahedra ate not free to rotate in the P-white modification. 

Electron diffraction studies indicate that phosphoms pentoxide vapor consists of P O q molecules. The vapor usually condenses to the hexagonal crystalline modification but under rapid cooling can be condensed to an amorphous soHd (P-form). The Hquid obtained by melting the stable orthorhombic modification cools to form a glass which is the P-form. The Hquid obtained from the H modification also can be supercooled to a glass. 

Binary Compounds. The mthenium fluorides are RuF [51621 -05-7] RuF [71500-16-8] tetrameric (RuF ) [14521 -18-7] (15), and RuF [13693-087-8]. The chlorides of mthenium are RUCI2 [13465-51-5] an insoluble RuCl [10049-08-8] which exists in an a- and p-form, mthenium trichloride ttihydrate [13815-94-6], RuCl3-3H2 0, and RuCl [13465-52-6]. Commercial RuCl3-3H2 0 has a variable composition, consisting of a mixture of chloro, 0x0, hydroxo, and often nitrosyl complexes. The overall mthenium oxidation state is closer to +4 than +3. It is a water-soluble source of mthenium, and is used widely as a starting material. Ruthenium forms bromides, RuBr2 [59201-36-4] and RuBr [14014-88-1], and an iodide, Rul [13896-65-6]. 

Binary Compounds. The fluorides of indium are IrF [23370-59-4] IrF [37501-24-9] the tetrameric pentafluoride (IiF ) [14568-19-5], and JIrFg [7789-75-7]. Chlorides of indium include IrCl, which exists in anhydrous [10025-83-9] a- and p-forms, and as a soluble hydrate [14996-61-3], and IrCl [10025-97-5], Other haUdes include IrBr [10049-24-8], which is insoluble, and the soluble tetrahydrate IrBr -4H20 IrBr [7789-64-2]-, and Irl [7790-41-2], Iridium forms indium dioxide [12030-49-8], a poorly characteri2ed sesquioxide, 11203 [1312-46-5]-, and the hydroxides, Ir(OH)3 [54968-01-3] and Ir(OH) [25141-14-4], Other binary iridium compounds include the sulfides, IrS [12136-40-2], F2S3 [12136-42-4], IrS2 [12030-51 -2], and IrS3 [12030-52-3], as well as various selenides and teUurides. 

PtCl2, and platiaum tetrachloride [37773-49-2]. Platiaum dichloride exists in an a- and P-form, the latter containing a Pt core and edge-bridging chlorides. Platinum trichloride [25909-39-1], PtCl, contains Pt(II) and Pt(IV) centers. Other haHdes include two bromides, PtBr2 [13455-12-4] and PtBr ... 

Physical Properties of PBT. Unlike PET, the polymer PBT exists in two polymorphs called the a- and p-forms, which have distinctly different crystal stmctures. The two forms are interconvertible under mechanical stress (158,159). Both crystal forms are triclinic and the crystal parameters are shown in Table 7. 

Use of HRh(CO)[P(CgH )2]3 as the catalyst and an excess of triphenylphosphine improves the y P ratio. For example, reaction of triethoxysilane with allylamine of equivalent moles at 150°C for 10 h, yields the y-form product ia more than 70% and the y P ratio is 26. Compared with this, when H2PtCl3 is used as the catalyst, the y P ratio is 4 (41). Furthermore, when Rh[(p.-P(C3H3)2-(cyclooctadiene)]2 is used as the catalyst, the yield of y-form product is selectively increased to 92% and that of P-form product is decreased to 1.1% (42). 

C), the yield of more than 90% purity L-glutamic acid crystals is very high. The glutamic acid crystals appear as both the metastable a- and stable P-forms. The a-form consists of prismatic crystals which are easy to filter, whereas the P-form needle crystals are difficult to filter. Control of crystallisation conditions of a-crystals are requited (13). The cmde L-glutamic acid crystals are suspended ia water and neutralized with caustic soda or sodium hydroxide. The solution is decolorized with activated carbon to produce a transparent solution and MSG is crystallized under reduced pressure. 

Aminophenol. This compound forms white plates when crystallized from water. The base is difficult to maintain in the free state and deteriorates rapidly under the influence of air to pink-purple oxidation products. The crystals exist in two forms. The a-form (from alcohol, water, or ethyl acetate) is the more stable and has an orthorhombic pyramidal stmcture containing four molecules per unit cell. It has a density of 1.290 g/cm (1.305 also quoted). The less stable P-form (from acetone) exists as acicular crystals that turn into the a-form on standing they are orthorhombic bipyramidal or pyramidal and have a hexamolecular unit (15,16,24) (see Tables 3—5). 

Silver iodide exists in one of three crystal stmetures depending on the temperature, a phenomenon frequently referred to as trimorphism. Below 137°C, silver iodide is in the cold cubic, or y-form at 137—145.8°C, it exists in the green-yeUow colored hexagonal, or P-form above 145.8°C, the yellow cubic or a-form of silver iodide is the stable crystal stmcture. Silver iodide decomposes into its elements at 552°C. 

The acid occurs both as colorless triclinic prisms (a-form) and as monoclinic prisms ( 3-form) (8). The P-form is triboluminescent and is stable up to 137°C the a-form is stable above this temperature. Both forms dissolve in water, alcohol, diethyl ether, glacial acetic acid, anhydrous glycerol, acetone, and various aqueous mixtures of the last two solvents. Succinic acid sublimes with partial dehydration to the anhydride when heated near its melting point. 

Properties. Physical properties of the three crystalline forms of dextrose are Hsted in Table 1. In solution, dextrose exists in both the a- and P-forms. When a-dextrose dissolves in water, its optical rotation, [cc], diminishes gradually as a result of mutarotation until, after a prolonged time, an... 

Titanium Trichloride. Titanium trichloride [7705-07-9] exists in four different soHd polymorphs that have been much studied because of the importance of TiCl as a catalyst for the stereospecific polymerization of olefins (120,124). The a-, y-, and 5-forms are all violet and have close-packed layers of chlorines. The titaniums occupy the octahedral interstices between the layers. The three forms differ in the arrangement of the titaniums among the available octahedral sites. In a-TiCl, the chlorine sheets are hexagonaHy close-packed in y-TiCl, they are cubic close-packed. The brown P-form does not have a layer stmcture but, instead, consists of linear strands of titaniums, where each titanium is coordinated by three chlorines that act as a bridge to the next Ti The stmctural parameters are as follows ... 

Titanium trichloride is almost always prepared by the reduction of TiCl, most commonly by hydrogen. Other reduciag agents iaclude titanium, aluminum, and 2iac. Reduction begias at temperatures of ca 500°C and under these conditions a-TiCl is formed. The product is cooled quickly to below 450°C to avoid disproportionation to the di- and tetrachlorides. P-TiCl is prepared by the reduction of titanium tetrachloride with aluminum alkyls at low (80°C) temperatures whereas y-TiCl is formed if titanium tetrachloride reacts with aluminum alkyls at 150—200°C. At ca 250°C, the P-form converts to d. d-TiCl is made by prolonged grinding of the d- or y-forms. 

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