Hydrates

7 Key excerpts on "Hydrates"

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  eBook - PDF

  Introduction to General, Organic, and Biochemistry

  • Morris Hein, Scott Pattison, Susan Arena, Leo R. Best(Authors)
  • 2014(Publication Date)
  • Wiley

    (Publisher)

  When certain solutions containing ionic compounds are allowed to evaporate, some water molecules remain as part of the crystalline compound that is left after evaporation is com- plete. Solids that contain water molecules as part of their crystalline structure are known as Hydrates. Water in a hydrate is known as water of hydration or water of crystallization. Formulas for Hydrates are expressed by first writing the usual anhydrous (without water) formula for the compound and then adding a dot followed by the number of water molecules present. An example is BaCl 2 # 2 H 2 O. This formula tells us that each formula unit of this compound contains one barium ion, two chloride ions, and two water molecules. A crystal of the compound contains many of these units in its crystalline lattice. In naming Hydrates, we first name the compound exclusive of the water and then add the term hydrate, with the proper prefix representing the number of water molecules in the formula. For example, BaCl 2 # 2 H 2 O is called barium chloride dihydrate. Hydrates are true compounds and follow the law of definite composition. The molar mass of BaCl 2 # 2 H 2 O is 244.2 g/mol; it contains 56.22% barium, 29.03% chlorine, and 14.76% water. Water molecules in Hydrates are bonded by electrostatic forces between polar water mol- ecules and the positive or negative ions of the compound. These forces are not as strong as covalent or ionic chemical bonds. As a result, water of crystallization can be removed by moderate heating of the compound. A partially dehydrated or completely anhydrous compound may result. When BaCl 2 # 2 H 2 O is heated, it loses its water at about 100°C: BaCl 2 # 2 H 2 O(s) ::: " 100°C BaCl 2 (s) + 2 H 2 O(g) When a solution of copper(II) sulfate (CuSO 4 ) is allowed to evaporate, beautiful blue crys- tals containing 5 moles water per 1 mole CuSO 4 are formed (Figure 13.10a).

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  Solid-State Properties of Pharmaceutical Materials

  • Stephen R. Byrn, George Zografi, Xiaoming (Sean) Chen(Authors)
  • 2017(Publication Date)
  • Wiley

    (Publisher)

  It has also been observed that hydration state of a crystal hydrate sometimes affects chemical stability of a compound. For example, the hydrated form of vitamin B 12 is chemically more stable to light and heat than the anhydrous form [12]. Some hydrated materials can become amorphous upon dehydration, a form which is generally less chemical stable than the various crystalline forms. For example, cephradine dihydrate is stable chemically but undergoes oxidation after dehydration due to the formation of the amorphous form [13]. 3.3 CLASSIFICATION OF PHARMACEUTICAL Hydrates Hydrates can be classified as three categories based on different structural aspects, based on how the water molecules are incorporated into the crystal lattice [13]. Class I represents Hydrates where the water molecules exist at isolated sites, that is isolated from other water molecules by surrounding drug molecules. In such cases, there is no direct hydrogen bonding between various water molecules. Instead, the water molecules form hydrogen bonds and van der Waals interactions only with drug molecules. This group of Hydrates is characterized by the appearance of sharp dehydration endotherms in thermal measurements by differential scanning calorimetry, small weight loss ranges in thermogravimetric analysis, and sharp hydroxyl bands in infrared spectroscopy [13]. Siramesine hydrochloride monohydrate is an example of a system that exists as an isolated site hydrate [14], as shown in the crystal structure illustrated in Figure 3.2. Here, it can be seen that each water molecule is hydrogen bonded to two nearby chloride ions. It is obvious that water molecules are relatively strongly bound and physically locked inside the crystal. It was further observed that the loss of water upon heating was associated with the disruption of the crystal lattice and the formation of an oily phase [14]

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  Handbook of Thermal Analysis and Calorimetry

  Applications to inorganic and miscellaneous materials

  • Michael E. Brown, Patrick K. Gallagher(Authors)
  • 2003(Publication Date)
  • Elsevier Science

    (Publisher)

  4 . Loss of coordinated water can be accompanied by the generation of “replacement” bonds in which the cation links with the anion and/or ligands may become shared through the formation of binuclear complex salts.

  The number of Hydrates known is far larger than is indicated above because many cation/anion combinations can form two or more stable crystalline structures that contain different stoichiometric proportions of H2 O molecules. Sometimes these appear as a sequence of dehydration intermediates containing progressively lower proportions of water as the original hydrate is dehydrated. MgSO4 .xH2 O is dehydrated [6 ] from x = 7, through x = 6, 4, 2, 1 and finally yields the x = 0, anhydrous, salt. There is the additional intervention of MgSO4 .2.5H2 O when a sufficient pressure of water vapour is present. The dehydration of NiSO4 .7H2 O depends on reaction conditions, several Hydrates are known [7 ], in which x = 7, 6, 4, 2 and 1 (also 0). In dehydrations that occur through a sequence of stepwise water losses, fundamental kinetic and mechanistic studies must regard each successive reaction as a distinct rate process to be investigated separately and individually.

  Water is also accommodated in many metal salts of organic acids, such as formates, acetates, oxalates, tartrates, etc., hydrated to various extents. Also a number of organic acids form crystalline Hydrates e.g. (COOH)2 .2H2 O. More important for practical applications are the organic compounds, including metal salts, that exhibit pharmaceutical activity. The stabilities of these compounds, in which water may influence the lengths of time during which each may be stored safely (the “shelf-life”), are of great importance. Loss of water from such Hydrates may be followed by deterioration, perhaps by breakdown of the active substance when dissolved in liquid water, if H2

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  Basics for Chemistry

  • David A. Ucko(Author)
  • 2013(Publication Date)
  • Academic Press

    (Publisher)

  Sodium sulfate, for example, forms Hydrates containing one molecule of water, N a 2 S 0 4 H 2 0 ; Hydrates containing seven molecules of water, N a 2 S 0 4 * 7 H 2 0 ; and Hydrates containing ten molecules of water, N a 2 S O 4 1 0 H 2 O . Note that the formula of a hydrate is written with a raised period followed by the number of attached water molecules. The name of a hydrate is written in either of the following two ways: N a 2 S 0 4 H 2 0 sodium sulfate monohydrate sodium sulfate 1-water The term anhydrous refers to the original substance with no water of hydration. For example, anhydrous sodium sulfate is N a 2 S 0 4 . Other examples of the names and formulas of Hydrates are given in Table 12-2. The water molecules in Hydrates are weakly held. The removal of water, called dehydration, can therefore be accomplished simply by heating. Certain compounds lose their water of hydration merely on TABLE 12-2 Examples of Hydrates Formula Name C a S 0 4 2 H 2 0 calcium sulfate dihydrate (calcium sulfate 2-water) C11SO4 5 H 2 0 copper sulfate pentahydrate (copper sulfate 5-water) F e S 0 4 7 H 2 0 iron(II) sulfate heptahydrate (iron(II) sulfate 7-water) N a 2 C 0 3 10H 2 O sodium carbonate decahydrate (sodium carbonate 10-water) 389 12.8 WATER OF HYDRATION exposure to air. This process is called efflorescence. For example, so-dium sulfate decahydrate ( N a 2 S O 4 1 0 H 2 O ) is an efflorescent com-pound. It has a significant vapor pressure (about 10 torr) at average room temperature and humidity. Hygroscopic compounds are compounds that pick up water vapor from the air. If enough water is absorbed so that the compound actually dissolves, the process is called deliquescence. Certain anhydrous com-pounds that are hygroscopic, such as CaCl 2 , M g S 0 4 , and N a 2 S 0 4 , are used as drying agents. As these compounds form their respective hy-drates, water is removed from the surroundings.

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  Clathrate Hydrates

  Molecular Science and Characterization

  • John A. Ripmeester, Saman Alavi, Saman Alavi, John A. Ripmeester, Saman Alavi, John A. Ripmeester(Authors)
  • 2022(Publication Date)
  • Wiley-VCH

    (Publisher)

  2 An Introduction to Clathrate Hydrates John A. Ripmeester1 and Saman Alavi

  1 , 2

  1 National Research Council of Canada, 100 Sussex Drive, Ottawa, ON, K1A 0R6, Canada 2 University of Ottawa, Department of Chemistry and Biomolecular Sciences, STEM Complex, 150 Louis‐Pasteur Pvt., Ottawa, ON, K1N 6N5, Canada

  2.1 Introduction

  It is just over 200 years ago in 1810 that a gas hydrate, frozen from a solution of gas in water, was first recognized as a new kind of material. This places the discovery of gas Hydrates in the period when the fundamentals of modern chemistry, e.g. Joseph Proust's law of definite proportions and John Dalton's atomic theory, were being formulated. Although the study of gas Hydrates ran parallel to the development of chemistry, and indeed work on gas Hydrates contributed significantly to the first applications of classical chemical thermodynamics, these substances presented peculiarities not found in the usual chemical compounds. Despite the best efforts of some of the most eminent chemists and physicists of the time, the nature of the interactions between the constituents of gas Hydrates, the non‐integer ratios of their constituting atoms/molecules, and their apparent compositional variations eluded explanation until the 1950s.

  Often clathrate Hydrates have been designated as “laboratory curiosities,” which leaves little appreciation of the fact that the early science of clathrate Hydrates is closely linked to the discovery and characterization of weak intermolecular forces. Neither covalent nor ionic, nevertheless such forces are able to control the assembly of complex structures. The work on clathrate Hydrates presaged the field of supramolecular chemistry – that is, “chemistry beyond the molecule.” Today, it is known that multiple weak interactions, e.g. van der Waals forces, hydrogen bonding, and halogen bonding, play a major role in the construction of complex materials in chemistry, biology, and materials science. This burgeoning field was duly recognized by the award of the 1987 Nobel Prize in Chemistry to Jean‐Marie Lehn, Donald J. Cram, and Charles J. Pederson.

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  Low Temperature Biology of Foodstuffs

  Recent Advances in Food Science

  • John Hawthorn, E. J. Rolfe(Authors)
  • 2016(Publication Date)
  • Pergamon

    (Publisher)

  It cannot be obtained by cooling ordinary ice. Its hydrogen-bonding arrangement is a modification of that of ice I, so that it crystallizes in a cubical form instead of the hexagonal crystal structure. The arrangement of the nearest neighbors and of the empty spaces around a molecule is very similar to that shown in Figure 2. On warming it changes over into ice I. 4 George Nemethy Gas Hydrates These crystals belong to the general class of clathrates, i.e.inclusion compounds, in which one substance is trapped in cavities of molecular size, found in a spatial network formed by the second substance. No chemical bonds are formed between the two substances. In the gas Hydrates, the framework consists of water molecules, hydrogen-bonded to each other. Each water molecule forms four hydrogen bonds, but the arrange-ment of the molecules is more open (Figure 3) than that in ice. Approxi-mately spherical cavities form with diameters of 5.2 and 5.9 Ä (class I Figure 3. Arrangement of molecules in the crystal of a gas hydrate (class I). Water molecules are represented by small circles, with hydrogen bonds along the heavy lines. Guest molecules occupy the points marked by large circles, (v. Stackelberg and Müller 7 . Reproduced with permission of the Zeitschrift für Elektrochemie.) Hydrates) or of 4.8 and 6.9 Ä (class II Hydrates) 7 . All or most of the cavities are filled with one molecule each of the second component (or components), which must usually satisfy two conditions: (a) It must be inert, i.e. it should not physically interact strongly with water molecules. Hydrocarbons, alkyl halides, halogens, argon and the higher rare gases, hydrogen sulphide and its alkyl derivatives, etc. satisfy these conditions. The presence of OH or NH 2 groups prevents the forma-tion of gas Hydrates, because of the hydrogen-bonding interaction with neighbouring water molecules. (b) It must be small enough to fit into the cavities of the structure without significant distortion, i.e.

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  Hydrogen Bonding

  Papers Presented at the Symposium on Hydrogen Bonding Held at Ljubljana, 29 July–3 August 1957

  • D. Hadži(Author)
  • 2013(Publication Date)
  • Pergamon

    (Publisher)

  In the first group the water molecules occur singly or in clusters; they may be 16 J. D. B E R N A L called, by analogy with silicates, the meso-Hydrates, exemplified by some of the lower Hydrates of inorganic salts like AL 2 F 6 H 2 0 and zeolites. With these may be taken the Hydrates coordinated around single atoms, such as those of nickel sulphate hexahydrate. Next would come the Hydrates containing columns of water mole-cules, such as are found in the ettringite group, Ca 6 [Al(OH) 6 ] 2 (S0 4 ) 3 -26H 2 0; then sheets, such as those of the clay minerals or of gypsum; and finally the cryoHydrates with complete networks of linked waters, as in the case of the Hydrates of the gases already mentioned. b FIG. 6. Structure of monobasic fatty acid C 1 4 H2 9 CO OH, showing arrangement of double molecules linked by hydrogen bonds Of greater interest to the Conference is the study of the second great division, that of hydrogen-bonded structures of organic complexes in which the other bonds are essentially of the van der Waals or dispersion kind. Here again the same type of classification can be used, but the physical properties will be inversely affected, the tecto-structures being here the strongest instead of the weakest. We might begin the classification by including, as a kind of null group, molecules with purely internal hydrogen bonds, in which the pattern of the molecules in the crystal is entirely independent of hydrogen bonding. Next to these come structures such as those of formic acid, or of many of the monobasic acids, in which a pair of molecules are joined together F I G . 7. Structure o f dibasic fatty acid C O O H -C 4 H 6 C O O H . L o n g h y d r o g e n b o n d e d chains are f o r m e d in the c direction in the crystal Facing p. 10 This page intentionally left blank FUNCTION OF THE HYDROGEN BOND 17 by hydrogen bonds forming effectively a double molecule.

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