Covalent molecules solubility

Generally, they do not dissolve in water. However, water is an excellent solvent and can interact with and dissolve some covalent molecules better than others. Covalent substances are generally soluble in organic solvents. For a further discussion of solubility of substances in organic solvents see Chapters 14 and 15. 

Covalent compounds are generally insoluble in water. This is shown by the fact that our proteins, skin and cell materials do not dissolve in the rain Covalent compounds can dissolve in other covalent liquids like oils or fats. Thus the effectiveness of any medication containing covalent or ionic molecules depends upon their solubility, the type of molecules present in the drug, and the parts of the cells being targeted. Some medications are water-soluble (usually containing ions) while others are fat-soluble (usually containing covalent molecules). 

A dilute solution of mercuric ( hloride (about 0.1%) is used as a disinfectant. Any somewhat soluble mercuric salt would serve equally well, except for the tendency of mercuric ion to hydrolyze and to precipitate basic salts. Mercuric chloride has only a small tendency to hydrolyze because its solution contains only a small concentration of mercuric ion, the mercury being present mainly as unionized covalent molecules... 

The exact nature of the Ziegler catalysts is somewhat obscure. The essential features seem to be that the TiCU (a covalent liquid, soluble in hydrocarbon solvents) is alkylated by the trialkylaluminum and also at least partly reduced to titanium (III). Titanium(III) chloride is an ionic solid, insoluble in organic solvents, so it is not surprising that the material so formed is not readily soluble in the reaction medium. This material, which is apparently somewhat variable in structure and composition but which contains Al as well as Ti, can add an olefin molecule at a Ti center, presumably in much the same way that ethylene adds to platinum (II) chloride to form Zeise s salts (Section 18.2), and an alkyl group already present on the Ti can then migrate onto the olefin to form a new, longer alkyl group ... 

Because there are no ions in covalent compounds, you do not expect them to be electrical conductors. Ionic compounds tend to be soluble in water while molecular compounds do not. This difference is also explained by interparticle forces. Ions are attracted by water molecules, but many covalent molecules are not and, therefore, do not dissolve. Solubility in water and the nature of water solutions is a major topic in chemistry. You will learn more about solutions in Chapter 13. 

The electronic and molecular geometries of covalent molecules, and hence their resulting polarities, can thus be predicted fairly accurately. Armed with these tools, one can predict whether or not a molecule should be soluble, reactive, or even toxic, see also Bonding Avogadro, Amedeo Bohr, Niels Cannizzaro, Stanislao Dalton, John Le Bel, Joseph-Achille Lewis, Gilbert N. Lewis Structures Pauling, Linus Thomson, Joseph John van t Hoff, Jacobus. 

Even large covalent molecules may be soluble in water if they contain sufficiently... 

I LEARNING CHECK 5.6 Write equations for the following reactions in total ionic and net ionic forms. Consider all ionic compounds to be soluble except CaCOs and BaS04, and remember that covalent molecules do not form ions when they dissolve. 

Certain proteins are found to be covalently linked to lipid molecules. For many of these proteins, covalent attachment of lipid is required for association with a membrane. The lipid moieties can insert into the membrane bilayer, effectively anchoring their linked proteins to the membrane. Some proteins with covalently linked lipid normally behave as soluble proteins others are integral... 

The solubilities of the ionic halides are determined by a variety of factors, especially the lattice enthalpy and enthalpy of hydration. There is a delicate balance between the two factors, with the lattice enthalpy usually being the determining one. Lattice enthalpies decrease from chloride to iodide, so water molecules can more readily separate the ions in the latter. Less ionic halides, such as the silver halides, generally have a much lower solubility, and the trend in solubility is the reverse of the more ionic halides. For the less ionic halides, the covalent character of the bond allows the ion pairs to persist in water. The ions are not easily hydrated, making them less soluble. The polarizability of the halide ions and the covalency of their bonding increases down the group. 

This is illustrated in Scheme VI. The protected glyceryl derivatives are insoluble in aqueous media and appear to be hydrolytically stable. The deprotected species (structure 27) is water-soluble and hydrolyzes in aqueous media at neutral pH at 37°C to give glycerol, phosphate, and ammonia. The free hydroxyl units of the deprotected polymer provide sites for the covalent attachment of drug molecules. Water insolubility can be imparted by the use of appropriate hydro-phobic cosubstituent groups to generate solid, erodible materials. 

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