Ion-dipole attractions

Crown ether (Section 16.4) A cyclic polyether that, via ion-dipole attractive forces, forms stable complexes with metal ions. Such complexes, along with their accompanying anion, are soluble in nonpolar solvents. 

The bonding in these complexes is the result of ion-dipole attractions between the hetero atoms and the positive ions. The parameters of the host-guest interactions can sometimes be measured by NMR." ... 

When a metal oxide surface is exposed to water, adsorption of water molecules takes place as shown in Equation 2.1. Cation sites can be considered as Lewis acids and interact with donor molecules like water through a combination of ion-dipole attraction and orbital overlap. Subsequent protonation and deprotonation of the surface hydroxyls produce charged oxide surfaces as shown in Equation 2.2 and Equation 2.3, respectively ... 

This indicates a lack of dynamic cohesion within the adducts i.e. the substrate has considerable freedom for reorientation within the receptor. The apparent reason for an absence of mechanical coupling is the nearly cylindrical symmetry of cucurbituril, which allows the guest an axis of rotational freedom when held within the cavity. Hence, the bound substrates show only a moderate increase in tc relative to that exhibited in solution. No relationship exists between values and the thermodynamic stability of the complexes as gauged by K (or K, cf. Tables 1 and 2). It must be concluded that the interior of cucurbituril is notably nonsticky . This reinforces previous conclusions that the thermodynamic affinity within adducts is chiefly governed by hydrophobic interactions affecting the solvated hydrocarbon components, plus electrostatic ion-dipole attractions between the carbonyls of the receptor and the ammonium cation of the ligands. 

Water, a protic solvent, helps separate the strongly attracting ions of the solid salt by solvation. Several water molecules surround each positive ion (Na ) by an ion-dipole attraction. The O atoms, which are the negative ends of the molecular dipole, are attracted to the cation. H,0 typically forms an H-bond with the negative ion (in this case Cl ). 

The way in which NaCI. a typical salt, dissolves in water, a typical protic solvent, was discussed in Problem 2,22. Dimethyl sulfoxide also solvates positive ions by an ion-dipole attraction the O of the S=0 group is attracted to the cation. However, since this is an aprotic solvent, there is no way for an H-bond to be formed and the negative ions are not solvated when salts dissolve in aprotic solvents. The S, the positive pole, is surrounded by the methyl groups and cannot get close enough to solvate the anion. 

The bonding in these complexes is the result of ion-dipole attractions between the hetero atoms and the positive ions. 

Figure 6-5 Environment of aqueous H304.13 Three H20 molecules are bound to H301 by strong hydrogen bonds (dotted lines), and one H20 (at the top) is held by weaker ion-dipole attraction (dashed line). The O H-O hydrogen-bonded distance of 252 pm (picomelers, 10 12 m) compares with an O H- O distance of 283 pm between hydrogen-bonded water molecules. The discrete cation (H20)3H30 found in some crystals is similar in structure to (H20) H30 with the weakly bonded HsO at the top removed.13...

So what happens to polar molecules, such as water molecules, when they are near an ionic compound, such as sodium chloride The opposite charges electrically attract one another. The positive sodium ions attract the negative side of the water molecules, and the negative chloride ions attract the positive side of the water molecules. This is illustrated in Figure 7.1. Such an attraction between an ion and the dipole of a polar molecule is called an ion—dipole attraction. 

Ion-dipole attractions are much weaker than ionic bonds. However, a large number of ion-dipole attractions can act collectively to disrupt an ionic bond. This is what happens to sodium chloride in water. Attractions exerted by the water molecules break the ionic bonds and pull the ions away from one another. 

Nonpolar grime attracts and is surrounded by the nonpolar tails of soap molecules, forming a micelle. The polar heads of the soap molecules are attracted by ion-dipole attractions to water molecules, which carry the soap-grime combination away. 

Which is stronger, the ion—dipole attraction or the induced dipole—induced dipole attraction ... 

How are ion—dipole attractions able to break apart ionic bonds, which are relatively strong ... 

Why are ion—dipole attractions stronger than dipole—dipole attractions ... 

However, 3-methyl-1,8-naphthyridine (37b) reacts with KNH2/NH3 in a completely different way than do 37a and 37c.15 H- and 13C-NMR spectroscopy unequivocally show the formation of the 1 1 cr-adduct 2-amino-3-methyldihydro-l,8-naphthyridinide (40) (Table V). No addition at C-7 is observed, although this position is also vulnerable to nucleophilic attack (see Section II,A,4). Similar behavior has been found in the Chichibabin animation of 3-methylpyridine the amide predominantly attacks C-2 and not C-6.33 This result has been explained by an ion dipole attraction between the incoming amide ion and the methyl substituent.34 This type of attractive interaction also possibly determines the attack on C-2 in 37b. 

The dissolution of a solid in a liquid can be visualized as shown in Figure 11.1 for NaCl. When solid NaCl is placed in water, those ions that are less tightly held because of their position at a corner or an edge of the crystal are exposed to water molecules, which collide with them until an ion happens to break free. More water molecules then cluster around the ion, stabilizing it by means of ion-dipole attractions. A new edge or corner is thereby exposed on the crystal, and the process continues until the entire crystal has dissolved. The ions in solution are said to be... 

Water molecules surround an accessible edge or corner ion in a crystal and collide with it until the ion breaks free. Additional water molecules then surround the ion and stabilize it by means of ion-dipole attractions. 

Ionic solids tend to dissolve (to varying degrees) in polar solvents such as water. Such solutions contain the separated ions, surrounded by solvent molecules. Ion-dipole attraction (in some cases with appreciable covalency as well) provides the energic compensation for the loss of electrostatic attraction which must accompany the dissolution of an ionic solid. Consider, as an example, the following thermochemical data ... 

The remainder of this chapter is concerned with the stabilities of ions (mainly cations) in aqueous solution, with respect to oxidation, reduction and disproportionation. Ions in solution are surrounded by solvent molecules, oriented so as to maximise ion-dipole attraction (although there may be appreciable covalency as well). The hydration number of an ion in aqueous solution is not always easy to determine experimentally it is known to be six for most cations, but may be as low as four for small cations of low charge (e.g. Li+) or as high as eight or nine for larger cations (e.g. La3+). 

It should be noted that self-ionisation is not an essential prerequisite for a satisfactory polar solvent. Liquids such as acetonitrile CH3CN or dimethylsulphoxide SO(CH3)2 appear not to ionise but they make very useful solvents for electrolytes as well as for polar molecular substances. As with H20, NH3, H2S04 etc., they owe their solvent powers to their polarity, leading to dipole-dipole interaction in the case of polar molecules as solutes and ion-dipole attraction in the case of electrolytes. There may in addition be considerable covalent bonding, via coordinate bond formation, in the case of cations. In solvents which do undergo appreciable self-ionisation, coordination often needs to be considered explicitly in discussing acid/base and other reactions and equilibria. 

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