Hard hydrogen-bonded protons

Figure 37 shows the structure of the water complex with the four-armed tetraaza[12]-crown-4 (10). The water molecule is located in the center of the cavity and serves as proton donor to two trans nitrogen atoms. The water molecule also accepts hydrogen bonds from OH side groups attached to the other trans nitrogen atoms. Thus a tetrahedral encapsulation of the water molecule occurs. A remarkable aspect is that the conformation of the twelve-membered ring hardly differs from that of the free ligand (10). 

The adsorption of a number of other simple monofunctionai compounds, e.gc amines, of low molecular weight, by inorganic oxides is also best explained by hydrogen bonding, because low molecular weight solutes containing a polar group which is a proton-acceptor, e.g. nitrobenzene, are hardly adsorbed at all. 

As to solvents in group (c) (nitroalkanes), which hardly solvate cations, higher reaction rates were observed than with other solvents [465] the rates increased with increasing dielectric constant and the sorption capacity of the resin for the solvent. This result indicates that a solvation of the anions — SOJ by the nitroalkane molecules might occur by the formation of a hydrogen bond — S03 -"-H6+ CH2N02 consequently, the activity of the protons of the sulphonic groups increases and the transesterification rate is enhanced. 

Hydrogen-bond formation is of importance also for various other properties of substances, such as the solubility of organic liquids in water and other solvents, melting points of substances under water,1 viscosity of liquids,14 second virial coefficient of gases,18 choice of crystal structure, cleavage and hardness of crystals, infrared absorption spectra, and proton magnetic resonance. Some of these are discussed in the following sections of this chapter. 

In pure water at 25°C, at any instant there are 1.0 x 10-7mol L 1 of H30+ and an equivalent concentration of OH- ions. It must be stressed that the proton is hardly ever bare in water because it has such a high affinity for water molecules. Hydrated complexes other than H30+ have been suggested, but since water is so extensively hydrogen-bonded, it is difficult to identify these species experimentally, and as a simplification the hydrated proton is often written as H +. ... 

In the latter type of solvent there will be no hydrogen bonds to anions, and so there is no envelope of solvent molecules hindering the attack of the base on the proton. This also has the effect of increasing the effective charge density on the base, as it is not dissipated by a solvent shell. This in turn increases the effective hardness of the base, and so favours elimination over substitution. Furthermore, in a solvent that has a low polarity, the stability of any charged intermediate would be reduced, and so bimolecular reactions would be favoured over unimolecular alternatives, and this also favours elimination. 

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