Polarisation, bonds, hydrolysis

The values of the 15N CP MAS chemical shift of Lys296 nitrogen bonded to retinal via the —C=N bond ( Schiff base) was equal to 155.4 ppm for rhodopsin and 282.8 ppm for metarhodopsin (relative to 5.6 M aqueous NH4C1).70 The results proved the imine bond polarisation, which facilitates Schiff base hydrolysis. The comparison between chemical shifts for metarhodopsin and model compounds suggested that Schiff base linkage of the all-frans retinal chromophore in Metall is in a polar environment. 

Most M3+(aq) and a number of M2+(aq) cations behave in much the same way as Fe3+(aq). Evidently the viabilities of simple aquo-cations are restricted by the possibilities for hydrolysis, as well as the redox processes discussed in Section 5.4. The simplest explanation for the hydrolysis of Fe3+(aq) acknowledges the considerable polarising power of Fe3+. When surrounded by six water molecules, the cation will tend to attract electron density from the O atoms. This makes the coordinated O atoms less attractive to protons than those in the bulk solvent, and encourages the transfer of protons from the coordination sphere. If the bonding in the complex [Fe(H20)6]3+ is regarded as covalent, with the formation of coordinate bonds, we can arrive at the same conclusion the delocalisation of the positive charge over the complex ion [Fe(H20)50H]2+ will tend to discourage recombination of the coordinated OH- ion with a proton. 

It is often found that imines are stabilised towards hydrolysis by co-ordination of the nitrogen to a 7t-bonding transition metal. Of course, in the absence of significant 7C-bonding interactions, ligand polarisation is expected to have the opposite effect and activate the imine towards nucleophilic attack. 

When we consider the reverse reactions, the metal-directed hydrolysis of amides, the polarising metal ion may also play a second, and often undesirable, role. In addition to polarising the carbonyl group and activating the carbon atom to nucleophilic attack, the metal may also polarise an amide N-H bond. If we consider the amino acid amide 3.4, the polarisation may be transmitted through the ligand framework to the amide N-H bond. This polarisation may be sufficient to lower the pKa so as to allow deprotonation under the desired reaction conditions (Fig. 3-13). 

Kuhn and co-workers reported the reaction of a stable carbene with POCl3 to yield the [(NHC)POCl2][Cl] salt which, following partial hydrolysis, affords 36 (Scheme 19).46 The solid state single crystal X-ray structure comprises a phosphorus-NHC bond length of 1.843(2) A, which is consistent with the P-C bond being polarised. 

The presence of groups which are easily polarised or capable of forming hydrogen bonds with water promotes solvent-solute interactions in the initial state at the expense of solvent-solvent interactions (Robertson, 1960). The reduction in the initial state solvation shell is thought to decrease the extent of its disruption on activation, consistent with the observation that AC for the hydrolysis of sulphonates is less negative than for the corresponding chlorides (see Table 6). This approach therefore considers that AG essentially reflects the interactions which occur in the initial state. 

With this assumption that the rate of hydrolysis is determined by the degree of polarisation of the carbon/halogen bond, then the alkyl chloride would be hydrolysed faster than the alkyl iodide. This hypothesis may be justified by saying that the more highly polarised the bond is initially, the closer it is to breaking heterolytically, and so the more readily it will actually break into the resultant ions. 

In spite of the fact that the Si—N, Si—F and Si—Cl bonds are reinforced, they (and also the Si—Br and Si—I bonds) can easily be changed into the Si—O bond in a solvolysis reaction. Hydrolysis or alcoholysis proceeds rapidly by an Sn2 reaction mechanism. A transition state is generated involving the vacant silicon d orbitals and thus increasing the silicon coordination number. The hydrolysis of the C—Br and C—I bonds, which are easily polarised, also proceeds by an 8 2 reaction mechanism, and is also rapid. For example, the hydrolysis constant K[CH3Br] [OH ] in an ethanol/water mixture (4 1) at 55°C is 0.0021 1-mol see [6]. 

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