Ketones polarisation

In some transitions, the polarity of the chromophore is weaker after absorption of radiation. One case of this is the n — jt absorption due to the carbonyl present in ketones in solution. Before absorption, the C+-0 polarisation stabilises in the presence of a polar solvent whose molecules will be clustered around the solute because of electrostatic effects. Thus, the n —> -rr electronic transition will require more energy and its maximum will be displaced towards a shorter wavelength, contrary to what would be observed in a nonpolar solvent. This is the hypsochromic effect. Because the excited state is readily formed, the solvent shell around the... 

The oxidation of alcohols to aldehydes, ketones or carboxylic acids is one of the commonest reactions in organic chemistry, and is frequently achieved by transition metal complexes or salts. However, in most cases the precise mechanisms are not known, and the intermediates not fully characterised. In general, metal complexes of the alcohols are formed as transient intermediates in these reactions, but we shall not deal with these extremely important reactions in any great detail. The precise mechanisms depend upon the accessibility of the various one- and two-electron reduction products of the particular metal ion which is involved in the reaction. However, we will outline a brief indication of the mechanism. The first step involves the formation of an alcohol complex of the metal ion (Fig. 9-14). This might or might not deprotonate to the alkoxide form, depending upon the pH conditions of the reaction, the pK of the alcohol and the polarising ability of the metal ion. 

The carbonyl group is polarised in such a way that the oxygen is slightly negative and the carbon is slightly positive. Both the polarity of the carbonyl group and the presence of the weak n bond explain much of the chemistry and the physical properties of aldehydes and ketones. The polarity of the bond also means that the carbonyl group has a resultant dipole moment. 

Optical detection of intermediates produced in the reactions of triplet carbonyl compounds with electron donors has some obvious limitations. However, the technique of CIDNP is proving particularly effective at elucidating the reaction pathways in these systems. The outstanding work of Hendriks et al. (1979) illustrates the power of the technique. Not only was the role of radical ions in the reactions of alkyl aryl ketones with aromatic amines defined but the rate constants for many of the processes determined. The technique has been used to show that trifluoracetyl benzene reacts with electron donors such as 1,4-diazabicyclo[2.2.2]octane and 1,4-dimethoxy-benzene by an electron-transfer process (Thomas et al., 1977 Schilling et al., 1977). Chemically induced dynamic electron polarisation (CIDEP) has been... 

Fig. 20.3. Reaction of the template molecule (Michler s ketone) with the polymer backbone upon irradiation with plane-polarised light.

These trends agree with the frontier orbital analysis. In particular, the delivery of hydride from carbon breaks a relatively unpolarised bond, making the hydride notably soft, as we saw earlier in its capacity to attack pyridinium salts preferentially at the 4-position. The metal hydrogen bond will be more polarised, and metal hydrides should therefore be harder. Similarly, the delivery of hydride from boron will make it softer than when it is delivered from the more electropositive metal, aluminium. It also seems that, among a,(3-unsaturated carbonyl compounds, the susceptibility to conjugate reduction increases in the sequence ketones < esters < acids < amides but there are too few examples to be sure. 

This coordination of the ketone at the Lewis acid site polarises the CO bond and activates the carbonyl carbon to nucleophilic attack by the hydrogen peroxide. By contrast, in the case of a titanium site, the Ti-OOH species is observed by UV-visible spectroscopy to be formed, and hydrogen peroxide is activated, and is then able to react selectively, for example to epoxidise C=C... 

Nucleophilic addition-elimination to the ester carbonyl of phenylmagnesium bromide occurs, with alkoxide as the leaving group, giving a phenyl ketone. This is still electrophilic (carbonyl polarised by electronegativity difference between C and 0) and a second addition of PhMgBr then occurs, which after protonation on work-up gives a tertiary alcohol. 

Imine formation proceeds by nucleophilic addition-elimination of NH3 to the ketone carbonyl group, with loss of water. Imines, like ketones, are polarised by the electronegativity difference between carbon and nitrogen. Nucleophilic addition of cyanide occurs to give an amino nitrile intermediate. 

Ketones exist in equilbrium with their enol forms, which are nucleophilic, and an acid anhydride is electrophilic with a good leaving group (trifluoromethansulfonate. Off). Pd(0) is readily oxidised to Pd(ll), and a polarised C-OTf bond is capable of accepting the released electrons, to form a Pd-C bond. 

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