Carbonyl bond polarisation

The NMR spectra of the product do not show these features. The highest C shift value is Sc = 160.9 and indicates a conjugated carboxy-C atom instead of the keto carbonyl function of an isoflavone (5c =175). On the other hand, a deshielded CH fragment at 5c/<5 = 138.7/7.i52 appears in the C NMR spectrum, which belongs to a CC double bond polarised by a -A/effect. The two together point to a coumarin 4 with the substitution pattern defined by the reagents. 

Dipole-dipole interactions can occur between polarised bonds other than N-H or O-H bonds. The most likely functional groups that can interact in this way are those containing a carbonyl group (C = O), the electrons in the carbonyl bond are polarised towards the more electronegative oxygen such that the oxygen acquires a... 

Formaldehyde possessing a strongly polarised carbonyl bond is very susceptible to either cationic or anionic addition polymerization. With a basic initiator the active centre is an alkoxide ion in contrast to the carbanion of a vinyl monomer. The readiness with which polymerization occurs makes initiation by relatively weak covalent bases possible. 

The reactant may be considered as a polarised carbonyl bond, reflected in contributions of covalent and ionic resonance structures. As the reaction proceeds, the contribution of both of these is replaced by the structure at the right of the diagram. Because of the greater concentration of positive charge on carbon in the reactant, donor substituents stabilise reactants more than transition state. In summary, donor substituents deactivate carbonyls. Cieplak acknowledged that the reactivity effect of a donor might be different from its stereochemical effect. 

The remainder of the biosynthesis takes place in the peroxisomes. The substrate is imported by the cassette-transporter comatose (CTS). The hydrogenation of the endocyclic double bond is carried out by 12-oxophytodienic acid reductase this enzyme belongs to a small group of flavin-dependent oxidoreductases. In accord with the reaction mechanism, which was suggested for the related Old Yellow Enzyme of yeast, two hydrogen bridges from histidine (His-186 and His-189 in the 12-oxophytodienic acid reductase from mouse-ear cress Arabidopsis thaliana)) towards the carbonyl group polarise the double bond, so that a hydride of the reduced flavin cofactor can be transferred to C-10. The carbanion in the 11 -position is then protonated by Tyr-191. 

As well as the C->-0 inductive effect in the bond joining the two atoms, the more readily polarisable it electrons are also affected (cf p. 22) so that the carbonyl group is best represented by a hybrid... 

Rate enhancements of 104 - 106 are typically associated with the formation of chelated complexes in which the carbonyl oxygen atom is also co-ordinated to the metal (3.2). This results in a considerably greater polarisation of the C-0 bond. 

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). 

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. 

The polarisation of the carbonyl group is away from carbon towards oxygen in the bonding orbital, and the opposite in the antibonding orbital, as usual. The wire-mesh pictures in Fig. 1.53 show more realistically an outer contour of these two orbitals in formaldehyde itself. Note that in Figs 1.52 and 1.53, it appears from the shape of the outer contour that the electron population in the bonding orbital is very similar on oxygen to that on carbon. 

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