Carbonyl groups orbital picture

Display the lowest-unoccupied molecular orbital (LUMO) for equatorial methylcyclohexanone. This is the orbital into which the nucleophile s pair of electrons will go. Is it larger on the axial or equatorial face A clearer picture follows from the LUMO map, which gives the value of the LUMO on the electron density surface, that is, the accessible surface of the molecule. Display the LUMO map for equatorial methylcyclohexanone. Which face of the carbonyl group is more likely to be attacked by a nucleophile Which alcohol will result ... 

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. 

The facts are consistent with the orbital picture of the carbonyl group. Electron diffraction and spectroscopic studies of aldehydes and ketones show that carbon, oxygen, and the two other atoms attached to carbonyl carbon lie in a plane the three bond angles of carbon are very close to 120°. The large dipole moments (2.3 2.8 d) of aldehydes and ketones indicate that the electrons ot me cybonvl group arc quite unequally shared. We shall see how the physical and chemical properties of aldehydes and ketones are determined by the structure of the carbonyl group. 

If the carbonyl group is conjugated with a n bond, as in a,/3-unsaturated ketones, the fraction of positive charge on the carbon atom is shared with the n bond, and the (3 carbon becomes partly cationic in nature, as symbolised in Figs. 2.3 and 2.4 by the relatively small p orbitals on the (3 carbon atom in Li and ip2 of acrolein as a model for a Z-substituted alkene. Looking again at those pictures, it is clear that the sum of the coefficients of the filled p orbitals on the carbonyl carbon is smaller than the sum at the (3 carbon, and that... 

By analogy to the carbon-carbon double bond, we might expect sp hybridization for the carbon atom in the carbonyl group. The 2p orbitals on carbon and oxygen form the n bond, and two lone pairs of electrons remain on oxygen. Figure 16.2 gives a schematic orbital picture of both the o and n systems for the simplest carbonyl compound, formaldehyde. 

The third factor to consider involves the orbitals making up the carbonyl 7t bond. If we look at the molecular orbitals involved, we can see how they influence the direction in which the reaction occurs. As in all organic reactions, we look for the combination of a nucleophile (here, water) and an electrophile. The electrophile in this case must be the empty if orbital of the carbonyl group. Figure 16.22 shows the orbital interactions graphically, and gives pictures of the orbital lobes involved. 

Figure 17-1 Orbital picture of the carbonyl group. The sp hybridization and the orbital arrangement are similar to those of ethene (Figure 11-2). However, both the two lone electron pairs and the electronegativity of the oxygen atom modify the properties of the functional group.

However, this is not a complete picture of the factors that contribute to the reactivity of functional groups. For example, the electronegativity difference between carbon and iodine is relatively small. In the much larger iodine atom the bonding orbitals are further from the nucleus than in chlorine and are more polarizable during the course of a reaction. These differences affect both the a- and ii-bonds. Thus many of the reactions of the alkyl halides and of carbonyl compounds may be rationalized in terms of the polarization of the bonds and the polarizability of the component atoms. 

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