Structure and Bonding The Carbonyl Group

The carbonyl group makes aldehydes and ketones rather polar, with dipole moments that are substantially higher than alkenes. 

The bonds to the carbon of the carbonyl group lie in the same plane, and at angles of approximately 120 with respect to each other. 

Both (a) ethylene and b) formaldehyde have the same number of electrons, and carbon is sp -hybridized in both. 

In formaldehyde, one of the carbons is replaced by an sp -hybridized oxygen. Like the carbon-carbon double bond of ethylene, the carbon-oxygen double bond of formaldehyde is composed of a a component and a component. The values given correspond to the C=C and the C=0 units respectively. 

The various ways of representing this polarization include 

Two notable aspects of the carbonyl group are its geometry and its polarity. The carbonyl group and the atoms directly attached to it lie in the same plane. Formaldehyde, for example, is planar. The bond angles involving the carbonyl group of aldehydes and ketones are close to 120°. 

Verify their geometries by making models of formaldehyde, acetaldehyde, and acetone. Make sure you execute the minimization routine. 

At 122 pm, the carbon-oxygen double bond distance in aldehydes and ketones is significantly shorter than the typical carbon-oxygen single bond distance of 141 pm in alcohols and ethers. 

In resonance terms, electron delocalization in the carbonyl group is represented by contributions from two principal resonance structures  

Compare the dipole moments and electrostatic potential maps of 1-butene and propanal on Learning By Modeling. 

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The 5-oxazolones or oxazolin-5-ones are very interesting heterocyclic compounds that have been used as intermediates in the synthesis of a variety of organic molecules. Two structural classes are possible, the 5(27T)-oxazolones (or 3-oxazo-lin-5-ones) and 5(47T)-oxazolones (or 2-oxazolin-5-ones). These structures differ only in the position of the double bond. Apart from the presence of the heteroatoms (N and O), the carbonyl group and the double bond, the 2- or 4-position, respectively, can be saturated or unsaturated. The isomeric 5-oxazolones are... 

For simplicity, we have thus far represented the structures of aldoses and ketoses as straight-chain molecules (Figs 7-3, 7-4). In fact, in aqueous solution, aldotet-roses and all monosaccharides with five or more carbon atoms in the backbone occur predominantly as cyclic (ring) structures in which the carbonyl group has formed a covalent bond with the oxygen of a hydroxyl... 

Figure 10.2 A polypeptide can form a helical structure in which the carbonyl group of the zth peptide bond is aligned with the amide group of the (i + 4)th peptide bond and forms a hydrogen bond (A) if all the three amino acid residues, i.e., Ca, are in their helical state (B). Image in (A) reprinted from Berg et al. [21] with permission.

Coordination of urea to a single Ni(II) ion via its O atom (mode D) does not appreciably alter the structural parameters of the carbonyl group and causes only a moderate shift of v(CO) from 1683 cm (121) to 1640-1660 cm This result suggests that the substrate is poorly activated in this binding mode. Nevertheless, the two C—N bonds may become significantly different in the bound substrate. 

Based on the structure of the rihozyme complexed with the adduct, it was proposed that in the active site of the rihozyme, orientation of substrate anthracene was defined by stacking with two purine bases and also the carbonyl group of the maleimide portion was hydrogen bonded with exocyclic amine on base. Thus, these interactions not only stabilized the transition state of [4 + 2] cycloaddition, but also activated diene and dienophile substrates. The same strategy was observed in the antibody counterpart. 

The structural features especially the very polar nature of the carbonyl group point clearly to the kind of chemistry we will see for aldehydes and ketones in this chapter The partially positive carbon of C=0 has carbocation character and is electrophilic The planar arrangement of its bonds make this carbon relatively uncrowded and susceptible to attack by nucleophiles Oxygen is partially negative and weakly basic... 

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