Carbonyl group electronic structure

The resonance effect of the carbonyl group Electron delocalization expressed by resonance between the following Lewis structures causes the negative charge in acetate to be shared equally by both oxygens Electron delocalization of this type IS not available to ethoxide ion... 

These three conjugation types may correspond to aromaticity or antiaromaticity, in accordance with the Hiickel rule. They can be depicted using the partial resonance structures given in Fig. 4. An additional requirement is that each branch required to participate in cyclic conjugation not contain a formally two-electron heteroatom site (such as -NH-, -O-, etc.) or exocyclic double bonds (such as carbonyl groups). These structural elements are incompatible with Kekule-type canonical structures. Finally, it should be noted that in the case of type C structures macrocyclic conjugation will be preserved, even when one of the a and b branches is not fully conjugated. 

The six unpaired electrons of the atoms of chromium, molybdenum and tungsten may form three double bonds with three carbonyl groups. Three unfilled p orbitals remain, which may be filled by the donation of an electron pair from each of three carbonyl groups. The structure of these compounds will therefore be represented by the valence-bond structure XXII ... 

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. 

The enol forms of -diketones and jS-ketoesters are stabilized by intramolecular hydrogen bonding and conjugation of the carbon-carbon double bond with the other carbonyl group. Electron diffraction studies of the enol form of acetylacetone in the gas phase are consistent with a symmetrical structure having a linear O-H-O... 

Because the carbon atom attached to the ring is positively polarized a carbonyl group behaves m much the same way as a trifluoromethyl group and destabilizes all the cyclo hexadienyl cation intermediates m electrophilic aromatic substitution reactions Attack at any nng position m benzaldehyde is slower than attack m benzene The intermediates for ortho and para substitution are particularly unstable because each has a resonance structure m which there is a positive charge on the carbon that bears the electron withdrawing substituent The intermediate for meta substitution avoids this unfavorable juxtaposition of positive charges is not as unstable and gives rise to most of the product... 

The carbonyl carbon of a ketone bears two electron releasing alkyl groups an aldehyde carbonyl group has only one Just as a disubstituted double bond m an alkene is more stable than a monosubstituted double bond a ketone carbonyl is more stable than an aldehyde carbonyl We 11 see later m this chapter that structural effects on the relative stability of carbonyl groups m aldehydes and ketones are an important factor m then rel ative reactivity... 

What structural features are responsible for the reactivity order of carboxylic acid derivatives Like the other carbonyl containing compounds that we ve studied they all have a planar arrangement of bonds to the carbonyl group Thus all are about the same in offering relatively unhindered access to the approach of a nucleophile They differ m the degree to which the atom attached to the carbonyl group can stabilize the carbonyl group by electron donation... 

Section 20 2 The structure and reactivity of carboxylic acid derivatives depend on how well the atom bonded to the carbonyl group donates electrons to it... 

A hydrogen attached to the a carbon atom of a p keto ester is relatively acidic Typical p keto esters have pA values of about 11 Because the a carbon atom is flanked by two electron withdrawing carbonyl groups a carbanion formed at this site is highly stabi hzed The electron delocalization m the anion of a p keto ester is represented by the res onance structures... 

The reaction starts with the nucleophilic addition of a tertiary amine 4 to the alkene 2 bearing an electron-withdrawing group. The zwitterionic intermediate 5 thus formed, has an activated carbon center a to the carbonyl group, as represented by the resonance structure 5a. The activated a-carbon acts as a nucleophilic center in a reaction with the electrophilic carbonyl carbon of the aldehyde or ketone 1 ... 

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