Structure of the Carbonyl Group

The carbonyl carbon atom is sp hybridized and bonded to three other atoms through coplanar sigma bonds oriented about 120° apart The unhybridized p orbital overlaps with a p orbital of oxygen to form a pi bond. The double bond between carbon and oxygen is similar to an alkene C=C double bond, except that the carbonyl double bond is shorter, stronger, and polarized 

The first resonance form is more important because it involves more bonds and less charge separation. The contribution of the second structure is evidenced by the large dipole moments of the ketones and aldehydes shown here. 

This polarization of the carbonyl group contributes to the reactivity of ketones and aldehydes The positively polarized carbon atom acts as an electrophile (Lewis acid), and the negatively polarized oxygen acts as a nucleophile (Lewis base). 

Carbonyl compounds are a broad group of compounds that includes aldehydes, ketones, carboxylic acids, esters, and amides. 

The carbonyl An aldehyde A ketone A carboxylic An ester An amide group acid 

The carbonyl carbon atom is sp hybridized thus it and the three atoms attached to it lie in the same plane. The bond angles between the three attached atoms are what we would expect of a tfigonal planar structure they are approximately 120°  

The carbon-oxygen double bond consists of two electrons in a a- bond and two electrons in a 77 bond. The tt bond is formed by overlap of the carbon p orbital with a p orbital from the oxygen atom. The electron pair in the tt bond occupies both lobes (above and below the plane of the a bonds). 

The 77 bonding molecular orbital of formaldehyde (HCHO). The electron pair of the 77 bond occupies both lobes. 

If we think of the carbonyl group as an oxygen analog of the alkene functional group, we can correctly predict its orbital description, the structures of aldehydes and ketones, and some of their physical properties. However, the alkene and carbonyl double bonds do differ considerably in reactivity because of the electronegativity of oxygen and its two lone pairs of electrons. 

The carbonyl group contains a shorty strong, and very polar bond 

Comparison with the electronic structure of an alkene double bond reveals two important differences. First, the oxygen atom bears two lone electron pairs located in two sp hybrid orbitals. Second, oxygen is more electronegative than carbon. This property causes an appreciable polarization of the carbon-oxygen double bond, with a partial positive 

---

All three structures have 0(, symmetry and are very similar. The bond length from the central atom to the carbonyl group is slightly different in each compound, and it is longest for the molybdenum substituent. The internal structure of the carbonyl groups is essentially unchanged by substitution. ... 

The electronic structure of the carbonyl group is perturbed by the rest of the molecule. In classical terms, a carbonyl compound may be represented as a resonance hybrid of the canonical structure I to V, viz. 

Structure of the Carbonyl Group 817 18-3 Nomenclature of Ketones and Aldehydes 818 18-4 Physical Properties of Ketones and Aldehydes 820 18-5 Spectroscopy of Ketones and Aldehydes 822 18-6 Industrial Importance of Ketones and Aldehydes 828 18-7 Review of Syntheses of Ketones and Aldehydes 829 18-8 Synthesis of Ketones from Carboxylic Acids 833 18-9 Synthesis of Ketones and Aldehydes from Nitriles 833... 

Enolates react with other carbonyl groups at the electrophilic carbonyl carbon. These reactions are more complicated because the initial addition adduct goes on to form different products depending on the structure of the carbonyl group. These reactions form the subject of Chapter 24. 

Let us examine the structure of the carbonyl group. Carbonyl carbon is joined to three other atoms by a bonds since these bonds utilize sp orbitals (Sec. 1.10), they lie in a plane, and are 120 apart. The remaining p orbital of the carbon overlaps a p orbital of oxygen to form a n bond carbon and oxygen are thus... 

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. 

In the last chapter, we saw how the structure of the carbonyl group—a multiple bond that is also highly polar—gives rise to a characteristic combination of functional behaviors addition reactions mediated by electrophilic attack (usually by protons) on the Lewis basic oxygen and attack by nucleophiles on the carbon. We turn now to a third site of reactivity in aldehydes... 

---