Carbonyl compounds orbitals

The initial bond formation between the -> ir excited carbonyl compound and an alkene can occur by interaction of the half-filled n -orbital of the [I CO] with the ir-system of the alkene, in a sense transferring a tt-electron to the -orbital and making a bond between an alkene carbon and the carbonyl oxygen. In this process (common for electron rich olefins) the plane formed by the alkene carbons and their four substituents is perpendicular to the plane of the carbonyl groups and its two substituents (Figure 1). In the... 

The photochemical reactions of organic compounds attracted great interest in the 1960s. As a result, many useful and fascinating reactions were uncovered, and photochemistry is now an important synthetic tool in organic chemistry. A firm basis for mechanistic description of many photochemical reactions has been developed. Some of the more general types of photochemical reactions will be discussed in this chapter. In Section 13.2, the relationship of photochemical reactions to the principles of orbital symmetry will be considered. In later sections, characteristic photochemical reactions of alkenes, dienes, carbonyl compounds, and aromatic rings will be introduced. 

For conjugated carbonyl compounds, such as a,) -enones, the orbital diagram would be similar, except for the recognition that the HOMO of the ground state is ij/2 of the enone system, rather than the oxygen lone-pair orbital. The excited states can sometimes be usefully represented as dipolar or diradical intermediates ... 

As was mentioned in Section 13.2, the [27t + 27i] photocycloaddition of alkenes is an allowed reaction according to orbital symmetry considerations. Among the most useful reactions in this categoty, from a synthetic point of view, are intramolecular [27t + 2ti] cycloadditions of dienes and intermolecular [2ti + 2ti] cycloadditions of alkenes with cyclic a, -unsaturated carbonyl compounds. These reactions will be discussed in more detail in Section 6.4 of Part B. 

Excitation of the ii-tt transition of carbonyl compounds entails transfer of an electron from an orbital essentially localized on oxygen to... 

Wavelengths between X, = 280 nm and 290 nm or longer are absorbed by carbonyl compounds. Here the free n-electrons of the oxygen enter the antibonding n molecular orbital. 

Alkyl substituents accelerate electrophilic addition reactions of alkenes and retard nucleophilic additions to carbonyl compounds. The bonding orbital of the alkyl groups interacts with the n bonding orbital, i.e., the HOMO of alkenes and raises the energy (Scheme 22). The reactivity increases toward electron acceptors. The orbital interacts with jt (LUMO) of carbonyl compounds and raises the energy (Scheme 23). The reactivity decreases toward electron donors. 

The excited state of the carbonyl compound is the (n, it ) state where one electron is excited from the HOMO to the LUMO. The SOMO is the n-orbital on the carbonyl oxygen atom. The SOMO is the antibonding jt -orbital. 

Ab initio molecular orbital calculations are being used to study the reactions of anionic nucleophiles with carbonyl compounds in the gas phase. A rich variety of energy surfaces is found as shown here for reactions of hydroxide ion with methyl formate and formaldehyde, chloride ion with formyl and acetyl chloride, and fluoride ion with formyl fluoride. Extension of these investigations to determine the influence of solvation on the energy profiles is also underway the statistical mechanics approach is outlined and illustrated by results from Monte Carlo simulations for the addition of hydroxide ion to formaldehyde in water. 

As discussed in Section 10.4 of Part A, concerted suprafacial [2tt + 2tt] cycloadditions are forbidden by orbital symmetry rules. Two types of [2 + 2] cycloadditions are of synthetic value addition reactions of ketenes and photochemical additions. The latter group includes reactions of alkenes, dienes, enones, and carbonyl compounds, and these additions are discussed in the sections that follow. 

In a, P-unsaturated carbonyl compounds and related electron-deficient alkenes and alkynes, there exist two electrophilic sites and both are prone to be attacked by nucleophiles. However, the conjugated site is considerably softer compared with the unconjugated site, based on the Frontier Molecular Orbital analysis.27 Consequently, softer nucleophiles predominantly react with a, (i-unsaturated carbonyl compounds through conjugate addition (or Michael addition). Water is a hard solvent. This property of water has two significant implications for conjugate addition reactions (1) Such reactions can tolerate water since the nucleophiles and the electrophiles are softer whereas water is hard and (2) water will not compete with nucleophiles significantly in such... 

Fig. 7.2 indicates the electron distribution of HO of carbon monoxide which largely localizes at the carbon atom 79>. This orbital resembles a lone-pair AO on the carbon atom and leads to the expectation that the carbon atom would behave as the electron-donating centre. As a matter of fact, the CO molecule coordinates with a metal cation by M—C—O type linkage (M represents a metal cation) in various metal carbonyl compounds. It is of interest to remark that the total electron population of the CO molecule has been shown by recent reliable calculation 80> to be rich on the oxygen atom in place of the carbon atom. 

In this section the effect of spin-orbit coupling on radiative and radiationless intercombinational transitions (transitions occurring between states of different multiplicity) will be investigated. We will be particularly concerned with the use of internal and external heavy atoms to induce spin-orbit coupling. The effect of heavy atoms on intercombinational processes occurring in aromatic hydrocarbons, carbonyl compounds, and heterocyclic compounds will be discussed. 

A second category of silene reactions involves interactions with tt-bonded reagents which may include homonuclear species such as 1,3-dienes, alkynes, alkenes, and azo compounds as well as heteronuclear reagents such as carbonyl compounds, imines, and nitriles. Four modes of reaction have been observed nominal [2 + 2] cycloaddition (thermally forbidden on the basis of orbital symmetry considerations), [2 + 4] cycloadditions accompanied in some cases by the products of apparent ene reactions (both thermally allowed), and some cases of (allowed) 1,3-dipolar cycloadditions. 

Figure 3.75 A model carbonyl compound with the amine group twisted for maximum delocalization of nN into the half-filled carbonyl n0 orbital of the 3n->-7T excited state.

Solvent polarity may affect the absorption characteristics, in particular since the polarity of a molecule usually changes when an electron is moved from one orbital to another. Solvent effects of up to 20 nm may be observed with carbonyl compounds. Thus the absorption of acetone occurs at 279 nm in n-hexane, 270 nm in... 

Carbon-heteroatom bonds of a-substituents to a carbonyl function are cleaved at less cathodic potentials than those required in the absence of the carbonyl group. The polarographic reduction of these systems has been extensively studies in aqueous media. Tie half-wave potentials are also less negative than those of the non-substituted carbonyl compounds. Electron addition involves an interaction between the n -orbital of the carbonyl group and the o-orbital of the departing group. 

Problem 16.4 Write resonance structures for the COOH group and show how these and orbital hybridization account for (a) polarity and dipole moments (1.7-1.9D) of carboxylic acids (b) their low reactivity toward nucleophilic additions, as compared to carbonyl compounds. ... 

The orbital interaction treatment of C=C n bonds and other types of p-p n bonds is given here. Carbonyl compounds are treated separately in Chapter 8, and organo-metallic 7i-type bonding is briefly described in Chapter 13. 

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