Carbonyl compounds atomic charges

The proton transfer equilibrium that interconverts a carbonyl compound and its enol can be catalyzed by bases as well as by acids Figure 18 3 illustrates the roles of hydroxide ion and water m a base catalyzed enolization As m acid catalyzed enolization protons are transferred sequentially rather than m a single step First (step 1) the base abstracts a proton from the a carbon atom to yield an anion This anion is a resonance stabilized species Its negative charge is shared by the a carbon atom and the carbonyl oxygen... 

The well-known photopolymerization of acrylic monomers usually involves a charge transfer system with carbonyl compound as an acceptor and aliphatic tertiary amine, triethylamine (TEA), as a donor. Instead of tertiary amine such as TEA or DMT, Li et al. [89] investigated the photopolymerization of AN in the presence of benzophenone (BP) and aniline (A) or N-methylaniline (NMA) and found that the BP-A or BP-NMA system will give a higher rate of polymerization than that of the well-known system BP-TEA. Still, we know that secondary aromatic amine would be deprotonated of the H-atom mostly on the N-atom so we proposed the mechanism as follows ... 

Figure 22.5 Mechanism of enolate ion formation by abstraction of an a proton from a carbonyl compound. The enolate ion is stabilized by resonance, and the negative charge (red) is shared by the oxygen and the a carbon atom, as indicated by the electrostatic potential map.

We shall see that most of the reactions of simple carbonyl compounds, like formaldehyde, are a consequence of the presence of an electron-deficient carbon atom. This is accounted for in resonance theory by a contribution from the resonance structure with charge separation (see Section 7.1). The second example shows the so-called conjugate acid of acetone, formed to some extent by treating acetone with acid (see Section 7.1). Protonation in this way typically activates acetone towards reaction, and we... 

In a, /J-unsaturated carbonyl compounds [282, 283], carbonyl carbons are shielded relative to those in comparable saturated compounds. Electron withdrawal of the carbonyl oxygen at the carbonyl carbon is attenuated by electron donation of the double bond. This, in turn, generates a positively charged [1 carbon atom which is deshielded as a result ... 

In all the cases considered, stabilization of the cis isomers of semidiones is observed when the cation is present. As a rule, for the nonchelated semidione the trans isomer is more stable than the cis isomer. However, the interaction with cations produces the opposite effect, and the cis isomer appears to be more stable than the trans form. Carbonyl compounds capture an electron and are converted into ketyls, which contain a negatively charged oxygen atom. This atom is a particularly powerful proton acceptor. Therefore, a proton, which steps forward as a cation, can provide the contour closure for ion-pair formation. 

The addition of a proton to a metal carbonyl compound may occur in either of two modes the formation of metal-hydrogen bond, or protonation of a ligand attached to the central metal atom. If the ligand protonated is an organic radical, a carbonium ion is produced, which may be stabilized by suitable delocalization of charge over the complex, including the central metal atom. Consequently, such protonated species may be legitimately considered as examples of cationic metal carbonyl compounds. 

The reaction of carbonyl compounds with secondary amines cannot give imines as there is no NH proton to be lost in the final step of the mechanism. However, there is another way in which the positive charge on the nitrogen can be neutralised. It involves loss of a proton from a neighbouring carbon atom (Following fig.). Water acts as a base to remove the proton and the electrons that make up the C-H o bond are used to form a new n bond to the neighbouring carbon. 

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