Carbonyl compounds conjugated, resonance

Carbanions derived from carbonyl compoimds are often referred to as etiolates. This name is derived from the enol tautomer of carbonyl compounds. The resonance-stabilized enolate anion is the conjugate base of both the keto and enol forms of carbonyl... 

Since Stork et al. introduced as a new synthetic method the alkylation and acylation of carbonyl compounds via enamines, this class of compounds has been the subjeet of intensive studies 1-3). The exceptional physical and chemical behavior of the enamine structure can be ascribed to resonance by conjugation of the unshared pair of electrons of the nitrogen atom with the 77 electrons of the double bond ... 

The acidity of a-hydrogen atoms of carbonyl compounds is due to the strong electron withdrawing effect of the carbonyl group and resonance stabilisation of the conjugate base. 

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... 

The 1,4-addition (or conjugate addition) of resonance-stabilized carbanions. The Michael Addition is thermodynamically controlled the reaction donors are active methylenes such as malonates and nitroalkanes, and the acceptors are activated olefins such as a,P-unsaturated carbonyl compounds. 

Isoprene can be polymerized in the laboratory by a radical chain mechanism. As shown in the following equations, the odd electron of the initially produced radical is delocalized onto both C-2 and C-4 by resonance. Either of these carbons may add to another isoprene monomer to continue the chain reaction. If C-2 adds, the process is called 1,2-addition if C-4 adds, the process is called 1,4-addition. (This is similar to the addition of electrophiles to conjugated dienes discussed in Section 11.13 and the addition of nucleophiles to a,/8-unsaturated carbonyl compounds described in Section 18.10.)... 

Because the two Jt bonds are conjugated, the electron density in an a,P-unsaturated carbonyl compound is delocalized over four atoms. Three resonance structures show that the carbonyl carbon and the P carbon bear a partial positive charge. This means that a,P-unsaturated carbonyl compounds can react with nucleophiles at two different sites. 

The Michael reaction is addition of a carbon nucleophile to the /3 position of an a,/3-unsaturated carbonyl compound or its equivalent. It also may be called a 1,4-addition reaction (the carbonyl oxygen is counted as 1 and the )3-carbon as 4). The conjugation of the v bond with thp carbonyl group imparts positive character to the /3 position, making it susceptible to reaction with a nucleophile. The product of this reaction, an enolate ion, also is stabilized by resonance. 

The influence of cyclopropyl on the gas phase stability of carbocations as measured by ion cyclotron resonance is shown in Table 14, along with data for some reference compounds. The results are given as gas phase basicities, GB, and proton affinities, PA, defined as AG° and AH°, respectively, for dissociation of the protonated molecule, as in equation 11. In addition hydride affinities D(BH H ) for some cations defined as — AH° for equation 18 are included. For the gas phase basicities and proton affinities the products B are alkenes, amines, nitriles or carbonyl compounds, and thus for these values the stability of the cation is compared to a derivative where the substituent is conjugated with a carbon-carbon or carbon-oxygen double bond, or a nitrogen lone pair, whereas for hydride affinities the products are saturated. 

Obtain infrared and nuclear magnetic resonance spectra following the procedures of Chapters 19 and 20. If these spectra indicate the presence of conjugated double bonds, aromatic rings, or conjugated carbonyl compounds obtain the ultraviolet spectrum following the procedures of Chapter 21. Interpret the spectra as fully as possible by reference to the sources cited at the end of the various spectroscopy chapters. 

The tendency for a,j9-unsaturated carbonyl compounds to undergo nucleophilic addition is thus due not simply to the electron-withdrawing ability of the carbonyl group, but to the existence of the conjugated system that permits formation of the resonance-stabilized anion 1. The importance in synthesis of a,j3-un-saturated aldehydes, ketones, acids, esters, and nitriles is due to the fact that they provide such a conjugated system. 

Although pyrone derivatives are a type of a,/3-unsaturated carbonyl compound, their reactivity to nucleophiles is greatly decreased owing to the inherent resonance stabilization of the pyrone ring and the conjugation of... 

The resonant anion can act as a typical carbanion and add to C=0 or C=N bonds, just as a Gtignard reagent does. In a true aldol condensation, the anion after formation proceeds to interact with another molecule of the starting aldehyde or ketone, giving a product that is a beta-hydroxy aldehyde or ketone known as an aldol (Scheme 4.29). Frequently, water is eliminated from this compound to produce an a, P-unsaturated carbonyl compound this process is made easy by the resonance stabilization of such conjugated systems. 

Carbonyl compounds having carbon-carbon double bonds adjacent to the carbonyl group represent another system in which certain structural properties can be attributed to resonance interactions. While only a single uncharged structure can be drawn for a conjugated unsaturated carbonyl compound, a second charged... 

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