Resonance structures enolates

There have been extensive investigations on the reaction mechanism. In most cases the reaction proceeds via initial nucleophilic addition of ammonia 2 to formaldehyde 1 to give adduct 5, which is converted into an iminium ion species 6 (note that a resonance structure—an aminocarbenium ion can be formulated) through protonation and subsequent loss of water. The iminium ion species 6 then reacts with the enol 7 of the CH-acidic substrate by overall loss of a proton ... 

Q Base removes an acidic hydrogen from the a position of the carbonyl compound, yielding an enolate anion that has two resonance structures. 

As the following resonance structures indicate, enamines are electronically similar to enolate ions. Overlap of the nitrogen lone-pair orbital with the double-bond p orbitals leads to an increase in electron density on the a carbon atom, making that carbon nucleophilic. An electrostatic potential map of N,N-6imethyl-aminoethvlene shows this shift of electron density (red) toward the a position. 

Show how resonance can occur in the following organic ions (a) acetate ion, CH,CO, (b) enolate ion, CH,COCH5, which has one resonance structure with a C=C double bond and an —O group on the central carbon atom (c) allyl cation, CH,CHCH,+ (d) amidate ion, CH,CONH (the O and the N atoms are both bonded to the second C atom). 

Fig. 3 Protonation states, isomerism and mesomerism of the HBI chromophore (p-hydroxybenzi-lidene-imidazolinone). The chromophore is shown in its most stable Z ( cw ) conformation, conventionally associated to a 0° value of the dihedral angle t, while the E ( trans ) conformation corresponds to t = 180°. For model compound HBDI (4 -hydroxy-benzylidene-2,3-dimethyl-imidazolinone), Ri = R2 = CH3, for chromophore in GFP, Ri, and R2 stand for the peptidic main chains toward N-terminus and C-terminus, respectively, (a) Possible protonation states of HBI (a) neutral, (b) anionic, (c) enolic, (d) cationic, and (e) zwitterionic. (b) Two resonance structures of the anionic form of HBI...

The two new bonds can be obtained by a Diels-Alder reaction. First, deprotonation gives an enolate that has an ortho-xylylene resonance structure. Diels-Alder reaction followed by retro-Diels-Alder reaction gives the product. 

This is an equilibrium reaction, and it raises a couple of points. First, there are two a-positions in the ketone, so what about the COCH3-derived enolate anion The answer is that it is formed, but since the CH3 group is not chiral, proton removal and reprotonation have no consequence. Racemization only occurs where we have a chiral a-carbon carrying a hydrogen substituent. Second, the enolate anion resonance structure with charge on carbon is not planar, but roughly tetrahedral. If we reprotonate this, it must occur from just one side. Yes, but both enantiomeric forms of the carbanion will be produced, so we shall still get the racemic mixture. 

The acidity of the a-hydrogen atom and the resonance structures of the conjugate base, the enolate ion. 

The structure can be thought of as a resonance-stabilized enolate anion with a proton bound between the two oxygen atoms and equidistant from them.156157... 

Due to the properties of the cx-hydrogen and carbonyl ketones and aldehydes exist at room temperature as enol tautomers. Tautomerization involves a proton shift, in this case from the a-carbon position to the carbonyl oxygen position. Both tautomers exist at room temperature, but the ketone or aldehyde tautomer is usually favored. Tautomerization is a reaction at equilibrium, not a resonance. (Remember, in resonance structures atoms don move and neither resonance structure actually exists.)... 

The resonance, which stabilizes the anion, creates two resonance structures — an enol and a keto form. In most cases, the keto form is more stable. 

Electronic configurations are the MO equivalents of resonance structures. Sometimes a molecular state cannot adequately be represented by a single configuration, just as benzene or an enolate ion cannot be represented by only one Kekule structure. The molecular state is then better described by a linear combination of several electronic configurations (configuration interaction method). 

The resonance structure having the charge on the oxygen atom (an enolate ion) is more stable than the original carbanion resonance structure. Therefore, the enolate ion will predominate over the carbanion. 

Because the enolate ion is the preferred resonance structure so a better mechanism for the acid base reaction shows the enolate ion being formed simultaneously as the acidic proton is lost (Following fig.). As the hydroxide ion forms its bond to the acidic proton, the C-H bond breaks, and the electrons in that bond form a rbond to the carbonyl carbon atom. Simultaneously, the carbonyl n bond breaks in such a way that both electrons move onto the oxygen. The electronegative oxygen is responsible for making the a proton acidic. 

However, the acidity of the a proton gets increased if it is flanked by two carbonyl groups rather than one, for example, 1, 3-diketones ((i-di ketones) or 1,3-diesters ([i-keto esters). This is due to the fact that the negative charge of the enolate ion can be stabilised by both carbonyl groups which results in three resonance structures (Following fig.). For example, the pKa of 2, 4-pentanedione is 9. 

As can be seen by the resonance structure, the double bond of the enol is electron rich, so bromine adds rapidly by an electrophilic mechanism. 

Two resonance structures can have exactly the same kinds of bonds, as they do in the carbocation in Sample Problem 1.5a, or they may have different types of bonds, as they do in the enolate in Sample Problem 1.5b. Either possibility is fine as long as the individual resonance structures are valid Lewis structures. 

I resonance-stabilized anion called an enolate. Either R C C resonance structure can be used to continue the Nu... 

Like other compounds with carbon-carbon double bonds, enols are electron rich, so they react as nucleophiles. Enols are even more electron rich than alkenes, though, because the OH group has a powerful electron-donating resonance effect. A second resonance structure can be drawn for the enol that places a negative charge on one of the carbon atoms. As a result, this carbon atom is especially nucleophilic, and it can react with an electrophile to form a new bond to carbon. Loss of a proton then forms a neutral product. 

C-H bond on the a carbon is more acidic than many other sp hybridized C-H bonds, because the resulting enolate is resonance stabilized. Moreover, one of the resonance structures is especially stable because it places a negative charge on an electronegative oxygen atom. 

Three resonance structures can be drawn for enolates derived from p-dicarbonyl compounds. 

Because enolates usually react at carbon instead of oxygen, the resonance structure that places the negative charge on oxygen will often be omitted in multistep mechanisms. 

Deprotonation. Treatment of diethyl malonate with OEt removes the acidic a proton between the two carbonyl groups. Recall from Section 23.3A that these protons are more acidic than other a protons because the enolate is stabilized by three resonance structures, instead of the usual two. Thus, OEt, rather than the stronger base EDA, can be used for this reaction. 

Base removes the acidic proton between the two carbonyl groups, forming the enolate In Step [1]. Only one of the three resonance structures is drawn. 

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