Resonance structures ketones

The reaction starts with the nucleophilic addition of a tertiary amine 4 to the alkene 2 bearing an electron-withdrawing group. The zwitterionic intermediate 5 thus formed, has an activated carbon center a to the carbonyl group, as represented by the resonance structure 5a. The activated a-carbon acts as a nucleophilic center in a reaction with the electrophilic carbonyl carbon of the aldehyde or ketone 1 ... 

The initial step of olefin formation is a nucleophilic addition of the negatively polarized ylide carbon center (see the resonance structure 1 above) to the carbonyl carbon center of an aldehyde or ketone. A betain 8 is thus formed, which can cyclize to give the oxaphosphetane 9 as an intermediate. The latter decomposes to yield a trisubstituted phosphine oxide 4—e.g. triphenylphosphine oxide (with R = Ph) and an alkene 3. The driving force for that reaction is the formation of the strong double bond between phosphorus and oxygen ... 

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. 

Stork enamine synthesis takes advantage of the fact that an aldehyde or ketone reacts with a secondciry cimine to produce an enamine. Enamines cire resonance stabilized (see Figure 15-25) and have multiple applications. In the first resonance structure, the nitrogen is the nucleophile, while in the second resonance structure, the Ccirbanion is the nucleophile. Some commonly used secondary amines, pyrrolidine, piperidine, and morpholine, are shown in Figure 15-26. 

The pinacol rearrangement is a dehydration of an alcohol that results in an unexpected product. When hot sulfuric acid is added to an alcohol, the expected product of dehydration is an alkene. However, if the alcohol is a vicinal diol, the product will be a ketone or aldehyde. The reaction follows the mechanism shown, below. The first hydroxyl group is protonated and removed by the acid to form a carboca-tion in an expected dehydration step. Now, a methyl group may move to fonn an even more stable carbocation. This new carbocation exhibits resonance as shown. Resonance Structure 2 is favored because all tire atoms have an octet of electrons. The water deprotonates Resonance Structure 2, forming pinacolone and regenerating the acid catalyst. 

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.)... 

IR spectra show that the C-0 band has a very low ketonic character. These results tend to show the significance of l-oxa-6,6a IV-dithia-pentalene resonance structures. However, compounds containing one oxygen atom have generally larger dipole moments than the com-pouuds containing only sulfur. 

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. 

The resonance structure on the right is much more stable than the one on the left because the octet rule is satisfied for all of the atoms. The cation is actually the conjugate acid of a ketone. Because this cation is so much lower in energy than the usual carbocation, the transition state leading to it is also lower in energy (Hammond postulate). Thus, it is formed readily and the initial addition of the proton is very fast. 

The only difference between this formulation and that of benzene is the historical view of benzene as a single molecule with two "main" overlapping resonance structures that were sufficiently similar that they could be coalesced into a single mental construct. On the other hand, the chemical difference between an alcohol and a ketone make such a viewpoint seem strange, if not completely untenable. Only because of the introduction of an extended bond set does such a perspective become viable. 

Metal rf-inline complexes with various transition metals [1-10] and lanthanides [11,12] are well known in the literature. Early transition metal if-imine complexes have attracted attention as a-amino carbanion equivalents. Zirconium rf-imine complexes, or zirconaaziridines (the names describe different resonance structures), are readily accessible and have been applied in organic synthesis in view of the umpolung [13] of their carbons whereas imines readily react with nucleophiles, zirconaaziridines undergo the insertion of electrophilic reagents. Accessible compounds include heterocycles and nitrogen-containing products such as allylic amines, diamines, amino alcohols, amino amides, amino am-idines, and amino acid esters. Asymmetric syntheses of allylic amines and a-amino acid esters have even been carried out. The mechanism of such transformations has implications not only for imine complexes, but also for the related aldehyde and ketone complexes [14-16]. The synthesis and properties of zirconaaziridines and their applications toward organic transformations will be discussed in this chapter. 

The two extremes of the Dewar-Chatt-Duncanson model for olefin coordination can also be applied to describe aldehyde, ketone, and imine complexes. Resonance structure A is an rf complex of Zr(II), while its resonance structure B is a zirconaaziridine containing Zr-C and Zr-N bonds (Fig. 6). X-ray structural studies of zirconaaziridines and their observed reactivity suggest that resonance structure B is more important. 

Because carboxylic acid derivatives (RCOZ) all contain an atom Z with a nonbonded electron pair, three resonance structures can be drawn for RCOZ, compared to just two for aldehydes and ketones (Section 20.1). These three resonance structures stabilize RCOZ by delocalizing electron density. In fact, the more resonance structures 2 and 3 contribute to the resonance hybrid, the more stable RCOZ is. 

This heteroannular dienone system causes rings A and B to assume a half-chair conformation. Carbon atoms 1, 3, 4, 5, 6, 8, 9, 10, 11 are all in one plane while carbon atom 2 projects above and carbon atom 7 below the plane. The rest of the molecule is below the plane. Addition of the second unsaturation to the A -3-ketone enone system provides an additional electron-rich unsaturated residue, i.e., it enhances the electron delocalization. The result will be an enlarged contribution of ionic resonance structures such as... 

The hybrid can remove a proton from the hydronium ion to give the ketone form of the tautomers. Although not strictly correct (because resonance structures do not exist), such reactions commonly are depicted as arising from the resonance structure that bears the charge on the atom that is adding the proton. 

Problem 21.15 Draw resonance structures to account for the unusual stability of an a,j8-unsaturated aldehyde or ketone. What is the significance of these structuies in terms of orbitals (See Sec. 8.17.)... 

The conjugate addition of a nucleophile to an a, -unsaturated ketone-or aldehyde is due to the same electronic factors that are responsible for direct addition. We ve seen that carbonyl groups are polarized so that the carbonyl carbon is positive, and we can even draw a dipolar resonance structure to underscore the point ... 

When we draw a similar resonance structure for an ,/3-unsaturated carbonyl compound, however, the positive charge is allylic and can be shared by the jS carbon. In other words, the /3 carbon of an ,j8-unsaturated carbonyl compound is an electrophilic site and can react with nucleophiles. A comparison of electrostatic potential maps of ethylene with an a,/3-unsaturated ketone shows that the double-bond carbon atoms of the unsaturated ketone are more positive (more green) than those of the isolated alkene ethylene. 

Some compounds have partial aromatic or antiaromatic character due to the presence of a minor aromatic or antiaromatic resonance structure. Tropolone (cy-cloheptadienone) is much more stable than one would expect from a highly unsaturated ketone because its C-O resonance structure is aromatic. On the other hand, cyclopentadienone is extremely unstable because its C-O resonance structure is antiaromatic. 

The ease of carbanion formation from the various carbonyl compounds decreases in the order aldehydes, ketones, esters, amides, and acids. This order is understandable if we consider the nature of Y. The more electron-releasing Y becomes, the less will the carbonyl group be able to withdraw electrons from the a-carbon atom (XI). Thus in comparing aldehydes and ketones, hyperconjugation in Y permits another resonance structure which does not place a positive charge on the carbonyl carbon atom (XIV). Consequently, both stabilization of the anion... 

Phenol can be considered as the enol of cyclohexadienone. While the tautomeric keto-enol equilibrium lies far to the ketone side in the case of aliphatic ketones, for phenol it is shifted almost completely to the enol side. The reason of such stabilization is the formation of the aromatic system. The resonance stabilization is very high due to the contribution of the ortho- and / ara-quinonoid resonance structures. In the formation of the phenolate anion, the contribution of quinonoid resonance structures can stabilize the negative charge. 

---