Acetate anion resonance forms

Look back at the resonance forms of the acetate ion and the acetone anion shown in the previous section. The pattern seen there is a common one that leads to a useful technique for drawing resonance forms. In general, any three-atom grouping with a p orbital on each atom has two resonance forms. 

Resolution (enantiomers), 307-309 Resonance, 43-47 acetate ion and, 43 acetone anion and. 45 acyl cations and, 558 allylic carbocations and, 488-489 allylic radical and, 341 arylamines and, 924 benzene and, 44. 521 benzylic carbocation and, 377 benzylic radical and, 578 carbonate ion and. 47 carboxylate ions and, 756-757 enolate ions and, 850 naphthalene and, 532 pentadienyl radical and. 48 phenoxide ions and, 605-606 Resonance effect, 562 Resonance forms, 43... 

The acetate anion has 24 valence electrons, leading to a Lewis structure with two resonance forms ... 

Figure 1-11. The resonance forms of some ligands that cannot be represented by a single valence bond structure (acetate anion, acetylacetonate anion and pyridine).

Note the hydrogen atoms on the a carbons. These hydrogens are fairly acidic due to resonance stabilization of the carbanion formed upon proton loss. Notice also the fact that the catalyst, sodium acetate, contains the acetate anion ... 

Trioxane can be polymerized cationically (catalyst BF3, HCIO4, etc.) or anionically (R3N, etc.). In the cationic polymerization, the hydrogen ion from the HCIO4, for example, protonates the acetal oxygen and forms an oxonium ion. The ring opens because the newly formed open-chain species is resonance-stabilized. The trimer eliminates formaldehyde up to an equilibrium concentration of about 0.07 mol of formaldehyde/liter. The actual chain growth probably involves the addition of formaldehyde, not trioxane. Thus, if the reaction is not too fast, an induction period is observed. The formaldehyde consumed in polymerization is replenished via the depolymerization of the trioxane ... 

Even though the formal process of converting one resonance structure to another moves electrons, resonance is not meant to indicate the motion of electrons. The acetate anion has a structure that is a composite of the two resonance forms that we have constructed. 

Whereas the pATa for the a-protons of aldehydes and ketones is in the region 17-19, for esters such as ethyl acetate it is about 25. This difference must relate to the presence of the second oxygen in the ester, since resonance stabilization in the enolate anion should be the same. To explain this difference, overlap of the non-carbonyl oxygen lone pair is invoked. Because this introduces charge separation, it is a form of resonance stabilization that can occur only in the neutral ester, not in the enolate anion. It thus stabilizes the neutral ester, reduces carbonyl character, and there is less tendency to lose a proton from the a-carbon to produce the enolate. Note that this is not a new concept we used the same reasoning to explain why amides were not basic like amines (see Section 4.5.4). 

Like the corresponding methylpyridines, 2- and 4-methylquinolines can be deprotonated by a base, such as sodium methoxide, forming resonance-stabilized anions (Scheme 3.9). The latter are useful in synthesis, providing nucleophilic reagents that allow extension of quinoline side chains through reactions with appropriate electrophiles. Activation of the 2-methyl group can also be achieved by the use of acetic anhydride (the same type of process occurs with 2-methylpyridine, Section 2.7.1, Worked Problem 2.3). 

The effect of resonance may be seen when the acidity of a simple carboxylic acid such as acetic acid is compared with the acidity of an alcohol such as ethanol. Both compounds can ionise to liberate a proton, but while the anion formed on ionisation of acetic acid is resonance-stabilised, the ethoxide anion formed on ionisation of ethanol is not so stabilised and the negative charge resides wholly on the oxygen atom (see Figure 3.3). 

The value of these reagents results from their specificity and the mildness of the reaction conditions. The reaction proceeds through the formation of an iodonium ion which, in the presence of carboxylate and silver ions, forms the resonance-stabilized cation 93 (5.93). Attack on the cation by the carboxylate anion in an Sn2 process gives the trans-diacyl compound. In the presence of water, however, a hydroxy acetal is formed this breaks down to gives the c/s-monoacylated diol. Note that with conformationally rigid molecules, or indeed with any alkene in which there is a preference for initial attack on one of the two faces of the double bond, the cis-diol obtained by the Woodward-Prdvost method may not have the same configuration as that obtained with osmium tetroxide. Related procedures, that avoid the use of expensive silver salts, have been reported with, for example, iodine and thallium(I) acetate or bismuth(III) acetate. 

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