Acetate anion resonance stabilization

The O-H hydrogen of acetic acid is more acidic than the C-H hydrogens. The -OH oxygen is electronegative, and, consequently, the -O-H bond is more strongly polarized than the -C-H bonds. In addition, the acetate anion is stabilized by resonance. 

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). 

Mechanism. Removal of an a-hydrogen from the ethyl acetate by NaOEt produces a resonance-stabilized enolate anion. 

Rablen, P. R. Is the acetate anion stabilized by resonance or electrostatics A systematic structural comparison. /. Am. Chem. Soc. 2000, 122, 357-368. 

This is the acetate anion. The curved arrows are used to help keep track of how electrons are moved to get from the first resonance structure to the second. An unshared pair of electrons on the lower oxygen is moved in to become the pi electrons in the second structure. The pi electrons are moved to become an unshared pair on the upper oxygen. Resonance structures must always have the same total charge — in this case — I. These structures happen to be equivalent in other respects also, so they contribute equally to the resonance hybrid. With two important resonance structures, the acetate anion has a large resonance stabilization. It is significantly more stable than would be predicted on the basis of examination of only one of the structures. 

This is acetic acid, a neutral molecule. Similar resonance structures can be written for acetic acid as are shown in part 0 for the acetate anion. In this case the two structures are not the same. The second structure is still neutral overall, but it has two formal charges. Therefore, the first structure is more stable and contributes much more to the resonance hybrid than the second does. Acetic acid has a smaller resonance stabilization than that of acetate anion — it is only a little more stable than the first structure would indicate. 

The other factor that is contributing to the dramatic increase in the acidity of acetic acid is resonance stabilization. Neither ethanol nor its conjugate base, which is called ethoxide ion, is stabilized by resonance. The following resonance structures can be written for acetic acid and its conjugate base, acetate anion ... 

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 ... 

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. 

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 ... 

The answer really depends on what each nucleophile is reacting with. However, because the electron-releasing methyl group pushes electron density toward oxygen, we expect that more electron density on MeO will make it more reactive (more nucleophilic) than hydroxide (HO ). Acetate is a resonance stabilized anion, so there is less electron density available for donation (weaker nucleophile). In ethoxide, the electron density is effectively concentrated on oxygen and more available for donation a stronger nucleophile. 

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