Acetate resonance structures

Resonance stabilization in the products is best illustrated by the reactant anhydrides (Figure 3.10b). The unpaired electrons of the bridging oxygen atoms in acetic anhydride (and phosphoric anhydride) cannot participate in resonance structures with both electrophilic centers at once. This competing resonance situation is relieved in the product acetate or phosphate molecules. 

Resonance forms differ only in the placement of their tt or nonbonding electrons. Neither the position nor the hybridization of any atom changes from one resonance form to another. In the acetate ion, for example, the carbon atom is sp2-hybridized and the oxygen atoms remain in exactly the same place in both resonance forms. Only the positions of the r electrons in the C=0 bond and the lone-pair electrons on oxygen differ from one form to another. This movement of electrons from one resonance structure to another can be indicated by using curved arrows. A curved arrow always indicates the movement of electrons, not the movement of atoms. An arrow shows that a pair of electrons moves from the atom or bond at the tail of the arrow to the atom or bond at the head of the arrow. 

A The O-H hydrogen in acetic acid is much more acidic than any of the C-H hydrogens. Explain this result using resonance structures. 

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). 

Lacking resonance stabilization, the chain radicals doubtless are very reactive, but owing to the corresponding lack of resonance structures in the transition state allyl acetate is a relatively unreactive monomer. These factors are conducive to the occurrence of the competitive reaction... 

Figure 3.8 Two resonance structures that can be written for acetic acid and two that can be written for acetate ion. According to a resonance explanation of the greater acidity of acetic acid, the equivalent resonance structures for the acetate ion provide it greater resonance stabilization and reduce the positive free-energy change for the ionization.

We have also shown some representations of acetate and methanesulfonate anions that have been devised to emphasize resonance delocalization these include partial bonds rather than double/single bonds. Although these representations are valuable, they can lead to some confusion in interpretation. It is important to remember that there is a double bond in these systems. Therefore, we prefer to draw out the contributing resonance structures. 

Reaction 11 involves hydrogen atom transfer as proposed by Halpern et al. (13) in the mechanism of formic acid oxidation by cobalt (III) in aqueous solutions. In this reaction one could consider that as peracetic acid approaches the coordination sphere of Co111 and transfers the hydrogen atom to the coordinated acetate, the Co111 atom is transformed into a Co11 complex of peracetoxy radical (or Co111 complex of peracetate anion). Complexes of free radicals with metal ions have been postulated by Kochi (16). The substitution rate in this complex could be intermediate between the rate of substitution of cobalt (III) and cobalt (II) complexes owing to the contribution of the resonance structures ... 

Cava and Pollack s elegant synthesis of benzo[c]thiophene52 has been extended to the synthesis of, for example, naphtho[l,2-c]thio-phene (47),52 methyl benzo[c]thiophene-5-carboxylate,54 and 1,3-dimethyl- (48)55 and l,3,4,6-tetraphenylthieno[3,4-c]thiophene (49).56 The last compound is of particular interest because it is a remarkably stable (cf. compound 48 which only exists transiently) nonclassical thiophene containing ten n electrons for which the only uncharged resonance structure (49) contains a tetravalent sulfur atom. When the sulfoxide (50) is pyrolyzed over aluminum oxide, it gives the parent cyclic sulfide and 51 by disproportionation.57 However, when 50 is heated in acetic anhydride in the presence of iV-phenylmaleimide,... 

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 ... 

The solution structure of the EGTA lanthanide complexes was investigated using the H and NMR spectra of the various lanthanide complexes and the Lanthanide Induced Shift (LIS) for one of the acetate resonances for a series of Ln-EGTA complexes. The spectra of the lighter lanthanides up to Sm show five resonances in the H spectrum and seven in the spectrum at room tempera-... 

With acetate (CH3COO"0, however, two resonance structures can be drawn. 

The difference in these two resonance structures is the position of a Jt bond and a ione pair. Although each resonance structure of acetate implies that the negative charge is localized on an... 

Remember that neither resonance form adequately represents acetate. The true structure is a hybrid of both structures. In the hybrid, the electron pairs drawn in different locations in individual resonance structures are delocalized. With acetate, a dashed line is used to show that each C-0 bond has a partial double bond character. The symbol 8 (partial negative) indicates that the charge is delocalized on both O atoms in the hybrid. 

We have already drawn resonance structures for the acetate anion (Section 2.5C) and the aUyl radical (Section 15.10). The conjugated allyl carbocation is another example of a species for which two resonance structures can be drawn. Drawing resonance structures for the allyl carbocation is a way to use Lewis stmctures to illustrate how conjugation delocalizes electrons. 

X, Y, and Z may all be carbon atoms, as in the case of an allylic carbocation (resonance structures A and B), or they may be heteroatoms, as in the case of the acetate anion (resonance structures C and D). The atom Z bonded to the multiple bond can be charged (a net positive or negative charge) or neutral (having zero, one, or two nonbonded electrons). The two resonance structures differ in the location of the double bond, and either the charge, the radical, or the lone pair, generalized by [ ]. 

Problem 16.8 For acetic acid (CH3CO2H) (a) Draw three resonance structures (b) draw a structure for the... 

Like acetate, phenoxide (CgHjO , the conjugate base of phenol) is also resonance stabilized. In the case of phenoxide, however, there are five resonance structures that disperse the negative charge over a total of four different atoms (three different carbons and the oxygen). 

Acetate is the most stable conjugate base because it has two equivalent resonance structures, both of which place a negative charge on an O atom. 

We said in Section 2.11 that acetic acid can be protonaled by H2SO4 either on its double-bond oxygen or on its singJe-bond oxygen. Draw resonance structures of the possible products to explain why the product of protonation on the double-bond oxygen is more stable. 

Use Spartan View to examine electrostatic potential maps of ethyl acetate and IT-methj l-2-pyrrolidinone. Identify the most basic atoms in each, and draw resonance structures of the protonaied forms. 

The existence of more than one resonance structure stabilizes the acetate ion and contributes to the greater acidity of acetic acid. 

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