Acetic acid resonance structures

Support for this suggestion comes from many quarters. Reduction of the jS-carboline anhydro-bases with sodium and alcohol or with tin and hydrochloric acid gives the 1,2,3,4-tetrahydro derivatives, as does catalytic reduction over platinum oxide in an alkaline medium. On the other hand, catalytic reduction with platinum oxide in acetic acid results in the formation of the 5,6,7,8-tetrahydro-j3-carbolinium derivatives (see Section III,A,2,a). It should be noted, however, that reduction of pyrido[l,2-6]indazole, in which the dipolar structure 211 is the main contributor to the resonance hybrid, could not be effected with hydrogen in the presence of Adams catalyst. 

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. 

The most frequently encountered hydrolysis reaction in drug instability is that of the ester, but curtain esters can be stable for many years when properly formulated. Substituents can have a dramatic effect on reaction rates. For example, the tert-butyl ester of acetic acid is about 120 times more stable than the methyl ester, which, in turn, is approximately 60 times more stable than the vinyl analog [16]. Structure-reactivity relationships are dealt with in the discipline of physical organic chemistry. Substituent groups may exert electronic (inductive and resonance), steric, and/or hydrogen-bonding effects that can drastically affect the stability of compounds. A detailed treatment of substituent effects can be found in a review by Hansch et al. [17] and in the classical reference text by Hammett [18]. 

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.

The use of surface-enhanced resonance Raman spectroscopy (SERRS) as an identification tool in TLC and HPLC has been investigated in detail. The chemical structures and common names of anionic dyes employed as model compounds are depicted in Fig. 3.88. RP-HPLC separations were performed in an ODS column (100 X 3 mm i.d. particla size 5 pm). The flow rate was 0.7 ml/min and dyes were detected at 500 nm. A heated nitrogen flow (200°C, 3 bar) was employed for spraying the effluent and for evaporating the solvent. Silica and alumina TLC plates were applied as deposition substrates they were moved at a speed of 2 mm/min. Solvents A and B were ammonium acetate-acetic acid buffer (pH = 4.7) containing 25 mM tributylammonium nitrate (TBAN03) and methanol, respectively. The baseline separation of anionic dyes is illustrated in Fig. 3.89. It was established that the limits of identification of the deposited dyes were 10 - 20 ng corresponding to the injected concentrations of 5 - 10 /ig/ml. It was further stated that the combined HPLC-(TLC)-SERRS technique makes possible the safe identification of anionic dyes [150],... 

PMR studies have been performed on a number of other ribosomal proteins isolated by the acetic acid/urea method (Morrison etal., 1977a). The results of these studies have shown that acedc acid/urea-extracted proteins contain little tertiary structure. However, some structure was seen in protein S4 and especially in protein S16 as indicated by the appearance of ring-current shifted resonances in the apolar region of the spectrum (Morrison et al., 1977b). These are due to the interaction of apolar methyl groups with aromatic amino acids in the tertiary structure of the protein. The PMR spectra were recorded either in water or in dilute phosphate buffer at pH 7.0—conditions under which the proteins were soluble. 

Ammonium acetate in acetic acid converts 55a into the imine (59). The hydrogen-bonded, resonance-stabilized form shown is consistent with its high melting point and intense color. The structure is further supported by the ability of the naphthalene analog, which is more soluble, to form a stable complex with cupric perchlorate. ... 

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 ... 

It was noted that chromatography of Cephalotaxus alkaloid fractions over neutral alumina resulted in considerable losses (111). Further elution with dilute aqueous acetic acid resulted in the isolation of a new alkaloid, desmethylcephalotaxinone ([a]D +2.3°). The IR spectrum of this alkaloid was consistent with the presence of a vinylic hydroxyl group (3520 cm-1) and a conjugated carbonyl group (1690 cm 1). The NMR spectrum obtained in deuterochloroform contained features of the cephalotaxine structure, but included a singlet attributable to an isolated methylene (<5 2.54 ppm). In DMSO-c/6 this resonance appeared as an AB quartet. Acetylation... 

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 ... 

As noted in Figure 3.16, acetic acid has only a small amount of resonance stabilization because the lower structure is only a minor contributor to the resonance hybrid. Acetate ion has a large amount of resonance stabilization because it has two equivalent contributors to the hybrid. 

The acetolysis of S-acetyl-l,2-0-isopropylidene-3,5-di-0-methyl-6-thio-a-D-glucofiiranose with acetic acid—acetic anhydride—sulfuric acid gave a small yield of the crystalline septanose triacetate (271). The septanose structure of 271 was established by the absence of the thiol band in its infrared spectrum, and by the signals for three O-acetyl and two O-methyl groups found in its nuclear magnetic resonance spectrum. 

A third factor that determines acidity is resonance. Recall from Section 1.5 that resonance occurs whenever two or more different Lewis structures can be drawn for the same arrangement of atoms. To illustrate this phenomenon, compare ethanol (CH3CH2OH) and acetic acid (CH3COOH), two different compounds containing O-H bonds. Based on Table 2.1, CH3COOH is a stronger acid than CH3CH2OH ... 

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

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. 

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