Acid anhydrides, ionization

DNA sequencing and. 1113 Electrospray ionization (ESI) mass spectrometry, 417-418 Electrostatic potential map, 37 acetaldehyde, 688 acetamide, 791,922 acetate ion. 43. 53, 56, 757 acetic acid. 53. 55 acetic acid dimer, 755 acetic anhydride, 791 acetone, 55, 56. 78 acetone anion, 56 acetyl azide, 830 acetyl chloride, 791 acetylene. 262 acetylide anion, 271 acid anhydride, 791 acid chloride, 791 acyl cation, 558 adenine, 1104 alanine, 1017 alanine zwitterion, 1017 alcohol. 75 alkene, 74, 147 alkyl halide, 75 alkyne. 74... 

SE.3.1.2. Desymmetrization of gem-Dwarboxylates An equivalent of asymmetric carbonyl addition can be achieved by the alkylation of gem-dicarboxylates (Scheme 8E.17). The alkylation of gem-dicarboxylates, which are easily prepared by the Lewis acid-catalyzed addition of acid anhydrides to an aldehyde, converts the problem of differentiating the two enantiotopic 7t-faces of a carbonyl group into that of asymmetric substitution of either enantiotopic C-O bond of the gem-dicarboxylate. Although asymmetric induction may be derived from enantio-discrimination in the ionization step or in the alkene coordination step, the fast and reversible nature of alkene coordination suggests that the ionization step is more likely to be the source of enantio-discrimination. 

No dimerization of acetic anhydride has been observed in either die liquid or solid state. Decomposition, accelerated by heat and catalysts such as mineral acids, leads slowly to acetic acid (2). Acetic anhydride is soluble in many common solvents, including cold water. As much as 10.7 wt % of anhydride will dissolve in water. The unbuffered hydrolysis rate constant k at 20°C is 0.107 min 1 and at 40°C is 0.248 min-1. The corresponding activation energy is about 31.8 kj/inol (7.6 kcal/mol) (3). Aldiougli aqueous solutions are initially neutral to litmus, they show acid properties once hydrolysis appreciably progresses. Acetic anhydride ionizes to acetylium, CH CO+, and acetate, CH - CO, ions in the presence of salts or acids (4). Acetate ions promote anhydride hydrolysis. A summary of acetic anhydride s physical properties is given in Table 1. 

The electrophile is the acylium ion, R-C +, generated by Lewis acid-catalyzed ionization of a leaving group (path Dn) from acyl halides or acid anhydrides (shown in the previous section). The proton that is lost comes from the same carbon that the electrophile attacked. The reaction fails for deactivated rings (Ai wg, meta directors). After the electrophile adds it deactivates the ring toward further attack. No rearrangement of the electrophile occurs. 

Acetic anhydride is a neutral molecule and therefore does not form ions. However, as the reaction progresses, ionic species are produced tty the hydrolysis of the anhydride to form acetic acid, an ionizable molecule. As a result, the electrical conductivity of the solution will change with the concentration of the acetic acid formed (i.e. with the conversion of the anhydride), but not directly. The reason for this complication is that the acetic acid produced does not dissociate completely but forms an equilibrium with its ions ... 

In connection with the method for preparing the esters from reaction of the perfluoroalkanesulfonic anhydrides with the parent or other acids, it seems reasonable to assume that the mechanism is quite similar to that of equation 80. P2Os or the acid promotes ionization of the acid anhydride to give the unstable cation RfS02 + and then Rf + reacts with the sulfonate anion to produce the desired ester104 (equation 81). The previously postulated ionic bimolecular mechanism for the formation of the esters... 

The process of substitution undertaken on carboxylic acids and the derivatives of carboxylic acids (anhydrides, acid halides, esters, amides, and nitriles) generally involves a series of replacement processes. Thus, individually, substitution may involve replacement of (a) the proton attached to oxygen of the -OH group (i.e., ionization of the acid) (b) the hydroxyl (-OH) portion of the carboxylic acid (or derivative) (e.g., esterification) (c) the carbonyl oxygen and the hydroxyl (-OH) (e.g., orthoester formation, vide infra) (d) the entire carboxylic acid functionality (e.g., the Hunsdiecker reaction, already discussed Scheme 9.101) and the decarboxylation of orotic acid (as orotidine monophosphate) to uracil (as uridine monophosphate)—catalyzed by the enzyme orotidine monophosphate decarboxylase (Scheme 9.115) or (e) the protons (if any) on the carbon to which the carboxylic acid functional group is attached (e.g., the Dieckman cycUzation, already discussed earlier, c Equation 9.91). Indeed, processes already discussed (i.e., reduction and oxidation) have also accomplished some of these ends. Some additional substitutions for the carboxylic acid group itself are presented in Table 9.6, while other substitutions for derivatives of carboxylic acids are shown in Tables 9.7-9.10 and discussed subsequently. 

The specific rates of solvolysis of the acid anhydride, acetyl p-toluenesulfonate (30) were measured by a rapid-response conductivity technique at temperatures in the range 263-218 K. For 13 solvents at 233.4 K, an extended Grunwald-Winstein equation correlation led to sensitivities to changes in solvent nucleophilicity of 0.65 and to changes in solvent ionizing power of 0.61. In 89.1 % acetone at 253 K, the comparison with acetyl bromide solvolysis led to a koj /k ratio of 1.4. In methanol and methanol-d at 233.4 K, the solvent deuterium isotope effect A MeOH MeOD was 0.99. These results are consistent with an 5 1 reaction with appreciable nucleophilic solvation or an 5 2 reaction with a loose TS. ... 

The performance of many metal-ion catalysts can be enhanced by doping with cesium compounds. This is a result both of the low ionization potential of cesium and its abiUty to stabilize high oxidation states of transition-metal oxo anions (50). Catalyst doping is one of the principal commercial uses of cesium. Cesium is a more powerflil oxidant than potassium, which it can replace. The amount of replacement is often a matter of economic benefit. Cesium-doped catalysts are used for the production of styrene monomer from ethyl benzene at metal oxide contacts or from toluene and methanol as Cs-exchanged zeofltes ethylene oxide ammonoxidation, acrolein (methacrolein) acryflc acid (methacrylic acid) methyl methacrylate monomer methanol phthahc anhydride anthraquinone various olefins chlorinations in low pressure ammonia synthesis and in the conversion of SO2 to SO in sulfuric acid production. 

Maleamic acid, cyclization of, 293 Maleic anhydride, 59 Maleimido azine, 307 Manganese diacetate catalysts, 71 Mark-Houwink-Sakurada equation, 57 Material safety data sheets (MSDSs), 246 Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS), 385, 388 McGrath, J. E., 327 MDI isomers, 210 MDIs. See Methylene diphenyl diisocyanates (MDIs)... 

The anhydride of sulfurous acid is sulfur dioxide, which is very soluble in water. Although most of the gas is physically dissolved, the ionization takes place slightly as indicated by the equations... 

The evidence for this self-ionization is the conductivity of the anhydride which, though low, exceeds that of acetic acid.204 208 The ionization of acetic anhydride into acetylium and acetate ions is analogous to the ionization of water molecules into protons and... 

---