Acetic anhydride, ionization

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. 

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... 

The behavior of acetic anhydride as a solvent system indicates an ionization 201 208... 

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... 

It is generally accepted that nitric acid-acetic anhydride mixtures contain acetyl nitrate (Equation 4.4). Acetyl nitrate is a weak nitrating agent but in the presence of a strong acid, like nitric acid, it can ionize to nitronium and acetate ions. Protonated acetyl nitrate could also be an active nitrating agent under these conditions. 

For the nitration of aromatic hydrocarbons with acetylnitrate, there is a clearly linear correlation between the ionization potentials of these hydrocarbons and the rate constants relative to benzene (Pedersen and others 1973). Table 4-4 juxtaposes spin densities of the cation radicals and the partial rate factors of ring attacks in the case of the nitration of isomeric xylenes by means of the (nitric acid-acetic anhydride) mixture. 

The results for nitration by solutions of nitric acid in acetic anhydride were omitted from the above discussion since this reaction has several special features. Unlike the other solvents, acetic anhydride reacts with nitric acid with the result that the stoichiometric nitric acid is actually present as acetyl nitrate as shown in (30). However, the marked retardation of the reaction by trace amounts of ionized nitrates shows that the actual electrophile is a positive ion present in very low concentration (Paul, 1958 Ridd, 1966). 

V-101 Flame ionization Simultaneous analysis of salicylamide, phenylpropanolamine HC1, caffeine, chlorpheniramine maleate phenylephrine HC1 and pyrilamine maleate, the sample should be treated with l+-(dimethylamino)-pyridine in pyridine-acetic anhydride 1 1. (62)... 

The process for the production of nitrobenzene from benzene involves the use of mixed acid (Fig. 1), but there are other useful nitrating agents, e.g., inorganic nitrates, oxides of nitrogen, nitric acid plus acetic anhydride, and nitric acid plus phosphoric acid. In fact, the presence of sulfuric acid in quantity is vital to the success of the nitration because it increases the solubility of the hydrocarbon in the reaction mix, thus speeding up the reaction, and promotes the ionization of the nitric acid to give the nitronium ion (N02+), which is the nitrating species. Absorption of water by sulfuric acid favors the nitration reaction and shifts the reaction equilibrium to the product. 

In the reaction with acetic anhydride in an inert solvent, pyrrole gives a mixture of 1- and 2-acetyl derivatives.139 The substitution at the 2-carbon seems to involve the neutral molecule of pyrrole, whereas that at nitrogen probably involves the dissociated anion. In fact, the C/N isomer ratio is decreased by adding sodium acetate (which favors ionization) and increased by adding acetic acid (which opposes it). 

Using acetic anhydride as an example, Feldmann et al. examined the feasibility and properties of a new method for studying IRMPE/D dynamics which involves single-photon ionization to monitor the time evolution of parent molecules and neutral intermediates. They discussed in some detail alternative detection schemes, such as mass spectrometry with ions formed by electron impact or MPI/REMPI, but decided upon single-photon ionization because of its wide applicability, its sensitivity, and its selectivity. VUV ionization ensures that the perturbation of the system is minimal as well as the extent of ion fragmentation. They used frequency-tripled 3rd harmonic (355 nm) radiation from a Nd-YAG laser as a source of 118.4 nm light. 

The occurrence of the ionization mechanism (eqns. (6) and (18)) in preference to the alternative addition mechanism (eqn. (24)) has been demonstrated by a study of the initiating activities of the series of bases pyridine, a-picoline, 2,6-lutidine [21]. Mechanism (24) is analogous to the normal tertiary base-catalysed acylation mechanism encountered with anhydrides such as acetic anhydride [41]. Steric factors determine the... 

Lindberg et al. [7] performed a derivatization of budesonide with acetic anhydride to the 21-acetyl derivative in order to enhance its ionization efficiency in TSP ionization. The same procedure in combination with positive-ion APCI was used by others for budesonide [27], and the catechol estrogens 2- and 4-hydroxy-estrone and 2- and 4-hydroxy estradiol in rat brain [28]. 

It is a consequence of this concept of displacement at an allylic position that conditions which favor an Sy2 attack should tend to decrease the amount of the other isomer formed. Correspondingly, conditions which would be expected to produce a preliminary dissociation should favor rearrangement. In the extreme case, when the reaction proceeds exclusively by ionization into a carbonium ion, the same proportion of products should be obtained regardless of whether compounds of type I or II were employed as starting materials. These consequences have been very nicely verified for the crotyl-methylvinylcarbinyl system which has already been mentioned. In acetic anhydride or acetone (solvents... 

---