Acid-base reactions carboxylic acids

Acid/base reactions Carboxylic acids, phenols, amines... 

Another important elimination reaction involves an eclipsed rotamer. It proceeds by what is effectively an intramolecular acid-base reaction. Carboxylic acids with a carbonyl at C3 (the carboxyl carbonyl is Cl) lose CO2 to form an enol... 

The first item in Table 9.6, the reaction between propanoic add (CH3CH2CO2H) and ammonium hydroxide, corresponds to an add-base reaction. Carboxylic acids are, as noted from the pKa s in Table 9.11, acidic and while, in the gas phase, the low molecular weight carboxylic acids contain significant quantities of dimer (Figure 9.16), it is likely that solvated monomer is the major spedes (at least in solvents capable of hydrogen bonding, e.g., H2O). 

The acid-base reactions that occur after the amide bond is broken make the overall hydrolysis ineversible in both cases. The amine product is protonated in acid the carboxylic acid is deprotonated in base. 

The most common method of resolution uses an acid-base reaction between a racemic mixture of chiral carboxylic acids (RC02H) and an amine base (RNH2) to yield an ammonium salt. 

It is worth mentioning that both the carboxylation of epoxides and anilines are acid-base reactions, which do not entail redox processes. Therefore a catalyst active in these reactions must provide acid-base functionality. In this perspective, positively charged gold could be the real player, although a co-catalytic or promotion effect of ze-rovalent gold could also be important. Therefore the catalysts for the oxidative carbonylation of aniline, supported on Merck Ion-exchanger IV, could be actually bifunctional. On one side, Au could catalyze the oxidation of CO with O2 to CO2, a reaction for which it is... 

In contrast to redox reactions, only proton transfer takes place in acid-base reactions (see also p.30). When an acid dissociates (1), water serves as a proton acceptor (i. e., as a base). Conversely, water has the function of an acid in the protonation of a carboxylate anion (2). 

The CD fragment 1s synthesized starting with resolved bicyclic acid 129. Sequential catalytic hydrogenation and reduction with sodium borohydride leads to the reduced hydroxy acid 1. The carboxylic acid function is then converted to the methyl ketone by treatment with methyl-lithium and the alcohol is converted to the mesylate. Elimination of the latter group with base leads to the conjugated olefin 133. Catalytic reduction followed by equilibration of the ketone in base leads to the saturated methyl ketone 134. Treatment of that intermediate with peracid leads to scission of the ketone by Bayer Villiger reaction to afford acetate 135. The t-butyl protecting... 

Positive-Tone Photoresists based on Dissolution Inhibition by Diazonaphthoquinones. The intrinsic limitations of bis-azide—cyclized rubber resist systems led the semiconductor industry to shift to a class of imaging materials based on diazonaphthoquinone (DNQ) photosensitizers. Both the chemistry and the imaging mechanism of these resists (Fig. 10) differ in fundamental ways from those described thus far (23). The DNQ acts as a dissolution inhibitor for the matrix resin, a low molecular weight condensation product of formaldehyde and cresol isomers known as novolac (24). The phenolic structure renders the novolac polymer weakly acidic, and readily soluble in aqueous alkaline solutions. In admixture with an appropriate DNQ the polymer s dissolution rate is sharply decreased. Photolysis causes the DNQ to undergo a multistep reaction sequence, ultimately forming a base-soluble carboxylic acid which does not inhibit film dissolution. Immersion of a pattemwise-exposed film of the resist in an aqueous solution of hydroxide ion leads to rapid dissolution of the exposed areas and only very slow dissolution of unexposed regions. In contrast with crosslinking resists, the film solubility is controlled by chemical and polarity differences rather than molecular size. 

In engine oil acid-base reactions with proton transfer, logKBH+A. > 103, occurs in systems, e.g., sulfuric acid + carbonate or sulfuric acid + phenate however, the majority of reactions belong to the group without proton transfer, logKB HA < 103, e.g., carboxylic acid + succinimide or phenol + succinimide. 

The adsorption of organic compounds on nascent surfaces can be considered as an acid-base reaction. According to the hard-soft acid and bases HSAB principle (Ho, 1977), polar compounds such as carboxylic acid and amine (with lone pair electrons on oxygen or nitrogen) are classified as "hard bases". A hard base reacts more easily with a hard acid than with a soft acid. Metals are classified as soft acids which react much more easily with soft bases than hard bases. The results in Table 5.1 can be explained with this concept. The soft bases (benzene, 1-hexene, diethyl disulfide) react easily with the nascent surface as a soft acid. On the other hand, the hard bases such as propionic acid, stearic acid, propyl amine and trimethyl phosphate exhibit a very low activity (Fischer et al., 1997a and 1997b Mori and Imazumi, 1988). 

The same group also showed that mono(cyclopentadienyl) mixed hydride/ aryloxide dimer complexes of several lanthanide elements (Y, Dy, Lu) could be synthesized easily by the acid-base reaction between the mixed hydride/alkyl complexes and an aryl alcohol [144]. These complexes reacted with C02 to generate mixed formate/carboxylate derivatives, which were moderately active initiators for the copolymerization of C02 and cyclohexene oxide, without requiring a co-catalyst. The lutetium derivative 21 was the most active (at 110°C, TOF = 9.4 h ), yet despite a good selectivity (99% carbonate linkages), the molecular weight distribution remained broad (6.15) (Table 6). 

These acid-base reactions allow a simple way distinguishing between most carboxylic acids, phenols, and alcohols. Since the salts formed from the acid-base reaction are water soluble, compounds containing these functional groups can be distinguished by testing their solubilities in sodium hydrogen carbonate and sodium hydroxide solutions. This solubility test is not valid for low molecular weight structures like methanol or ethanol since these are water soluble and dissolve in basic solution because of their water solubility rather than their ability to form salts. 

Secondly, the acidic proton is no longer present and so an acid-base reaction is prevented. Thirdly, the original OH group is converted into a good leaving group and is easily displaced once the chloride ion makes its attack. The reaction of a carboxylic acid with thionyl chloride follows the general mechanism shown below ... 

Fig. Acid-base reaction of a Grignard reagent with a carboxylic acid.

Base-mediated ester hydrolyses have a high driving force. This is because of the acid/base reaction between the carboxylic acid formed in the reaction, and the base used as the reagent. The resonance stabilization of the carboxylate is approximately 30 kcal/mol, which means a gain of about 16 kcal/mol compared to the starting material, the carboxylic ester (resonance stabilization 14 kcal/mol according to Table 6.1). Accordingly, the hydrolysis equilibrium lies completely on the side of the carboxylate. 

An amino acid protected only at the N atom and a different amino acid in which only the C02H group is protected do not react with each other to form a peptide bond. On the contrary, they form an ammonium carboxylate in a fast acid/base reaction (Figure 6.29). Ammonium car-boxylates can in principle be converted into amides by strong heating. Thus, for example, in the industrial synthesis of nylon-6,6, the diammonium dicarboxylate obtained from glutamic acid and hexamethylenediamine is converted to the polyamide at 300 °C. However, this method is not suitable for peptide synthesis because there would be too many undesired side reactions. 

On excitation the phenolic group becomes more strongly acidic while the carboxylic acid group becomes a stronger base and proton exchange then occurs between the two. In confirmation of this explanation, methyl salicylate was found to behave similarly, but methyl 2-methoxybenzoate, with no transferable proton, showed a normal Stokes shift. Quenching experiments demonstrated that at room temperature the proton transfer reaction reached equilibrium within the lifetime of the excited state. 

Tire nitrogen atom in the pyridine ring is planar and trigonal with the lone pair in the plane of the ring. This makes it an imine. Most of the imines you have met before (in Chapter 14, for example), have been unstable intermediates in carbonyl group reactions, but in pyridine we have a stable imine—stable because of its aromaticity. All imines are more weakly basic than saturated amines and pyridine is a weak base with a pK of 5.5. This means that the pyridinium ion as about as strong an acid as a carboxylic acid. 

In the following we shall focus on heterogeneous acid-base reactions, which were among the first processes to be studied. The Italian scientist Pellizzari studied the reaction of dry ammonia vapour with dried and pulverized phenols and carboxylic acids [79]. This subject was taken up and extended by Paul, Curtin and... 

Positive Photoresists. Positive resists are entirely different from negative resists. For the purposes of this discussion we restrict ourselves to visible-light-sensitive materials. Typically, these materials are mixtures of low-molecular-weight phenol-formaldehyde polymers and derivatives of naphtho-1,2-quinone diazide, the photosensitive component. The former is soluble in aqueous alkali, but the presence of the latter, a hydrophobic species, inhibits attack of this developer on the film. On irradiation the "sensitizer" is converted to a ketene, which, after reaction with water, forms a base-soluble carboxylic acid. Thus the irradiated part of the film is rendered soluble in the developer and it can be removed selectively. The important feature of this system is that the unirradiated areas are not swollen by the developer and the resolution of this material is quite high. It is possible to prepare gratings having several... 

Simulation Results. A onc-dimensional simulation model based on the Nernst-Planck and Poisson equations [14, in which all the acid-base reactions occurring in the membrane are taken into account, has been used to give a qualitative description of the pH step titration process. In these simulations, a pH step is applied outside a 2 mm thick stagnant layer, which is assumed to be present in front of an 8 mm thick membrane. Diffusion coefficients in the membrane are assumed to be 4/10 of those in water (this value is based on experience with ion step experiments). Lysozyme, used as a model protein, is assumed to contain 11 carboxylic groups (pKa = 4.4), 2 imidazole groups (pKa = 6.0), and 9 amino groups (pKa = 10.4) per molecule. Concern... 

General Acid-Base Reaction of Carboxylic Acids (19.9)... 

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