Carboxylate anions basic solution

The enthalpy and entropy of complex formation between Zn11 and picolinate and dipicolinate anions in aqueous solution have been determined by calorimetry and from formation constant data. The greater stability of the dipicolinate complex compared to the picolinate complex reflects an entropy effect, and Ais actually less favourable. These anions are well known to have a low basicity to H+ compared to their complexing ability to metals. In the present case, this probably reflects the coplanarity of the carboxylate anions and the pyridine ring, so that the oxygen atoms are in a favourable position to coordinate.800... 

It is more common to hydrolyze esters under basic conditions because the equilibrium is favorable. The mechanism for this process, called saponification, is presented in Figure 19.4. The production of the conjugate base of the carboxylic acid, the car-boxylate anion, which is at the bottom of the reactivity scale, drives the equilibrium in the desired direction. To isolate the carboxylic acid the solution must be acidified after the hydrolysis is complete. Some examples are provided in the following equations. We saw another example of this hydrolysis reaction in Chapter 10, where it was... 

Under acidic conditions the equilibrium for the hydrolysis of an amide is driven toward the products by the protonation of the ammonia or amine that is formed. Under basic conditions the equilibrium is driven toward the products by the formation of the carboxylate anion, which is at the bottom of the reactivity scale. The pH of the final solution may need to be adjusted, depending on which product is to be isolated. If the carboxylic acid is desired, the final solution must be acidic, whereas isolation of the amine requires that the solution be basic. Several examples are shown in the following equations. Also, note that the last step of the Gabriel amine synthesis, the hydrolysis of the phthalimide (see Section 10.6 and Figure 10.5 on page 365), is an amide... 

The dipolar ion has both an acidic group, the ammonium cation, and a basic group, the carboxylate anion, so it can act as either an acid or a base. Such compounds are termed amphoteric. The species that is present in aqueous solution depends on the pH. In the pH range near neutral the amino acid is present in the form of the dipolar ion. In acidic solution the carboxylate group becomes protonated and the amino acid is present as a cation, whereas in basic solution the ammonium group gives up a proton and the molecule exists as an anion ... 

Carboxylates as the phenylacetate anion also eject electrons in methanol [75] giving benzyl anion after recombination between solvated electron and benzyl radical [76]. In phenyl substituted carboxylate anions (from benzoate to phenylbutyrate) in water the quantum yield of photoejected electron was found between 0.002 and 0.03 these values increase with increasing excitation energy and with the number of CH2 separating the phenyl and carboxylate group [77], In the case of phenylalanine and tryptophan in water, the mechanism seems to differ according to the conditions biphotonic and from a triplet state in neutral solution or monophotonic in basic medium [78, 79, 80]. In certain cases, the quantum yield for electron ejection is found to increase with pH [79], The anion of bromouracil also gives hydrated electron [81]. 

There has been intensive study of 1-hydropyridinyl radicals bearing one or more carboxylic acid substituents.72,195,198-202 In basic solution these radicals are anionic, but the charge is associated with the ionization of the acid functions and not with the radical moiety. Equilibrium and kinetic data have been reported for the acid ionizations 198-202 and the variation of persistence with structure discussed.195... 

In basic solution the nucleophile is the excellent methoxide ion, which attacks the anhydride to give a tetrahedral intermediate having the carboxylate anion as its best leaving group. This carboxylate aamot be protonated and is not attacked by methoxide anion. 

The first example is an esterification (or lactonization as the product is a cyclic ester or lactone cr the forwards direction and an ester hydrolysis in the backwards direction. Ester hydrolysis a catalysed by acid or base but esterification by acid only. In addition, even a weak base will be enough to turn the starting material into the carboxylate anion, which will not cyclize. The equilibrium is to the right in acid solution and to the left in basic solution. 

In acidic solutions, organic amines protonate to form cations with hydrophobic tails. These ions will seek out and blanket cathodic surfaces, much as carboxylate anions seek out and cover anodic spots in neutral or basic media. The usual choices include amylamine (C5H11NH2), cyclohexyl-amine (C6H11NH2), pyridine (C5H5N), and morpholine [0(CH2CH2)2NH]. Metallic iron behaves as a soft acid in terms of the HSAB classification (Section 2.9), despite the indubitable hard behavior of its trivaJent ion, and so molecules with soft donor atoms adsorb more strongly than hard bases (S > N > 0). This principle can be applied to the design of inhibitors. 

In basic solution, the product is the carboxylic acid anion ... 

Aldehydes and ketones can be distinguished on the basis of differences in their reactivity. The most common laboratory test for aldehydes is the Tollens test. When exposed to the Tollens reagent, a basic solution of Ag(NH3)2, an aldehyde undergoes oxidation. The silver ion (Ag+) is reduced to silver metal (Ag ) as the aldehyde is oxidized to a carboxylic acid anion. 

The aldehyde group of aldoses is readily oxidized by the Benedict s reagent. Recall that the Benedict s reagent is a basic buffer solution that contains Cu " ions. The Cu ions are reduced to Cu+ ions, which, in basic solution, precipitate as brick-red CU2O. The aldehyde group of the aldose is oxidized to a carboxylic acid, which undergoes an acid-base reaction to produce a carboxylate anion. 

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