Trichloroacetic acid, ionization

It is always important to keep in mind the relative nature of substituent effects. Thus, the effect of the chlorine atoms in the case of trichloroacetic acid is primarily to stabilize the dissociated anion. The acid is more highly dissociated than in the unsubstituted case because there is a more favorable energy difference between the parent acid and the anion. It is the energy differences, not the absolute energies, that determine the equilibrium constant for ionization. As we will discuss more fully in Chapter 4, there are other mechanisms by which substituents affect the energy of reactants and products. The detailed understanding of substituent effects will require that we separate polar effects fiom these other factors. 

Carboxylic acids vary considerably in strength. Among the strongest is trichloroacetic acid (Ka = 0.20) a 0.10 M solution of C13C—COOH is about 73% ionized. Trichloroacetic acid is an ingredient of over-the-counter preparations used to treat canker sores and remove warts. 

Most of the chemistry of PA is determined by its acidic nature. It is a strong acid whose ionization constant of 1.6 x 10"1 (Ref 31) makes it comparable in acid strength to pyrophosphoric acid and trichloroacetic acid. PA readily forms salts with bases and esters with alcohols. The salts are known as Picrates. Many of them are expl and will be described in a separate article in this Vol. The esters are phenol ethers, eg, Trinitro-anisolc (see Vol t, A450-L)... 

PDMS = polydimethylsiloxane. PA = polyacrylate. CW = Carbowax. DVB = divinylbenzene. FID = flame ionization detection. NPD = nitrogen-phosphorus detection. TSD = thermionic-specific detection. LOQ = limit of quantitation. LOD = limit of detection. TCA = trichloroacetic acid. PICI-MS = positive ion chemical mass spectrometry. SIM = selected ion monitoring. 

Trichloroacetic acid decomposes in water, alcohol and aniline and other basic solvents but it does not decompose in non-basic solvents such as benzene, carbon tetrachloride, sulfuric and acetic acid. Furthermore, the ethyl ester of trichloroacetic acid dissolves in alcohol but it does not decompose. It is known that this ester does not ionize in alcohol. Trichloroacetic acid, the sodium salt, the barium salt, and the anilinium salt all decompose in water at the same rate and all give beautiful first order constants throughout the whole course of the reaction. It is known that all these salts... 

B) Ionization Constants. Determine the pH of 0.1 solutions of acetic, chloroacetic, and trichloroacetic acids. Convert the pH to hydrogen-ion concentration as moles per liter then calculate the ionization constants for acetic and monochloroacetic acid. 

Calculate the analytical and equilibrium molar concentrations of the solute species in an aqueous solution that contains 285 mg of trichloroacetic acid, CI3CCOOH (163.4 g/mol), in 10.0 mL (the acid is 73% ionized in water). 

Derivatization is also useful to detect volatile metabolites. Liu et al. [282] described a specific, rapid, and sensitive in situ derivatization solid-phase microextraction (SPME) method for determination of volatile trichloroethylene (TCE) metabolites, trichloroacetic acid (TCA), dichloroacetic acid (DCA), and trichloroethanol (TCOH), in rat blood. The metabolites were derivatized to their ethyl esters with acidic ethanol, extracted by SPME and then analyzed by gas chromatography/negative chemical ionization mass spectrometry (GC-NCI-MS). After validation, the method was successfully applied to investigate the toxicokinetic behavior of TCE metabolites following an oral dose of TCE. Some of the common derivatization reagents include acetyl chloride and TV-methyl-iV- ft-b u (y Idi methyl si I y I) tro (1 uoroacctam i nc (MTBSTFA) for phenols and aliphatic alcohols and amines, dansyl chloride and diazomethane for phenols, dansyl chloride for amines, acidic ethanol and diazomethane for carboxylic acids, and hydrazine for aldehydes. 

The data in Table 10.4 show that even among carboxylic acids (where the ionizing functional group is kept constant), acidities can vary depending on what other groups are attached to the molecule. Compare, for example, the of acetic acid with those of mono-, di-, and trichloroacetic acids, and note that the acidity varies by a factor of 10,000. 

In this experiment, we will compare the acidities of acetic, chloroacetic, and trichloroacetic acid. This experiment could be approached in the same fashion as the relative rates in Experiment 20B, using the ionization energies to determine the relative acidities. 

Chlorine-36 is a convenient radiotracer. It is a weak beta CQ emitter, with ty2 = 3 X 10 yr. Describe how you would use this radiotracer to carry out each of the following e>q3eriments. (a) Determine whether trichloroacetic acid, CCI3COOH, undergoes any ionization of its chlorines as chloride ion in aqueous solution, (b) Demonstrate that... 

What is the (a) degree of ionization and (b) percent ionization of trichloroacetic acid in a 0.035 M CCI3COOH solution ... 

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