Acetic acid ionizable hydrogen atoms

Predict Salicylic acid, shown in Figure 18.32, is used to manufacture acetylsalicylic acid, commonly known as aspirin. Evaluate the hydrogen atoms in the salicylic acid molecule based on your knowledge about the ionizable hydrogen in the acetic acid molecule, CH3COOH. Predict which of salicylic acid s hydrogen atoms is likely to be ionizable. 

In organic compoxmds, acidity is quite often associated with the presence of a carboxylic acid group (CO2H) and it is relatively easy to determine the number of ionizable hydrogen atoms in the system. Acetic acid, 7.1, is a monobasic acid since it can donate only one proton. Ethanedioic acid (oxalic acid), 7.2, can donate two protons and so is a dibasic acid. The tetrabasic acid, 7.3, and the anions derived from it are commonly encountered in coordination chemistry the trivial name for this acid is A, MA, A -ethylenediamine-tetraacetic acid (see Table 7.7) and is generally abbreviated to H4EDTA. 

Acids contain ionizable hydrogen atoms, which are generally identified by the way in which the formula of an acid is written. Ionizable H atoms are separated from other H atoms in the formula either by writing them first in the molecular formula or by indicating where they are found in the molecule. Thus, there are two ways that we can show that one H atom in the acetic acid molecule is ionizable and the other three H atoms are not. 

Contrast the following two formulas for acetic acid HC2H302 and CH,COOH. Explain the advantages of each. Which hydrogen atom is lost upon ionization of acetic acid in water ... 

It must be pointed out, however, that physical shielding takes place only when the nitrogen atom is not in an ionized form. Hydrogenations of pyridine and 2-substituted pyridines when carried out in acid solution or as salts show that pyridine is reduced first in comparison with other pyridines. For example, under the conditions used by Skita (9) pyridine was reduced more readily than 2-picoline. The same order of reducibility was observed in hydrogenations in acetic acid in the presence of Adams catalyst (55). In another comparison of the reduction of mixed hydrochloride salts (39) it was found that pyridine reduced first. This is further amplified in the reduction of equimolecular binary mixtures of hydrochloride salts in the presence of platinum oxide (40). 

From the chemical formulas of HCl and HF, you know that each acid has one hydrogen ion per molecule. An acid that can donate only one hydrogen ion is called a monoprotic acid. Other monoprotic acids are perchloric acid (HCIO4), nitric acid (HNO3), hydrobromic acid (HBr), and acetic acid (CH3COOH). Because acetic acid is a monoprotic acid, its formula is often written HC2H3O2 to emphasize the fact that only one of the four hydrogen atoms in the molecule is ionizable. 

Solution (a) The acid ionization constant for pyruvic acid should be somewhat greater than that of acetic acid because the carbonyl function on the o-carbon atom exerts an electron-withdrawing effect on the carboxylic acid group. In the C—O—H bond system the electrons are shifted from hydrogen, facilitating loss of the hydrogen as a proton. (Section 16.10)... 

Acetic acid melts at 17°C and boils at 118°C. It is soluble in water and alcohol. The molecule contains one hydrogen atom that ionizes from it in water, producing the acetate ion, C2H302. The acid reacts with bases... 

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