Water reaction with weak acids

The hydrolysis of silicate minerals involves chemical reactions with weakly acidic or alkaline aqueous solutions. With natural waters that contain dissolved C02-that is, C02(aq) which can be written as carbonic acid (H2C03)-the hydrolysis of magnesium silicate (olivine) proceeds as... 

The catalytic activity of rhodium diacetate compounds in the decomposition of diazo compounds was discovered by Teyssie in 1973 [12] for a reaction of ethyl diazoacetate with water, alcohols, and weak acids to give the carbene inserted alcohol, ether, or ester product. This was soon followed by cyclopropanation. Rhodium(II) acetates form stable dimeric complexes containing four bridging carboxylates and a rhodium-rhodium bond (Figure 17.8). 

Both inorganic (e.g., metals) and organic substances may be subject to a hydrolysis reaction in waters. Examples of several hydrolyzable functional groups are given in Table 13.2. Water is a weak acid and the acidity of the water molecules in the hydration shell of a metal ion usually is greater than that of the water. The acidity of aqueous metal ions is expected to increase with a decrease in the radius and an increase in the charge of the central ion. In the case of Fe(III), for example. 

Sodium hydroxide is a strong base. The most important reactions are the neutraliztion reactions with acids that form salts and water. Thus with sulfuric, hydrochloric, and nitric acids, the corresponding sodium salts are obtained when solutions are evaporated for crystallization. Neutralization with weak acids forms basic salts. Reactions with organic acids produce their soluble sodium salts. 

Hydrolysis is the general term of the chemical reaction of anions with water. Salts of weak acids and bases hydrolyze in aqueous solutions. 

Both the acidic character and the covalent character of different oxides of the same element increase with increasing oxidation number of the element. Thus, sulfur(VI) oxide (S03) is more acidic than sulfur(IV) oxide (S02). Reaction of S03 with water gives a strong acid (sulfuric acid, H2SC>4), whereas reaction of S02 with water yields a weak acid (sulfurous acid, H2S03). The oxides of chromium exhibit the same trend. Chromium(VI) oxide (Cr03) is acidic, chromium(III) oxide (Cr203) is amphoteric, and chromium(II) oxide (CrO) is basic. 

Section 19.1 discusses the Brpnsted theory of acids and bases, which extends the concepts of add and base beyond aqueous solutions and also explains the acidic or basic nature of solutions of most salts. Dissociation constants, the equilibrium constants for the reactions of weak acids or bases with water, are introduced in Section 19.2. The concept of the ionization of covalent compounds is extended to water itself in Section 19.3, which also covers pH, a scale of acidity and basicity. Section 19.4 describes buffer solutions, which resist change in their acidity or basicity even when some strong acid or base is added. Both the preparation and the action of buffer solutions are explained. Section 19.5 discusses the equilibria of acids containing more than one ionizable hydrogen atom per molecule. 

For reactions of weak acids or weak bases with water, the specialized equilibrium constant is denoted or Ab, respectively. Neither nor explicitly includes the concentration of water in its equilibrium constant expression. If initial concentrations and one equilibrium concentration are given, the equilibrium constant can be calculated. If initial concentrations and the value of the equilibrium constant are given, the equilibrium concentrations can be calculated (Section 19.2). 

Their amalgam studies showed that the rate for very strong acids and some of the weak acids, under conditions in which the H2O reaction with the amalgam can be neglected, is zero-order in metal concentration. The water reaction with Na-Hg is order in metal but independent of acid. 

DOT CLASSIFICATION 6.1 Label Poison SAFETY PROFILE Poison by ingestion, intravenous, and intraperitoneal routes. Moderately toxic by intramuscular route. Mutation data reported. Acute symptoms of exposure are headache, nausea, vomiting, weakness and stupor, cyanosis and methemoglobinemia. Chronic exposure can cause liver damage. Experimental reproductive effects. Combustible when exposed to heat or flame. See NITRATES for explosion and disaster hazards. To fight fire, use water spray or mist, foam, dry chemical, CO2. Vigorous reaction with sulfuric acid above 200°C. Reaction with sodium hydroxide at 130°C under pressure may produce the explosive sodium-4-nitrophenoxide. When heated to decomposition it emits toxic fumes of NOx. See also m-NITROANILINE, o-NITROANILINE, NITRO COMPOUNDS OF AROMATIC HYDROCARBONS, and ANILINE DYES. 

The reactions of arylmagnesium and aryllithium compounds are similar and are analogous to the reactions of the corresponding alkyl species. These reagents are strong bases, particularly the lithium compounds, and react rapidly with weak acids such as water and alcohols to form the arene. It is therefore important when using these reagents that reactants, solvents and apparatus are dry and free of acid. 

Schemes using only weak-acid and weak-base ion exchangers have been considered as well [52,53]. In the first column illustrating this (Fig. 23) water is treated with a weak-base anion exchanger in the bicarbonate form. All anions are replaced by bicarbonate ion. In the second column the bicarbonate solution obtained is treated with weak-acid ion exchanger in the H form. Because carbon dioxide is formed all the reactions of the type...

The dehydration of alcohols was studied over acidic catalysts (for review see refs. [1,2]). This reaction requires weak acid sites and produces alkenes and water. Karge et al. [3] studied the dehydration of cyclohexanol on cationic forms of mordenite, clinoptilolite and Y zeolite and observed only dehydration products. The results of the catalytic test were compared with the IR results concerning zeolite acidity and good correlations were observed. In the presence of strong acid sites alkenes may undergo further reactions. 

The reactions of weak acids and bases with water do not go to completion. So, to calculate the pH of their solutions, we use or K, and the laws of chemical equilibrium. The calculations follow the pattern of Example 14.10 for gas equilibria. In that case the initial gas-phase pressures P° are known, and we calculate the pressures of products resulting from the incomplete reaction. Here we know the initial concentration of acid or base, and calculate the concentrations of products resulting from its partial reaction with water. 

Reactions of weak acids with strong bases also produce salts and water, but there is a significant difference in the balanced ionic equations because weak acids are only slightly ionized. 

We write equilibrium constant expressions for the reactions of weak acids with water, just as we did for regular equilibria, but remember that liquids such as water in dilute solutions are not included in the equilibrium constant expression. The expressions for... 

Alcohols are very weak acids and bases, slightly weaker than water. The extremely weak acidity of alcohol is evidenced by the negligible degree of reaction with a strong base such as NaOH. The equilibrium for the reaction... 

Water and HCI are the acids in a and b, respectively. The reaction of methylamine with water is shown as an equilibrium reaction with the equilibrium only partially to the right-hand side because amines are weak bases and water is a weak acid. The reaction with HCI goes completely to the right-hand side because HCI is a strong acid. 

Strong bases probably can also catalyze rapid formation of D-glucosylamines at ambient temperatures. A classic method for preparation of iV-(D-glucosyl)-amino acids employs conversion of an amino acid into its sodium salt in water or methanol followed by introduction of D-glucose into the alkaline solution at room temperature. The reaction is complete as soon as the solid carbohydrate dissolves. Basic conditions, however, may favor a radier direct attack of an amine at the anomeric carbon, with water or anodier weak acid synchronously providing donation of a proton to 0-5. Such a mechanism may explain the inefficiency of A-aryl D-glucosylamine formation diat is observed under alkaline conditions. 

The oxides and hydroxides of the metals of Group 3 and higher tend to be only weakly basic, and most display an amphoteric nature. Most of these compounds are so slightly soluble in water that their acidic or basic character is only obvious in their reactions with strong acids or bases. 

Dissolution may be aided by hydrolysis, which means the reaction of one of the liberated ions with water forms a weak acid or a weak base to set free H+ or OH . For example, carbonates dissolve to form a bicarbonate ion by the reaction... 

It should be clear that these reactions of weak acids and bases are a special case of the equilibrium that we have been exploring throughout this chapter. When any acid dissolves in water, hydronium ions are always formed as one of the products. This species is so common that an additional way to describe its concentration has been devised. To avoid dealing with small numbers in scientific notation, we often use the pH scale, in which pH is defined as the negative logarithm of the hydronium ion concentration ... 

Water reacts with HCl, and the organic acid known as formic acid (HCOOH, 1) also reacts with water as a weak acid, as shown. Formic acid is a much weaker acid than HCl. When 1 reacts with water, the conjugate base is the formate anion, 2, and the conjugate acid is the hydronium ion. If 1 is a weaker acid than HCl, the equilibrium for 1 + HgO lies to the left in the reaction shown when compared to the reaction of HCl + H2O in Section 2.1. Note the (aq) term indicates solvation by the solvent water. Note also that the term reaction is used for the acid-base equilibrium. The acid-base equilibria shown for HCl and HCOOH are chemical reactions that generate two products the formate anion (HCOO , 2) and the hydronium ion (from 1) or the hydronium ion and the chloride ion (from HCl). 

Because alkynes are weaker bases than alkenes and given the discussion in Section 10.1 concerning the reactivity of alkenes, it is not surprising that alkynes do not react directly with weak acids such as water, alcohols, etc. As with alkenes in Section 10.3, alkynes can be made to react with a weak acid such as water if a strong acid catalyst is added to generate the reactive vinyl carbocation intermediate in situ. Therefore, hydration of alkynes with aqueous acid is a viable reaction. 

Epoxides will not react directly with water or with alcohols because they are weak acids. However, in the presence of a strong acid catalyst, an epoxide will react first to form an oxonium ion, and then with the weak nucleophile to open the three-membered ring. In the case of 126, reaction with the acid catalyst will generate oxonium ion 127. In the presence of the water solvent, the oxonium ion tends to open to give tertiary carbocation 128, and the oxygen atom of water attacks the positive carbon of 128 to give 129. Loss of the proton from 129 gives... 

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