Weak acids nitrous acid

The first step involves the formation of the weak acid nitrous acid (HN02 or, more helpfully, HONO) from the sodium salt and the strong acid HC1. Nitrous acid is itself protonated and then loss of water creates the reactive electrophile NO+. 

Nitric acid, nitrous acid and hypochlorous acid are examples of monobasic acids HNO3 is essentially fully ionized in aqueous solution (equation 6.13), but HNO2 and HOCl behave as weak acids (equations 6.14 and 6.15). 

Practically all soluble salts, acids (such as sulfuric, nitric, and hydrochloric acids), and bases (such as sodium, potassium, calcium, and barium hydroxides) are strong electrolytes. Weak electrolytes include numerous other acids and bases such as acetic acid, nitrous acid, carbonic acid, and ammonia. The terms strong acid, strong base, weak acid, and weak base refer to whether an acid or a base is a strong or weak electrolyte. A brief list of strong and weak electrolytes is given in Table 15.3. 

Nitrous acid, HNOj, is known as a gas, but otherwise exists only in solution, in which it is a weak acid. Hence addition of a strong acid to a solution of a nitrite produces the free nitrous acid in solution. 

The addition of even a weak acid (such as ethanoic acid) to a nitrite produces nitrous acid which readily decomposes as already indicated. Hence a nitrite is distinguished from a nitrate by the evolution of nitrous fumes when ethanoic acid is added. 

The weak effect of nitrous acid upon nitration in nitric acid is a consequence of the already considerable concentration of nitrate ions supplied in this case by the medium. 

Alkaline solutions of mononitroparaffins undergo many different reactions when stored for long periods, acidified, or heated. Acidification of solutions of mononitro salts is best effected slowly at 0°C or lower with weak acids or buffered acidic mixtures, such as acetic acid—urea, carbon dioxide, or hydroxyl ammonium chloride. If mineral acids are used under mild conditions, eg, dilute HCl at 0°C, decomposition yields a carbonyl compound and nitrous oxide (Nef reaction). 

HNO2 Nitrous acid Unstable weak acid, HONO (p. 461) stable salts (nitrites) are known... 

The reactive species for the transfer of the nitrosyl cation NO+ is not the nitrous acid 2 but rather N2O3 4 which is formed in weakly acidic solution. Other possible nitrosating agents are NOCl or H2N02 ", or even free NO+ in strong acidic solution. The initially formed N2O3 4 reacts with the free amine 1 ... 

Molecules containing an ionizable hydrogen atom. This type of weak acid was discussed in Chapter 4. There are literally thousands of molecular weak acids, most of them organic in nature. Among the molecular inorganic weak acids is nitrous add ... 

These trends are general ones, observed with other oxoadds of the nonmetals. Recall, for example, that nitric acid, HNO3 (oxid. no. N = +5), is a strong acid, completely ionized in water. In contrast, nitrous add, HN02 (oxid. no. N = +3), is a weak acid (Ka = 6.0 X 10-4). The electronegativity effect shows up with the strengths of the oxoadds of sulfur and selenium ... 

With the salts of certain weak acids, such as carbonic, sulphurous, and nitrous acids, an additional factor contributing to the increased solubility is the actual disappearance of the acid from solution either spontaneously, or on gentle warming. An explanation is thus provided for the well-known solubility of the sparingly soluble sulphites, carbonates, oxalates, phosphates(V), arsenites(III), arsenates(V), cyanides (with the exception of silver cyanide, which is actually a salt of the strong acid H[Ag(CN)2]), fluorides, acetates, and salts of other organic acids in strong acids. 

It is decompd even by weak acids with evolution of brown fumes of nitrous anhydride (Ref 1) KN02 detonates violently when heated in the presence of thiosulfates or cyanides. Ref 1 states that when mixed with K cyanide, the mixt burns, and at 450°, it explds violently. Ref 6 details instances of molten KN02 being violently decompd by boron, and of a wgorous reaction attended by flame caused by the addition of a 0 little amm sulfate to fused KNOa... 

Testing for excess of nitrous acid at the end of the reaction. For this purpose starch-potassium iodide papers are best used, and these indicate nitrite in acid solution by turning blue instantaneously. With some practice, the nitrite reaction can be clearly distinguished from the coloration caused by certain diazo compounds, such as those bearing nitro substituents. The latter react only after 0.5 to 2 seconds. Often the difference becomes more marked after dilution of the diazo solution with concentrated hydrochloric acid. A properly conducted diazotization should exhibit on completion a very weak nitrite reaction, corresponding to an excess of about 10 4 m. 

The major problem of these diazotizations is oxidation of the initial aminophenols by nitrous acid to the corresponding quinones. Easily oxidized amines, in particular aminonaphthols, are therefore commonly diazotized in a weakly acidic medium (pH 3, so-called neutral diazotization) or in the presence of zinc or copper salts. This process, which is due to Sandmeyer, is important in the manufacture of diazo components for metal complex dyes, in particular those derived from l-amino-2-naphthol-4-sulfonic acid. Kozlov and Volodarskii (1969) measured the rates of diazotization of l-amino-2-naphthol-4-sulfonic acid in the presence of one equivalent of 13 different sulfates, chlorides, and nitrates of di- and trivalent metal ions (Cu2+, Sn2+, Zn2+, Mg2+, Fe2 +, Fe3+, Al3+, etc.). The rates are first-order with respect to the added salts. The highest rate is that in the presence of Cu2+. The anions also have a catalytic effect (CuCl2 > Cu(N03)2 > CuS04). The mechanistic basis of this metal ion catalysis is not yet clear. 

At first sight this seems surprising, as nitrous acid is a fairly weak acid (pKa = 3.15, Tummavuori and Lumme, 1968) and therefore the low equilibrium concentration of nitrite ions in Scheme 3-9 does not appear to favor the formation of dinitrogen trioxide. 

Mesitylene was studied using the range 5-7 M nitric acid, and when the nitrous acid concentration is small (< 0.014 M) nitronium ion nitration appears to occur, giving zeroth-order kinetics weakly retarded by nitrous acid. At rather higher nitrous acid concentrations the reaction is catalysed by nitrous acid and the kinetics go over to first-order (at constant nitrous acid concentration). 

Acetic acid is used as a solvent for some reactions between acids and bases, (a) Nitrous acid and carbonic acids are both weak acids in water. Will either of them act as a strong acid in acetic acid Explain your answer, (b) Will ammonia act as a strong or weak base in acetic acid Explain your answer. 

Nitrite is an extremely reactive chemical and is soluble in the aqueous phase of meat. It is usually used for curing in the form of the sodium salt. The nitrite ion is the conjugate base of nitrous acid (a weak acid) and has a PK of 3.36. The usually mild acid conditions found in meat give formation of only a small quantity of nitrous acid when nitrite is added to the meat ( ) ... 

Nitrous acid is a weak acid with Ka = 4.5 X 10-4. Because it contains nitrogen in an intermediate oxidation state, nitrous acid can function as both an oxidizing agent and a reducing agent as shown in the following equations ... 

The only substance present is nitrous acid, HN02, a weak acid. Since this is a weak acid, this must be a Ka problem. As a Ka problem, we can set up a simple equilibrium problem ... 

B—A (nitrous acid) and D (acetic acid) are weak acids, and E (ammonia) is a weak base. Weak acids and bases are weak electrolytes. C (ethanol) is a nonelectrolyte. Potassium nitrate (B) is a water-soluble ionic compound. 

The major problem of these diazotizations is oxidation of the initial aminophenols by nitrous acid to the corresponding quinones. Easily oxidized amines, in particular aminonaphthols, are therefore commonly diazotized in a weakly acidic medium (pH 3) so-called neutral diazotization or in the presence of zinc or copper salts. This process, which is due to Sandmeyer, is important in the manufacture of diazo components for metal complex dyes, in particular those derived from l-amino-2-naphthol-4-sulfonic acid. 

The classical discoveries of the reacting nitrosation reagents mentioned above were mainly the result of kinetic investigations. There were some surprising results, e.g. the dependence of the diazotization rate on the square of the nitrous acid concentration in sulfuric or perchloric acid up to 0.3 M. Nitrous acid is a fairly weak acid (pK = 3.1569). Therefore, the low concentration of nitrite ions in the last step of the nitrous acid equilibria system of Scheme 5 does not appear to favor the formation of dinitrogen trioxide. N2O3 is, however, strongly indicated by the second order of rate on the (analytical) nitrous acid concentration. 

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