Acids nitrous

As discussed in Chapter 1, nitrous acid is important in tropospheric chemistry because it photolyzes to form OH  

Measuring experimentally the quantum yields for reaction (15) as well as the absorption cross sections has been difficult because of contamination of the HONO by other, strongly absorbing species, particularly N02, necessitating corrections for their contributions to the absorption. However, it is believed, based on studies by Cox and Derwent (1976/1977) that the quantum yield for reaction (15) is 1.0 at A 400 nm. 

FIGURE 4.14 Absorption spectrum of HONO at 277 K (adapted from Bongartz et al., 1991). Note that the absolute values of the cross sections shown here should be multiplied by 0.855 as recommended by Bongartz et al. (1994). 

TABLE 4.13 Recommended Absorption Cross Sections (Base e) for HONO  

Wavelength (nm) io2V (cm2 molecule ) Wavelength (nm) 102° t (cm2 molecule-1) Wavelength (nni) io2V (cm2 molecule ) 

For the preparation of gaseous nitrous acid, arsenious acid, broken into pieces the size of a pea, is treated with nitric acid, sp. gr. 1.3, and heated gently on a wire gauze with a free flame (under the hood). In order to condense the nitric acid carried along with the gases, an empty wash-bottle, cooled by cold water, is employed. (See Fig. 81.) 

Preparation and Properties. 1. Dissolve a small amount of potassium nitrite in 1-2 ml of water, cool the solution with ice, and add two or three drops of concentrated sulphuric acid to it. What is observed Write the equations of the reactions. How can you explain the difierent strength of nitric and nitrous acids  

Pour a potassium nitrite solution into a test tube, acidify it with dilute sulphuric acid, and add several drops of potassium permanganate. What happens Write the equation of the reaction. Run 

Add several drops of starch and of a potassium nitrite solution to an acidified solution of potassium iodide. What do you observe Write the equation of the reaction. Run a similar experiment with potassium bromide, replacing the starch with an organic solvent. What role is played by the nitrous acid in these reactions  

Nitric acid is a strong oxidizing agent. Organic substances may ignite when reacted with the concentrated acid (fuming and anhydrous). Nitric acid vapour irritates the respiratory tracts. If nitric acid gets on one s skin, it causes serious 

Perform experiments with concentrated nitric acid in a fume cupboard-, wear gloves and eye protection. 

The first weak absorption band in the 300-400 nm range is diffuse, consisting of the V2 (-N=0) vibrational progression the second much stronger band in the 190-270 nm region is continuous.  

This suggests that there is a H0N0 coplanar transition state formed during this reaction. The initial parent vibrational excitation in -N=0(V2) has no effect on the energy content of the OH product, suggesting that dissociation is so rapid that the 2(-N=0 stretch) vibration does not interact effectively with the fragmentation coordinate v),(0-N stretch), that is, the vibration is localized. 

---

It is prepared by an intramolecular transformation of diazoaminobenzene in the presence of aniline hydrochloride, or in one stage by diazotizing a solution of aniline and aniline hydrochloride with an insufficient amount of nitrous acid. 

Usually prepared from the corresponding sulphonic acids by alkali fusion, methylation of phenol or from the aminotoluene by treatment with nitrous acid followed by boiling. Both o- and p-cresol are used as end components in azo dyes. 

The most important reaction of the diazonium salts is the condensation with phenols or aromatic amines to form the intensely coloured azo compounds. The phenol or amine is called the secondary component, and the process of coupling with a diazonium salt is the basis of manufacture of all the azo dyestuffs. The entering azo group goes into the p-position of the benzene ring if this is free, otherwise it takes up the o-position, e.g. diazotized aniline coupled with phenol gives benzeneazophenol. When only half a molecular proportion of nitrous acid is used in the diazotization of an aromatic amine a diazo-amino compound is formed. 

M.p. —80°C, b.p. 37°C. Prepared from sodium azide and acid or (N2Hj) plus nitrous acid, HNO2. Heavy-metal salts, azides, are used as detonators, alkali metal salts are stable and azides are used synthetically in organic chemistry. 

With nitrous acid it gives l-nitroso-2-naph-thol. It can also be chlorinated and sulphonated. Oxidized ultimately to phthalic acid on prolonged oxidation. 

Nitrous acid, HNO2. See separate entry. Hyponitric acid, H2N2O3, trioxo di-nitrate(Il). Sodium salt from HjNOH in MeOH with CiHiNOi many complexes are known the free acid is unstable. 

It reduces to 4-aminodimethylaniline and gives dimethylamine with hot sodium hydroxide solution. It is prepared by the action of nitrous acid upon dimethylaniline at 0 C. 

H—N—N=N. It is prepared by the oxidation of hydrazine in strongly acid solution the oxidising agent used is usually nitrous acid (i.e. sodium nitrite is added to the acid solution of hydrazine) ... 

The nitrous acid decomposes rapidly at room temperature, thus 3HNO2 HNO3 + 2NO + H2O (9.2)... 

Dinitrogen trioxide, the anhydride of nitrous acid, is very unstable. At low temperature it dissociates thus ... 

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. 

Nitrous acid is unstable, decomposing to give nitric acid and evolving nitrogen oxide ... 

These all contain the ion NO2. They are much more stable than nitrous acid, and those of the alkali metals can be fused without... 

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. 

Dissolve a few drops of nitromethane in 10% sodium hydroxide solution. Add a few crystals of sodium nitrite and shake. Now add dilute sulphuric acid drop by drop. A brownish-red coloration develops, but fades again when an excess of acid is added. The sulphuric acid has thus liberated nitrous acid, which has in turn reacted with the nitromethane to give a nitrolic acid, the sodium salt of which is CH3NO2 + ONOH = CH(N02) N0H + HgO reddish-brown in colour, probably owing to mesomeric ions of the type ... 

CH,),CHNO, -h HONO = (CH,),C(NO)NO, -h H,0 producing a blue solution. A tertiary nitro paraffin such as trimethylnitro-methane, (CH )jCNO, gives no reaction with nitrous acid. 

Aromatic primary amines differ markedly from aliphatic amines in their reaction with nitrous acid. Thus a cold aqueous solution of mono thylamine hydrochloride reacts with nitrous acid to give mainly the corresponding primary alcohol ... 

In preparing an aqueous sol ution of a diazonium salt, such as benzene-diazonium chloride, it is usual to dissolve the amine in a slight excess (about 2 2 molecular equivalents) of dilute hydrochloric acid (or alternatively to dissolve the crystalline amine hydrochloride in i 2 equivalents of the acid) and then add an aqueous solution of a metallic nitrite. Nitrous acid is thus generated in situ, and reacts with the amine salt to give the diazonium compound. For a successful preparation of an aqueous solution of the diazonium salt, however, two conditions must always be observed ... 

The solution of the aniline hydrochloride should be cooled to 5°C., and this temperature maintained throughout the addition of the sodium nitrite solution. External cooling has to be maintained, otherwise the heat of the reaction would cause the temperature to rise, with the consequent decomposition of the diazonium chloride and the production of phenol. If, on the other hand, the temperature is reduced to about o , diazotisation becomes extremely slow and unchanged nitrous acid may remain in the solution for an impracticably long time. 

To ensure the presence of a slight excels of nitrous acid, potassium iodide-starch paper is sometimes used as an external indicator, a drop of the solution being removed from time to time during the addition of the sodium nitrite, and then dropped on to the paper. When an excess of nitrous acid is present, iodine is liberated, and gives the familiar... 

Dissolve 15 ml. (15-4 g.) of aniline in a mixture of 40 ml. of concentrated hydrochloric acid and 40 ml. of water contained in a 250 ml. conical flask. Place a thermometer in the solution, immerse the flask in a mixture of ice and water, and cool until the temperature of the stirred solution reaches 5°. Dissolve I2 5 g. of powdered sodium nitrite in 30 ml. of water, and add this solution in small quantities (about 2-3 ml. at a time) to the cold aniline hydrochloride solution, meanwhile keeping the latter well stirred by means of a thermometer. Heat is evolved by the reaction, and therefore a short interval should be allowed between consecutive additions of the sodium nitrite, partly to allow the temperature to fall again to 5°, and partly to ensure that the nitrous acid formed reacts as completely as possible with the aniline. The temperature must not be allowed to rise above 10°, otherwise appreciable decomposition of the diazonium compound to phenol will occur on the other hand, the temperature... 

Secondary amines of both the aliphatic and the aromatic series react similarly with nitrous acid, giving nilrosamines ... 

Nitrous acid does not react with aliphatic tertiary amines, such as triethyl-amine, (CaHj)aN, nor does it usually react with aromatic tertiary amines such as triphenylamine, (CaHj)aN, which contain three aryl groups. 

If, however, a tertiary amine has two alkyl groups and also an aryl group having the para position unsubstituted, then the action of nitrous acid is to insert the nitroso group directly into this para position. Dimethylaniline, for example, when treated with nitrous acid readily gives p-nitrosodimethyl-... 

Tertiary amines containing one alkyl and two aryl groups, such as mono-I ncthyldiphenyhiniir.e, Cl l3(C, l l.diX , arc rarely encountered and arc unimportant. They usually react with nitrous acid with the insertion of a nitroso group into only one of the two available para positions monomethyl-diphenylamine thus gives monomethyl-mono - pnitroso-diphenylamine. Cl hj(C.ill .)N C l 1 jXO, or V-nicthyl-p-nitrosodiphcnylaniine. 

Diazonium salts couple readily with aromatic primary amines, giving diazoamino compounds. If for instance an aqueous solution of aniline sulphate is diazotised with a deficiency of nitrous acid, only part of it is converted into benzenediazonium sulphate and the latter then couples with the unchanged aniline to give diazoaminobenzene. The reaction is carried out at the opti-CeHsNHj.HjSO + HONO = CbHsNjHSO, + zHaO... 

It is important in this preparation to avoid an excess of nitrous acid before coupling occurs, otherwise the excess of nitrous acid will react directly with the dimethylaniline, and the deep green p-nitrosodimethylaniline so formed will contaminate the methyl-orange. 

In the present preparation, ethyl acetoacetate is treated with sufficient nitrous acid to convert half into the a-nitroso (or a-oximino) ester, which is reduced by zinc and acetic acid to the a-amino ester (I). The latter then condenses with... 

The colorations produced in this reaction arise from the action of nitrous acid on the phenol, giving />-nitrosophenol (I) which then reacts with excess of phenol to form an indophenol (II) which is an acid-base indicator ... 

The Reaction has the following limitations (i) a compound that can liberate nitrous acid in acid solution is required (e.g., a metallic nitrite or a nitroso-amine, p. 204). (2) Nitrophenols and />-substituted phenols do not give the test. (3) Among the dihydroxyphenols. only resorcinol gives a satisfactory positive test. 

In addition to General Reactions i and 2, all amides react with nitrous acid evolving nitrogen ... 

The evolution of nitrogen is not always entirely satisfactory as a test owing to the possible evolution of gaseous decomposition products of nitrous acid itself. The test may be performed as follows. To i ml. of chilled concentrated sodium nitrite solution add i ml. of dilute acetic acid. Allow any preliminary evolution of gas to subside, and then add the mixed solution to a cold aqueous solution (or suspension) of the amide note the brisk effervescence. 

---