Nitrous acid is a weak and monobasic acid known only in solution and in the
form of nitrite salts. Nitrous acid is used to make diazides
from amines; this occurs by nucleophilic attack of the amine onto the nitrite,
reprotonation by the surrounding solvent, and double-elimination of
water. The diazide can then be liberated to give a carbene or carbenoid.
Structure In the gas phase, the planar nitrous
acid molecule can adopt both a cis and a trans form. The trans form predominates
at room temperature, and IR measurements indicate it is more stable by around 2.3
kJ mol−1. Preparation
When cold, dilute solutions of nitrite ion, NO2− are carefully acidified, a
light blue solution of nitrous acid is produced. Free nitrous acid is unstable
and decomposes rapidly. 1. it can be prepared by dissolving dinitrogen
trioxide in water at 0°c. N2O3+H2O → 2HNO2
Decomposition In anything other than very dilute, cold
solutions, nitrous acid rapidly decomposes into nitrogen dioxide, nitric
oxide, and water: 2 HNO2 → NO2 + NO + H2O
Nitrogen dioxide disproportionates into nitric acid and nitrous acid in aqueous
solution: 2 NO2 + H2O → HNO3 + HNO2
In warm or concentrated solutions, the overall reaction amounts to production
of nitric acid, water, and nitric oxide: 3 HNO2 → HNO3 + 2 NO + H2O
Chemistry = Inorganic Chemistry =
Reduction of the acid gives different products, depending on the reducing
agent: With I− and Fe2+ ions, NO is formed:
2 KNO2 + 2 KI + 2 H2SO4 → I2 + 2 NO + 2 H2O + 2 K2SO4
2 KNO2 + 2 FeSO4 + 2 H2SO4 → Fe2(SO4)3 + 2 NO + 2 H2O + K2SO4
With Sn2+ ions, N2O is formed: 2 KNO2 + 6 HCl + 2 SnCl2 → 2 SnCl4 + 2
N2O + 6 H2O + 2 KCl With SO2 gas, NH2OH is formed:
2 KNO2 + 6 H2O + 4 SO2 → 3 H2SO4 + K2SO4 + 2 NH2OH
With Zn in alkali solution, NH3 is formed:
5 H2O + KNO2 + 3 Zn → NH3 + KOH + 3 Zn(OH)2
With N2H5+, HN3, and subsequently, N2 gas is formed:
HNO2 + [N2H5]+ → HN3 + H2O + H3O+ HNO2 + HN3 → N2O + N2 + H2O
Oxidation by nitrous acid has a kinetic control over thermodynamic control, this
is best illustrated that dilute nitrous acid is able to oxidize I− to I2, but
dilute nitric acid cannot. I2 + 2 e− ⇌ 2 I− {Eo = +0.54 V}
NO3− + 3 H+ + 2 e− ⇌ HNO2 + H2O {Eo = +0.93 V}
HNO2 + H+ + e− ⇌ NO + H2O {Eo = +0.98 V} It can be seen that the values of Ecello
for these reactions are similar, but nitric acid is a more powerful oxidizing
agent. Base on the fact that dilute nitrous acid can oxidize iodide into
iodine, it can be deduced that nitrous is a faster, rather than a more
powerful, oxidizing agent than dilute nitric acid.
= Organic Chemistry = Nitrous acid is used to prepare
diazonium salts: HNO2 + ArNH2 + H+ → ArN2+ + 2 H2O
where Ar is an aryl group. Such salts are widely used in organic
synthesis, e.g., for the Sandmeyer reaction and in the preparation azo
dyes, brightly colored compounds that are the basis of a qualitative test for
anilines. Nitrous acid is used to destroy toxic and potentially explosive
sodium azide. For most purposes, nitrous acid is usually formed in situ by the
action of mineral acid on sodium nitrite: It is mainly blue in colour
NaNO2 + HCl → HNO2 + NaCl 2 NaN3 + 2 HNO2 → 3 N2 + 2 NO + 2 NaOH
Reaction with two α-hydrogen atoms in ketones creates oximes, which may be
further oxidized to a carboxylic acid, or reduced to form amines. This process
is used in the commercial production of adipic acid.
Nitrous acid reacts rapidly with aliphatic alcohols to produce alkyl
nitrites, which are potent vasodilators: (CH3)2CH-CH2-CH2-OH + HNO2
→2CH-CH2-CH2-ONO + H2O Atmosphere of the earth
Nitrous acid is involved in the ozone budget of the lower atmosphere: the
troposphere. The heterogeneous reaction of nitric oxide and water produces
nitrous acid. When this reaction takes place on the surface of atmospheric
aerosols, the product readily photolyses to hydroxyl radicals.
See also Demjanov rearrangement
Nitric acid Tiffeneau-Demjanov rearrangement
References