231The Nitrite Ion
Nitrite Salts
Sodium nitrite is made industrially by passing "nitrous fumes" into aqueous sodium hydroxide or
sodium carbonate solution:
NO + NO 2 + 2NaOH (or Na 2 CO 3 ) → 2NaNO 2 +H 2 O ( or CO 2 )The product is purified by recrystallization. Alkali metal nitrites are thermally stable up to and
beyond their melting point (441 °C for KNO 2 ). Ammonium nitrite can be made from dinitrogen
trioxide, N 2 O 3 , which is formally the anhydride of nitrous acid:
2NH 3 + H 2 O +N 2 O 3 → 2NH 4 NO 2This compound may decompose explosively on heating. In organic chemistry nitrites are used in
diazotization reactions.
Structure
The nitrite ion has a symmetrical structure (C2v symmetry), with both N-O bonds having equal
length. In valence bond theory, it is described as a resonance hybrid with equal contributions from
two canonical forms that are mirror images of each other. In molecular orbital theory, there is a
sigma bond between each oxygen atom and the nitrogen atom, and a delocalized pi bond made
from the p orbitals on nitrogen and oxygen atoms which is perpendicular to the plane of the
molecule. The negative charge of the ion is equally distributed on the two oxygen atoms. Both
nitrogen and oxygen atoms carry a lone pair of electrons. Therefore, the nitrite ion is a Lewis base.
Moreover, it can act as an ambidentate ligand towards a metal ion, donating a pair of electrons
from either nitrogen or oxygen atoms.
Acid-base Properties
In aqueous solution, nitrous acid is a weak acid:
HNO 2 H+ + NO 2 - ; pKa = ca. 3.3 at 18 °CNitrous acid is also highly volatile - in the gas phase it exists predominantly as a trans-planar
molecule. In solution, it is unstable with respect to the disproportionation reaction:
3HNO 2 (aq) H 3 O+ + NO 3 - + 2NO
This reaction is slow at 0 °C. Addition of acid to a solution of a nitrite in the presence of a reducing
agent, such as iron (II), is a way to make nitric oxide (NO) in the laboratory.
Oxidation and reduction
The formal oxidation state of the nitrogen atom in a nitrite is +3. This means that it is can be either
oxidized to oxidation states +4 and +5, or reduced to oxidation states as low as -3. Standard
reduction potentials for reactions directly involving nitrous acid are shown in the table below: