15.4A: Nitrogen

• Nitrogen forms a range of different types of compounds:
  • In combination with the most electropositive element ionic nitrides (N^3-) are formed.
  • Also forms anionic species NR₂^– and NR^2-.
  • Forms cationic compounds NR₄⁺ for the maximum covalency of four bonds.
  • Forms compounds with single, double and triple covalent bonds to itself, carbon and oxygen.
  • There are some paramagnetic compounds of nitrogen, i.e. with an odd number of electrons, notably certain oxides.
• In its three-covalent compounds, the following generallities apply:
  • Nitrogen is almost always a Lewis donor (except in very special cases, e.g. NF₃).
  • Although based on a tetrahedral shape with the lone-pair occupying one site, optical isomers of potentially chiral compounds :NRR'R" cannot be isolated because the molecules invert so easily through a planar transition state (like an umbrella turning inside-out).
  • There are a few cases where the lone-pair is delocalized into empty orbitals on the attached groups in which nitrogen atom is planar. An example is N(SiMe₃)₃.
• Probably due to repulsion between the lone pairs, nitrogen-nitrogen single bonds are rather weak, e.g. 160 kJ mol^-1 in H₂N-NH₂ as compared to a carbon-carbon bond dissociation energy of around 350 kJ mol^-1 in CH₃-CH₃.
• The ability to form multiple bonds via pp - pp overlap is one of the things that distinguishes nitrogen from the other members of group 15. Because of the lone pairs, molecules will normally not be linear.

Occurence and Properties of the Element

• Dinitrogen constitutes 78% of the atmosphere. The ratio of the abundances of the two stable isotopes is ¹⁴N/¹⁵N = 272.0 so ¹⁵N at natural abundance is somewhat useful for nmr studies, and when it is enriched it, can be used in tracer studies.
• Nitrogen is obtained by low temperature distillation (fractionation) of air. It will normally still be contaminated with about 30 ppm of oxygen and some argon. It can be prepared pure by thermally decomposing sodium azide, NaN₃.
• The NºN bond is very stron with a dissociation energy of 944.7 kJ mol^-1. For this reason, many hitrogen compounds are "endothermic" that is they have positive heats of formation.
• Compared to other molecules with triple bonds, nitrogen is very unreactive. It will react with lithium at room temperature, and there are bacteria which are able to activate it by a mechanism that remains unclear.
• At high temperatures it is more reactive:

N₂ + 3H₂ 2NH₃

N₂ + O₂ 2NO

N₂ + 3Mg Mg₃N₂

N₂ + CaC₂ CaN

CN + C

Nitrides

• The ionic ones only occur with the most electropositive elements. Hydrolysis yields ammonia.
• Transition metals tend to form non-stoichiometric compounds.
• The important covalent nitrides are covered in the section concerning the other element

Hydrides

Ammonia

• This topic has been covered to some extent elsewhere.
• The following sequence of conversions is industrially important:
• Ammonia is very soluble in water due to extensive hydrogen bonding. Th eresulting solutions are conventionally called ammonium hydroxide, but:
  NH₃ + H₂O NH₄⁺ + OH^– Kb = 1.77x10^-5

  i.e. there is not much ammonium ion in solutions of ammonia!

• Ammonium salts are mostly soluble, and the ammonium ion, r = 1.48 Å, resembles potassium ion, r = 1.33, Å or rubidium ion, r = 1.48 Å.
• Ammonium salts sublime via this type of reaction:
  NH₄Cl(s) NH₃(g) + HCl(g)
  NH₄NO₃(s) NH₃(g) + HNO₃(g)

  But with oxidizing anions, decomposition can occur:

  NH₄NO₃(l) N₂O(g) + 2H₂O
  (NH₄)₂Cr₂O₇(s) N₂(g) + 4H₂O(g) + NH₃

Hydrazine

• Hydrazine bp 114 ^oC is explosive when pure but organo-substitued hydrazines are more stable.
• Preparation:

  NH₃ + NaOCl NaOH + NH₂Cl NH₃ + NH₂Cl + NaOH N₂H₄ + NaCl + H₂O but the following reaction is catalysed by traces of transition metal ions, notable Cu²⁺:

  NH₂Cl + NH₂Cl 2NH₄Cl + +N₂ Therefore, the reaction is perfomed in the presence of gelatin which complexes the metal ions and suppresses their catalytic effect.

• Hydrazine is a "bifunctional" base:
  N₂H₄ + H₂O N₂H₅⁺ + OH^– K_b1 = 8.5x10^-7
  N₂H₅⁺ + H₂O N₂H₆²⁺ + OH^– Kb2 = 8.9x10^-15

  These equilibria leasd to two sets of hydrazinium salts.

• Hydrazine and the substituted hydrazines are used as reducing agents. In combination with N₂O₄ as oxidant, alkylhydrizines are used in teh vernier rocket engines on things like the space shuttle.