Nitric acid covalence

Carius method The quantitative determination of S and halogens in covalent (organic) compounds by complete oxidation of the compound with cone, nitric acid and subsequent estimation of precipitated AgX or BaS04. 

The Raman spectrum of nitric acid shows two weak bands at 1050 and 1400 cm. By comparison with the spectra of isolated nitronium salts ( 2.3.1), these bonds were attributed to the nitrate and nitronium ion respectively. Solutions of dinitrogen pentoxide in nitric acid show these bands , but not those characteristic of the covalent anhydride , indicating that the self-dehydration of nitric acid does not lead to molecular dinitrogen pentoxide. Later work on the Raman spectrum indicates that at —15 °C the concentrations of nitrate and nitronium ion are 0-37 mol 1 and 0 34 mol 1 , respectively. The infra-red spectrum of nitric acid shows absorption bands characteristic of the nitronium ion. The equivalence of the concentrations of nitronium and nitrate ions argues against the importance of the following equilibrium ... 

When acetic anhydride was in excess over nitric acid, acetyl nitrate and acetic acid were the only products. When the concentration of nitric acid was greater than 90 moles %, dinitrogen pentoxide, present as (N02+)(N0a ), was the major product and there were only small traces of acetyl nitrate. With lower concentrations of nitric acid the products were acetic acid, acetyl nitrate and dinitrogen pentoxide, the latter species being present as covalent molecules in this organic medium. A mixture of z moles of nitric acid and i mole of acetic anhydride has the same Raman spectrum as a solution of i mole of dinitrogen pentoxide in 2 moles of acetic acid. 

Moving now to nitrogen we see that it has four covalent bonds (two single bonds + one double bond) and so its electron count is 5(8) = 4 A neutral nitrogen has five electrons m its valence shell The electron count for nitrogen m nitric acid is one less than that of a neutral nitrogen atom so its formal charge is +1... 

FIGURE 1.5 Counting electrons in nitric acid. The electron count of each atom is equal to half the number of electrons it shares in covalent bonds plus the number of electrons in its own unshared pairs. 

Although these compds are readily formed, they are also readily decompd, eg by the addition of a solvent that dissolves one of the two constituents. The nature of these non-covalent compds is discussed by Urbanski (Ref 35) Picric Acid is not resistant to strong oxidizing agents. It is oxidized by boiling nitric acid to yield oxalic add. Boiling with ammonium persulfate results in a complete oxidation, according to the equation ... 

The standard reduction potential for Be2+ is the least negative of the elements in the group and by the same token beryllium is the least electropositive and has the greatest tendency to form covalent bonds. The bulk metal is relatively inert at room temperature and is not attacked by air or water at high temperatures. Beryllium powder is somewhat more reactive. The metal is passivated by cold concentrated nitric acid but dissolves in both dilute acid and alkaline solutions with the evolution of dihydrogen. The metal reacts with halogens at 600°C to form the corresponding dihalides. 

In the solid state, dinitrogen pentoxide is ionic, existing as N02+N03 and sometimes called nitronium nitrate. The same is true of dinitrogen pentoxide in polar solvents like nitric acid where complete ionization to nitronium and nitrate ions is observed. In the vapour phase, and in nonpolar solvents, a covalent structure is observed. This dichotomy of behavior in both physical state and in solution means that no single nitrating agent is as diverse and versatile as nitrogen pentoxide. 

Lead nithopluiuhaie, Pb-PbOj, ted lead, is similarly described ns a sail, in Ihi.s ease an orihoplumbalc of divalent lead. Pb PbO i. because on treatment with nitric acid, two-thirds of the lead dissolves and one third remains us PbO . It is prepared in the red farm by atmospheric heating of PbO. and in a black form by reaction nl" pbO with pure oxygen. Red lead is formed of PhOj, octahedra (with one common edge I linked by lead atoms covalently bonded to three oxygen atoms. 

These esters of nitric acid form one of the two groups of nitrates, the covalent group, which are also exemplified by nitryl hypofluorite and... 

Conversely, neutral extracting agents possessing nitrogen electron-donor atoms in their structure (soft bases) will easily discriminate between An(III) and Ln(III) even from nitric acid feeds, thanks to covalently hinted An(III)-N interactions, the best example being the terdentate bis-triazinyl-pyridines (BTPs) or the tetradentate bis-triazinyl-bis-pyridines (BTBP). 

The nitryl halides (C2 symmetry) are planar in analogy to the isoelectronic COs. The gas phase and solid-state structures for CINO2 both show covalently bound molecules (see Table 40). In contrast to the nitrosyl hahdes, the halogen-nitrogen bond distances in the gas phase and in the solid state are similar. The nitryl halides NO2F and NO2CI are the mixed anhydrides of nitric acid and hydrofluoric or hydrochloric acid, respectively. 

Attempts to prepare protactinium pentanitrate by reacting penta-halides with liquid dinotrogen pentoxide have resulted in the formation of HPalNOglfl, possibly as a result of traces of anhydrous nitric acid present in the N Os 49). The presence of the jiroton has not been confirmed by electron spin resonance studies, but infrared results have shown that all the nitrate is covalently bound and vibrations associated with the nitronium and nitrosonium cations were not observed. Niobium and tantalum pentahalides react under similar conditions to form the anhydrous oxytrinitrates, M 0(N0g)3 20, 87). 

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