Group 15 (Pnicogens) - The Elements

! N2 comprises 78% by volume of air. •N2(g) is obtained by fractional distillation of air (b.p. 77K). • The very stable N /N bond ( D = 941 kJ/mol) accounts for

the inertness of N 2(g) and its utility as a means of achieving a chemically inert atmosphere. (Only Li reacts with it at room temperature.)

! The only molecular species containing just nitrogen were N 2 –. 1 and the , N 3 , but in 1999 Christe et al. synthesized + N5 by the following reaction + – HF + – [N 2F] [AsF 6] + HN 3 -78 oC N5 [AsF 6] + HF 2 • Ab initio calculations predict the following C2v structure:

• Structure confirmed from low-temperature i.r. and Raman. • Idealized Lewis structure: + N NN NN

+ – •N5 [AsF 6] is a strongly oxidizing material that can detonate violently.

1K. O. Christe, W. W. Wilson, J. A. Sheehy, and J. A. Boatz, Angew. Chem., Int. Ed. , 1999 , 38 , 2004.

2Calculated at B3LYP and CCSD(T)/6-311G(2d) levels of theory. Group 15 - The Elements - Nitrogen (cont.)

3 ! In 2001 Auffermann et al. synthesized SrN 2, containing the 2– N2 ion, from Sr 2N under nitrogen pressure at 5500 bar at a temperature of 920 K.

5500 bar/920 K 2Sr 2N + 3N 2 4SrN 2

2– •N2 is isoelectronic with O 2 and on the basis of MO considerations has a bond order of 2. 2 2 2 4 2 (σ2s) (σ*2s) (σ2p) (π2p) (π*2p)

• x-ray and neutron diffraction studies from microcrystalline powder samples show an N–N distance of 122.4 pm,

comparable to 120.7 for O 2.

• SrN 2 is air- and moisture-sensitive.

3G. Auffermann, Y. Prots, and R. Kniep, Angew. Chem. Int. Ed., 2001 , 40 , 547. Group 15 - The Elements - Phosphorous

! Phosphorous occurs in minerals, such as collophanite,

Ca 3(PO 4)2@2H 2O. • High temperature fusion with coke and silica yields the element:

2Ca 3(PO 4)2@2H 2O + 6SiO 2 + 10C ∆ 6CaSiO 3 + P 4 + 10CO + 4H 2O • There are three basic allotropes and many intermediate forms.

! White phosphorous is the most reactive.

• Contains strained P 4 tetrahedral units held together by van der Waals forces. • Exists in two forms α (cubic) and β (hexagonal). > -76.9 oC α-P 4 β-P 4 • Can be stored under water, but oxidizes with a yellow- green glow in air and combusts spontaneously above 50 oC or when finely divided: o P4 + 5O 2 ÷ P 4O10 ∆H = –2940 kJ/mol

! Black phosphorous is least reactive form, obtained by heat and pressure from white form. • Composed of trigonally coordinated atoms in a puckered sheet structure, similar to graphite. • Inert in air up to ~400 oC.

! Red phosphorous, an inert and nontoxic form, is obtained from white by heating in an inert atmosphere at 250 oC. • Sheet structure with a random network. Group 15 - The Elements - As, Sb, Bi

! Occur in sulfide minerals of Cu, Ag, and Pb.

! All have metallic forms, but As and Sb have unstable nonmetallic yellow forms, obtained by rapid condensation of

vapors, that probably contain M 4 units.

! Stable form of all is α-rhombahedral form, similar to black phosphorous.

! Bi is the heaviest element to have a stable, nonradioactive nucleus. Oxidation States and Bonding

! Principal oxidation states of the group are +5, +3, –3. • Nitrogen also has +1, +2, and +4 states, and phosphorous +4 compounds are known, too. • Bi does not form compounds with a –3 state.

! Bonding in nitrogen and phosphorous compounds is very different, owing to the different modes of effective bonding available to each.

Mode N P pπ-pπ strong unstable pπ-dπ rare weak-moderate hypervalence no yes Nitrogen Chemistry - -

! Ammonia, the most important of nitrogen, is made by the Haber process. 500 oC/250 atm/Fe

3H 2 + N 2 2NH 3 ∆Ho = –92 kJ, ∆Go = –33 kJ o • Liquid NH 3 b.p. = –33.35 C and ∆Hvap = 23.677 kJ/mol. • NH 3(l) is a moderately good ionizing solvent. + – –33 o 2NH 3 = NH 4 + NH 2 K = 1.9 x 10 at –50 C

+ ! In water, NH 3 hydrolyzes to give small amounts of NH 4 and OH –, but there is no evidence for the existence of “ammonium

hydroxide, NH 4OH.” + – –5 NH 3 + H 2O º NH 4 + OH Kb = 1.78 x 10

! Ammonia burns in air to give N 2. 4NH 3 + 3O 2 ÷ 2N 2 + 6H 2O • With Pt catalyst, NO is produced instead.

4NH 3 + 5O 2 ÷ 4NO + 6H 2O This is the basis of the Ostwald process for making HNO 3. Nitrogen Chemistry - Hydrides -

! N2H4 is a fuming liquid that functions as a diacidic base: + – –7 N2H4 + H 2O º N 2H5 + OH K1 = 8.5 x 10 + 2+ – –10 N2H5 + H 2O º N 2H6 + OH K2 = 8.9 x 10

o ! At 25 C it appears to have a gauche configuration ( C2).

! It is a powerful reducing agent. – – o N2H4 + 4OH º N 2 + 4H 2O + 4 e –E = +1.16 V

! Hydrazine is made by the Raschig synthesis: glue – – 2NH 3 + OCl N2H4 + Cl + H 2O • Reaction proceeds quantitatively at 0 oC with a 3:1 ratio of – NH 3 and OCl by a two-step process: NH 3 + NaOCl ÷ NaOH + NH 2Cl (fast) NH 2Cl + NH 3 + NaOH ÷ N 2H4 + NaCl + H 2O • Chloramine, NH 2Cl, is as a solid or liquid. • Without glue (or gelatin) a parasitic reaction predominates:

2NH 2Cl + N 2H4 ÷ 2NH 4Cl + N 2

o ! Hydrazine is metastable ( ∆H f = +50 kJ/mol) and burns in air, sometimes explosively. • It is used as rocket fuel. Nitrogen Chemistry - Hydrides - Hydrazoic Acid 114 o H NNN 124 pm 113 pm

–5 ! Hydrazoic acid, HN 3, is a weak acid ( Ka = 1.8 x 10 )

! HN 3 is obtained by ion exchange of solutions of NaN 3, prepared by either of the following syntheses: 175 oC 3NaNH 2 + NaNO 3 NaN 3 + 3NaOH + NH 3 190 oC 2NaNH 2 + N 2O NaN 3 + NaOH + NH 3

o ! Pure HN 3 (b.p. 37 C) is explosive, as are heavy-metal or organic (e.g., AgN 3, Pb(N 3)2).

! azides decompose smoothly when heated:

2NaN 3 ∆ 2Na + 3N 2

3LiN 3 ∆ Li 3N + 4N 2 (Li 3N = nitride) Nitrogen Chemistry - Oxides

o ! Nitrogen forms at least six oxides, all with ∆G f > 0.

! Nitrous oxide, N 2O, is “laughing gas”, made by heating ammonium nitrate. o NH 4NO 3 ∆ N2O + 2H 2O ∆H = -26 kJ

• Isoelectronic with CO 2 and also linear ( C4v). NNO

o • ∆G f = +103.6 kJ/mol, but it is kinetically quite inert (i.e., metastable).

• Propellant for whipped cream (pH neutral, moderately soluble in cream; FDA approved)

L Thermal decomposition of NH 4NO 3 at moderate temperature yields N 2O, but at high temperature it decomposes explosively by the following reaction: o NH 4NO 3 ∆ N2 + ½O 2 + 2H 2O ∆H = -206 kJ

! Nitric oxide, NO, is a colorless paramagnetic gas produced in – – a number of oxidations with NO 2 and NO 3 . 2 2 4 2 1 • Bond order is 2.5: (σ2s) (σ*2s) (π2p) (σ2p) (π*2p) • A lab preparation is melt 3KNO 2 + KNO 3 + Cr 2O3 ∆ 2K 2CrO 4 + 4NO Nitrogen Chemistry - Oxides (cont.)

! NO readily oxidizes in air to nitrogen dioxide, NO 2, which exists in equilibrium with its dimer. o N2O4 º 2NO 2 ∆H = 57 kJ/mol colorless brown

2 o • K = [N 2O4]/[NO 2] = 0.67 @25 C. • NO 2 is a free radical • Both forms are toxic components of smog. • The dimer has a long N–N bond. O O 118 pm NN 134 o 178 pm O O • MO calculations indicate a σ pair delocalized over the entire molecule. Nitrogen Chemistry - Oxides (cont.)

! N2O3 and N 2O5 are formal anhydrides of HNO 2 and HNO 3, respectively.

! N2O3 is an intense blue liquid, formed by oxidation of NO, which is not stable above –30 oC. < -30 oC 4NO + O 2 2N 2O3

o o ! N2O5 (m. 30 C, d. 47 C) can be made by dehydrating HNO 3 with P 4O10 . 12HNO 3 + P 4O10 ÷ 6N 2O5 + 4H 3PO 4

It is unstable (sometimes exploding) and is obtained by distillation in a stream of ozonized oxygen. Nitrogen Oxoacids - HNO 2

! , HNO 2, can be obtained in solution by acidifying solutions of :

Ba(NO 2)2(aq ) + H 2SO 4(aq ) ÷ 2HNO 2(aq ) + BaSO 4(s)

! HNO 2 is unstable and has not been obtained pure: 3HNO 2 º HNO 3 + 2NO + H 2O

! Structurally it is HONO, a bent molecule ( Cs) having structural parameters consistent with its Lewis structure and VSEPR predictions. H 146 pm HOON ON 120 pm 116 o O –6 ! It is a weak acid with Ka = 6.03 x 10 .

! Nitrites can function as oxidants or reductants: + – o HNO 2 + H + e ÷ NO + H 2O E = 1.0 V – + – o HNO 2 + H 2O ÷ NO 3 + 3H + 2 e –E = –0.94 V

! salts are obtained by reducing nitrates:

(Chile saltpeter) 2NaNO 3 ∆ 2NaNO 2 + O 2

(saltpeter) KNO 3 + Pb ∆ KNO 2 + PbO Nitrogen Oxoacids - HNO 3

! is made by a variety of processes.

! Today, most HNO 3 is made by acidifying saltpeter in concentrated , followed by distillation. 0 oC 2KNO 3 + conc-H 2SO 4 2HNO 3 + K 2SO 4

! The classic synthesis is the Ostwald process:

o 4NH + 5O 750-850 C 4NO + 6H O ∆H = –1170kJ 3 2 Pt 2

2NO + O 2 ÷ 2NO 2 3NO 2 + H 2O ÷ 2HNO 3 + NO

• NO produced in the third step is recycled for use in the second. • Product is concentrated to 70% (16 M) by distillation.

! HNO 3 photo-decomposes, causing the reagent to turn brown after prolonged exposure to light. hν 4HNO 3 4NO 2 + 2H 2O + O 2

! HNO 3 is not only a strong acid but also a strong oxidant. + – – o 4H + NO 3 + 3 e ÷ NO + 2H 2O E = +0.96 V

• Below 2M there is little oxidizing ability, and it behaves like any other strong acid. Nitrogen Halides

! The following pure halides are known: NF 3, NCl 3, N 2F4, NF 2Cl, NFCl 2, XN 3 (X = F, Cl, Br, I). • All except NF 3 are metastable.

! Unlike NH 3, NF 3 is a poor Lewis base due to the polarity of the N–F bonds.

! NBr 3 and NI 3 exist as ammonia complexes, which are shock sensitive .

NH 3(aq ) 5NH 3(aq ) + 3I 2 NI 3@NH 3 + 3NH 4I Compounds of P, As, Sb, Bi - Hydrides

! All MH 3 hydrides are known. • Stability falls off rapidly through the series, and SbH 3 and BiH 3 exist in only trace amounts.

! The Marsh test, a classic qualitative test for the presence of arsenic, is based on the thermal instability of arsine. + 2+ 4Zn( s) + 8H (aq ) + H 3AsO 4(aq ) ÷ AsH 3(g) + 4Zn (aq ) + 4H 2O( l)

2AsH 3(g) ∆ 2As( s) + 3H 2(g) • As( s) forms as a silver mirror on the walls of a heated tube through which the gas is passed.

! Unlike NH 3, these other hydrides are neither acid nor base in water.

! Although pyramidal ( C3v) they have small bond angles consistent with LCP considerations and a p-only bonding model. H 93.7 o PH H

• Less accessible 3 s lone pair on P makes PH 3 a poorer Lewis base than NH 3. Compounds of P, As, Sb, Bi - Halides

! Both MX 3 and MX 5 halides can be formed by direct combination.

! Except PF 3, all MX 3 halides hydrolyze in water. • PX 3 and AsX 3 hydrolyze to the +3 oxoacids: PX 3 + 3H 2O ÷ 3HX + H 3PO 3 (not PF 3) AsX 3 + 3H 2O ÷ 3HX + H 3AsO 3 • SbX 3 and BiX 3 hydrolyze to give solutions containing the antimonyl and bismuthyl cations, respectively. + – SbX 3 + H 2O ÷ 2HX + SbO + X + – BiX 3 + H 2O ÷ 2HX + BiO + X

! The MX 5 halides include MF 5, PCl 5, PBr 5, and SbCl 5. • PX 5 halides hydrolyze to H 3PO 4, the +5 oxoacid. PX 5 + 4H 2O ÷ H 3PO 4 + 5HX • In limited water, phosphoryl halides are formed as molecular species.

PX 5 + H 2O ÷ POX 3 + 2HX

• Organic derivatives of phosphoryl halides can be formed:

POX 3 + 3ROH ÷ (RO) 3PO + 3HX POX 3 + 3RMgX ÷ R 3PO + 3MgX 2 Phosphorous Oxides and Oxoacids

! When P 4 is burned, either P 4O6 or P 4O10 is formed, depending on the oxygen supply.

• Both structures have Td symmetry. O P O O P O O O O P P ` P O P P O O O P O O O O O

! Adding P 4O10 to water gives several oxoacids, most importantly orthophosphoric acid, H 3PO 4 (usually called phosphoric acid).

P4O10 + 6H 2O ÷ 4H 3PO 4 (+ others) • In addition to P 4O10 hydrolysis, H 3PO 4 is commercially prepared as follows:

Ca 3(PO 4)3 + 3H 2SO 4 ÷ 2H 3PO 4 + 3CaSO 4 • Phosphoric acid is supplied as an 85% solution with a syrupy consistency.

• The structure is C3v. O

P HO HO OH

•H3PO 4 is triprotic: –3 –8 –13 K1 = 7.5 x 10 , K2 = 6.2 x 10 , K3 = 4.2 x 10 Phosphorous Oxides and Oxoacids (cont.)

! Adding P 4O6 to water gives orthophosphorous acid (also called phosphorus acid or phosphonic acid).

P4O6 + 6H 2O ÷ 4H 3PO 3 •H3PO 3 is a deliquescent, clear solid. • It has a tetrahedral structure with a non-acidic P–H bond

(Cs). O

P H HO OH

•H3PO 3 is a diprotic acid: –2 –7 K1 = 1.6 x10 , K2 = 7 x 10

! Hypophosphorous acid (also called phosphinic acid), H 3PO 2, is obtained as a white solid following oxidation of PH 3 with I 2 in water:

PH 3 + 2I 2 + 2H 2O ÷ H 3PO 2 + 4HI • It has a tetrahedral structure ( Cs) with two non-acidic P–H bonds. O

P H H OH –2 •H3PO 2 is a monoprotic acid with K . 10 . Higher Phosphoric Acids and Anions

Formula Name pK1 pK2 pK3 pK4 pK5

H4P2O6 hypophosphoric 2.2 2.8 7.3 10.0 —

H4P2O7 pyrophosphoric 0.85 1.49 5.77 8.22 —

H5P3O10 triphosphoric <0 0.89 4.09 6.98 9.93 O OH OO OH PP HO P P HO HOO OH O HO OH

O O O

P P P HO OH O O HOOH OH Oxides and Oxoacids of As, Sb, Bi

! As, Sb, and Bi form the +3 oxides when burned in air (n.b., greater stability of lower state for heavier group 15 elements).

• As 4O6 and Sb 4O6 are molecular and isostructural with P 4O6. • Bi 2O3 is ionic

! As and Sb +5 oxides are not isostructural with P 4O10 , and their formulas are usually written As 2O5 and Sb 2O5.

! Arsenic acid, H 3AsO 4, is analogous to H 3PO 4 but somewhat weaker. –3 –7 –12 K1 = 5.6 x 10 , K2 = 1.0 x 10 , K3 = 3.0 x 10 • As the Marsh test suggests, it is a moderately strong oxidizing agent. + – o H3AsO 4 + 2H + 2 e º H 3AsO 3 + H 2O E = +0.559 V

! Arsenous acid, H 3AsO 3, functions as a monoprotic acid with –10 Ka = 5.1 x 10 . • Raman spectra show that in acidic solutions of As 4O6 the only detectable species is pyramidal As(OH) 3 ( C3v).