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Ch 15. Group 15 N2 / O2 separation BP MW main uses N2 77K 28 inert gas/coolant O2 90K 32 fuel/medial 2 Elemental Forms N2 B(N≡N) = 946 KJ/mol (kinetically inert) N2 fixation: N2 + 3 H2 400C, 200 atm, Fe cat 2 NH3 Haber process, industrial source of all N compounds 3 N2 chemistry 6 Li + N2 Li3N 2 [Ru(NH3)5(H2O)]2+ (aq) Cat process to N cmpds? bacteria N2 NH4+ cat = nitrogenase enzyme w/ Fe4S4 cage + MoFe7S8 cofactors (ferrodoxins) 4 P allotropes black P white P Molecular, Td, rapidly oxidized to phosphate in air red P many polymorphs, air stable hexagonal puckered sheets prep from high P or Bi flux, air stable 5 History From “The 13th Element: The Sordid Tale of Murder, Fire, and Phosphorus” by John Emsley 6 Elemental forms As, Sb, Bi incr. metallic character Single vs. multiple bonds D(E-E) N 163 P 201 O 142 S 264 D(E=E) 409 D(E≡E) 946 447 431 NN -P-PO=O -S-S- generally in the p-block, -bonds are uncommon except with period 2 elements 7 Halides almost all group 15 halides are air sensitive: PCl3 + ½O2 O = PCl3 PCl3 + 4H2O H3PO4 + 3HCl + H2 oxidation + hydrolysis oxidation all pentahalides hydrolyze rapidly and generate HX N forms endoergic halides NF3 to “NI3” show decreasing stability NF4+ is isostructural to ammonium and is the only stable N(V) halide P to Bi MX3 MX5 C3v D5h MX6 all are known for X = F, most for Cl, some for Br,I Oh 8 Halides PF5 to BiF5 show increasing Lewis acidity ex : PF5 + F PF6 ΔH = - 340kJ/mol SbF5 + F SbF6 - 500kJ/mol SbF5 is an oligomeric, viscous, colorless liquid Heavier congeners tend to higher CN (SbF5)4 9 Group 15 Frost diagrams 10 Group 15 redox trends • NO3 and Bi(V) are strong oxidants • NO3 should be the strongest oxidant from general periodic trend down a group (higher χ and higher IE result in less stable high oxidation state). But there is no regular trend. • Bi(III) is unusually stable due to inert pair effect • PO43 is unusually stable due to strong P=O bonding • Low pH increases oxidation strength of nitrogen oxoanions and also often increases rate (via protonation of N-O bonds) • most reactions are slow and many species are kinetically stable ex: NO2 , N2O, NO, NO2 N2O4 11 N oxides N2O4 is isoelectronic w C2O42 (oxalate). Since C has lower χ than N, oxalate has a stronger M-M bond and there is no appreciable equ w/ monomer 12 N oxides 13 N oxides 4 HNO3 (aq) 4 NO2 (aq) + O2 (g) + 2 H2O (l) More rapid for conc. HNO3 due to presence of undissociated acid Easier to break N-OH vs N=O N2O (g) + 2 H+ (aq) + 2 e- N2 (g) + H2O (l) E = + 1.77 V at pH = 0, but it’s a poor oxidant due to slow reaction kinetics NO+ (solv) + e- NO (g) E ~ + 1.1 V, nitrosyl cation is a facile oxidant with rapid kinetics 14 Low oxidation state N Ox state pKb -3 -1 -2 Ammonia hydroxylamine hydrazine 4.8 8.2 7.9 also N3- (azide) which is isoelectronic with CO2 and N2O NaN3 Na (m) + 3/2N2 (g) 15 Pourbaix diagrams 16 P oxides 17 Phosphates 18 P oxides mostly strong reducing agents (except for P(V)), especially in base Generally labile reactions Ox state +1 H2PO2 (hypophosphite) H3PO2 is monoprotic +3 HPO32 (phosphite) H3PO3 is diprotic +5 PO43 (phosphate) Td H3PO4 is triprotic Anhydride P4 O 6 acid H3PO3 H2O P4O10 H3PO4 19 Sb2O4 20 PS compounds Matches: P4S3 + KClO3 + filler/glue/water = strike anywhere KClO3 (head) and red P (stripe) = safety P4S3 21 PN compounds Note that –P=N- is isoelectronic with –Si=O- (siloxanes) 130 C nPCl3 + nNH4Cl These hydrolyze in air to form phosphate and HCl (Cl2PN)n + 4n HCl n = 3 or 4 dichlorophosphazene trimer or tetramer P4(NR)6 planar but not aromatic oligomer 290 C + Lewis acid initiator An air stable polyphosphazene (Cl2PN)n polydichlorophosphazene, elastomeric at RT 2n NaOR (can be OR , NR2) [(RO)2PN]n 22 Arsine ligand 4 As + 6 CH3I → 3 (CH3)2AsI + AsI3 (CH3)2AsI + Na → Na+(CH3)2As- + NaI o-C6H4Cl2 / THF Na+(CH3)2As- → o-C6H4(As(CH3)2)2 soft LB, bidentate [PdCl6]2- 23 Organoarsine chemistry As(CH3)3 + CH3Br → As(CH3)4+Br- oxidative addition As(III) -> As(V) For As(Ph)3 , this does not work Ph3As=O + PhMgBr → Ph4As+Br(Td) + MgO acid-base exchange LiPh AsPh5 + LiBr 24 As-As bonding 2As(CH3)2Br + Zn → (CH3)2As-As(CH3)2 + ZnBr2 As5(CH3)5 25