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Ch 17. Group 17

Ch 17. Group 17. Prepn of elements. F 2 colorless gas Mineral source of F is CaF 2 or Na 3 AlF 6 (cryolite) 2 HF  H 2 + F 2 E  = -2.87 V (Moisson, 1886)

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Ch 17. Group 17

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  1. Ch 17. Group 17

  2. Prepn of elements F2 colorless gas Mineral source of F is CaF2 or Na3AlF6 (cryolite) 2 HF  H2 + F2 E = -2.87 V (Moisson, 1886) Fluorine is very reactive with almost all other elements, but not with materials such as (CF2)n, SiO2 (dry), Cx at RT Chemical prepn: K2MnF6 + 2 SbF5  2 KSbF6 + MnF3 + ½ F2 anhyd KF, electrolysis

  3. Fluorine vacuum line Ni, SS, or Monel, passivation layer such as NiF2

  4. Prepn of elements Cl2 pale yellow gas, source = brine 2 Cl Cl2 + 2e E = -1.36V 13M tons in 2004 Since H2O/O2 = 1.23 V (or higher at pH > 0), Cl2 production requires there be a greater overpotential for O2 than for Cl2 (RuO2 electrode works best) Br2 deep red liquid, source = brine Cl2 + 2 Br Br2 + 2 Cl E = +0.26V 0.4M tons in 2000 I2 violet solid that sublimes, source = brine, kelp, or NaIO3 Oxidation of iodide with Cl2 21k tons 2003

  5. Periodic Trends Xp Ea D(X2) D(HX) D(CX4) DHf(LiX) E(X2/X-)** F 4.0 328* 159 574 456 1037 2.8 Cl 3.0 344 243 428 327 832 1.4 Br 2.8 325 193 363 272 813 1.1 I 2.5 295 151 294 239 750 0.5 * all in kJ/mol ** V vs SHE for e- + ½ X2(g)  X- (aq)

  6. Periodic Trends Reactivity of F2 >> Cl2 > Br2 > I2 Reason for reactivity for F2 is the strong bond of F with other elements and weak F-F bond ex: Hhyd(F) - Hhyd(Cl) = 143 kJ/mol = 1.48 eV note that difference in E(X2/X-) is similar (1.4 V) High E to generate F2 means F ligands can often stabilize high ox states ex: PtF6, PbF4, BiF5, IF7 none of these exist with other halides High c means substitution can lead to incr. acidity CH3SO3H pKa ~ -2 CF3SO3H pKa ~ -15 inductive effect of F

  7. Interhalogens: Structures: Larger atom is always central VSEPR works well XY XY3 XY5 XY7 ClF ClF3 ClF5 BrF BrF3 BrF5 IF (IF3)n IF5 IF7 BrCl ICl (ICl3)2 IBr pentag. bipyr (D5h)

  8. Interhalogens: Prepn, reactions: I2 + Cl2 2 ICl Br2 + 3F2 2 BrF3 All interhalogens are good oxidizing agents, unstable in air ClF3 and BrF3 are very good fluorinating agents BrF3 + asbestos  burns brightly liberating O2(g) + Br LA LB 2 BrF3 BrF2+ + BrF4 KF is a LB, add to incr [BrF4] SnF4 is a LA, add to incr [BrF2+]+ (BrF2)2SnF6  SnF4 + 2 BrF3 (Sn + 2 F2 is not practical due to passivation layer) Sn + 2 Br2 SnBr4 (l)  BrF3

  9. Polyhalides and cations: XYnor Xn+1 (n even) n = 2 I3,Br3 (linear) ICl2, IBr2, BrCl2, IBrCl n = 4 I5 ClF4, ICl4, IBrCl3 sq. planar n = 6 I7 ClF6 XYn+ or Xn+1+ (n even) I3+ ClF2 +, BrF2+ (all C2v) XF4+ X = Cl, Br, I XF6+ (IF7 + SbF5  IF6+SbF6)

  10. Polyhalides I3- is linear (12 valence orbitals, 22 e-, so just maximize E of unfilled s* orbital)

  11. Polyiodide structures

  12. Conduction mechanism in polyiodides Li / I2 batteries: http://www.greatbatch.com

  13. :B I2 Iodine complexes transition E in uv for F2, but HOMO-LUMO transition decreases from F2 to I2

  14. Halogen Oxides F: OF2 MP ~ -224 C, BP ~ -145 C easily hydrolyzed to HF + O2 O2F2 very strong oxidant and fluorinator Pu (s) + 3 O2F2(g)  PuF6 (g) + 3 O2(g) Cl, Br, I: oxoacids, oxoanions ox. state pKa conj. base +1 HXO (hypochlorous acid) C∞v 7.5 XO (hypochlorite) +3 HXO2 (chlorous acid) C2v 2.0 XO2 (chlorite) +5 HXO3 (chloric acid) C3v -1.2 XO3 (chlorate) +7 HXO4 (perchloric acid) Td -10 XO4 (perchlorate)

  15. Halogen Oxides Trend in acidity similar for Cl, Br, I Pauling’s rules pKa = 8 - 5p for XOp(OH)q Note exception: IO4(periodate) has pKa = 3.3 it is actually HIO4 + 2H2O H5IO6 (p = 1)

  16. Frost diagrams all ox states above -1 are strong oxidants disproportionation favorable for many species oxyhalides are stronger oxidants in acidic solutions In acid: ClO4+ 2 H+ + 2e-  ClO3+ H2O E = +1.2 V In base: ClO4+ H2O + 2e- ClO3+ 2 OH E = +0.37 V

  17. Oxyhalides • Reaction rates: • XO4  XO3  XO2 XO X2 • ex ClO4is kinetically stable in aqueous soln (but can explosively decompose in organic soln or anhydrides) • (reaction mechanism often involves nucleophilic attack on “X” which is shielded in XO4 • but ClOis a labile oxidant (bleach) • Rates with central atom Cl  Br  I • ex ClOundergoes slow disproportionation • but IOhas only been detected as a reaction intermediate • Rates increase in acidic solutions • X = O  X - OH (weaker interaction) H+

  18. Oxyhalides Representative reactions: Cl2 (aq) + 2 OH- (aq)  ClO (aq) + Cl- (aq) + H2O (l) fast ClO (aq)  2 Cl (aq) + ClO3(aq) slower 4 ClO3 (aq) 3 ClO4 (aq) + Cl (aq) very slow

  19. Fluorocarbons cat CHCl3 + 2 HF → CHClF2 + 2 HCl X exchange 2 CHClF2 → C2F4 + 2 HCl HX elimination n C2F4 → (C2F4)n polymerization D cat

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