Chemistry of the Main Group Elements: Hydrogen, Alkali, and Alkaline Earth Metals PDF

Chemistry of the Main Group Elements: Hydrogen, Alkali and Alkaline Earth Metals Sections 8.1-8.4 Wednesday, November 4, 2015 M. Al-Anber  Hydrogen Obviously the simplest element, with a 1s1 elec...

Chemistry of the Main Group Elements: Hydrogen, Alkali and Alkaline Earth Metals Sections 8.1-8.4 Wednesday, November 4, 2015 M. Al-Anber  Hydrogen Obviously the simplest element, with a 1s1 electron configuration. placement on the Periodic Table is questionable: is it an alkali metal, is it a halogen, or should it be placed above carbon (half-filled valence shell)? Electron Affinity kJ/mol Ionization Energy Electronegativity  Hydrogen Production Laboratory Scale Zn s   2HCl aq   ZnCl2 aq   H 2 g  Steam Reforming CH 4 g   H 2O g  1000C  Nicat CO g   3H 2 g   kcal H rxn  49.3 mol Driven by C s   H 2O g  1000C  CO g   H 2 g  entropy!  kcal H rxn  31.4 mol Water-Gas Shift Reaction CO g   H 2O g  300C  CO2 g   H 2 g   kcal H rxn  9.8 mol  Hydrogen Use Ammonia Production N 2 g   3H 2 g  100 450C  atm 2NH 3 g   kcal H rxn  11 mol Medicinal Chemistry L-DOPA Food Chemistry (Parkinson’s)  The Hydrogen Economy Hydrogen is an attractive fuel because of its high heat of combustion and zero pollution 1 H 2 g   O2 g   H 2O g  2  kcal H rxn  57.8 mol The problem: our hydrogen comes from fossil fuels CH 4 g   H 2O g   CO g   3H 2 g   kcal H rxn  49.3 mol kcal CO g   H 2O g   CO2 g   H 2 g   H rxn  9.8 mol 2 4H 2 g   2O2 g   4H 2O g   kcal H rxn  231 mol kcal CH 4 g   2O2 g   CO2 g   2H 2O g   H rxn  192 mol Same overall result as burning methane: same energy out, same CO2 out. To be clean, H2 must come from something other than fossil fuels.  Types of Hydrogen Compounds Metallic Hydrides conducting hydrides (MgH2, NiHx) often non-stoichiometric, i.e., [MHx] where x < 1 (PdHx) Saline Hydrides salt-like solids of alkali and alkaline earth metals non-conducting characterized by a reduced hydrogen, i.e., [M+H–] Molecular Hydrides electron precise compounds: CH4, SiH4, GeH4 basic covalent hydrides: NH3, PH3, AsH3 weak-acid covalent hydrides: H2O, H2S strong-acids: HF, HCl, HI electron-deficient hydrides: B2H6 anionic hydrides: BH4–, AlH4–  Hydride Stability Formation of saline hydrides generally is exoergic (∆G < 0) Formation of acids is mostly exoergic ∆Gf of covalent hydride compounds can be exoergic or endoergic 2nd row and lower are endoergic – as such, compounds like SiH4 are extremely reactive Group 1 2 13 14 15 16 17 Period IA IIA IIIB IVB VB VIB VIIB 2 LiH(s) BeH2(s) B2H6(g) CH4(g) NH3(g) H2O(l) HF(g) –16.4 +4.8 +20.7 –12.1 –3.9 –56.7 –65.3 3 NaH(s) MgH2(s) AlH3(s) SiH4(g) PH3(g) H2S(g) HCl(g) –8.0 –8.6 ~0 +13.6 +3.2 –8.0 –22.8 4 KH(s) CaH2(s) Ga2H6(s) GeH4(g) AsH3(g) H2Se(g) HBr(g) –8.6 –35.2 >0 +27.1 +16.5 +3.8 –12.8 5 RbH(s) SrH2(s) SnH4(g) SbH3(g) H2Te(g) HI(g) –7.2 –33.6 +45.0 +35.3 >0 +0.4 6 CsH(s) BaH2(s) –7.6 –33.4 kcal/mol  Hydride Synthesis and Reactivity Synthesis Direct reaction (radical based) 2E  H 2  2HE Protonation (transfer of H+) E   H 2O   HE  OH  Metathesis (transfer of H–) EX  MH  MX  HE Reactivity Patterns Homolytic cleavage H   E HE  Hydride Transfer E  H  HE  Proton Transfer E  H  HE   Alkali & Alkaline Earth Metals Naturally occurring in various minerals rock salt (NaCl) carnallite (KCl MgCl2 6 H2O) beryl (Be3Al2(SiO3)6) dolomite (CaCO3 MgCO3) limestone (CaCO3)  Alkali & Alkaline Earth Metals Li, Na and K were discovered by electrolysis (1807-1818) Cs and Rb were discovered spectroscopically in mineral spa waters Lithium is a very important metal Li2CO3 – flux in porcelain enamels, hardening agent for glass, therapeutic for manic- depressive psychoses Li0 – lightens and strengthens aircraft aluminum, alloyed with Mg for armor plate LiOH – CO2 absorber in space capsules and submarines  Solvated Electron Solutions All alkali metals (plus Ca, Sr, & Ba) dissolve in liquid ammonia 3  M 0  M  NH 3 n  e NH 3 n NH l Dilute Solutions dark blue in color (λmax ≅ 1500 nm) diagnostic of a ‘solvated electron’ paramagnetic useful reducing agent (Birch reduction) Concentrated Solutions metallic bronze color conductivity like a molten metal weakly paramagnetic  Alkalides and Electrides Crown ethers and cryptands are special Lewis bases designed to selectively bind metal cations. 18-crown-6 dibenzo-14-crown-4 benzo-15-crown-5 [2.2.2] cryptand 260-280 pm 120-150 pm 170-220 pm K+, Sr2+ Li+, Mg2+ Na+, Ca2+ Alkalides K+ 2Na 0  2.2.2 cryptand  2   Na cryptand   Na  EtNH l  Electrides Cs crown   e  Cs 0  18crown6   Cs 15-crown-5 Sandwich Electride Cs+(15c5)2e-  Organometallic Chemistry Arene Reduction ethereal solvents (ether groups act as Lewis bases to prevent aggregation) reduced arene is deep green to deep blue similar to the solvated electron Organolithium Reagents 2Li 0  RX solvent  LiR  LiX alkane, arene, or ethereal solvents works best for alkyl derivatives most stable for R = Me, nBu, tBu LiR is actually a higher order cluster depending on R group LiR used as very strong base, or for nucleophilic addition of R-  More Organolithium Chemistry Aryl derivatives accessible by metal-halogen exchange tBuLi  PhI solvent  PhLi  tBuX Unsaturated derivatives accessible by transmetallation tetravinyl tin vinyl lithium General Reactivity RLi  X2  RX  LiX RLi  HX  RH  LiX RLi  R' X  RR' LiX  Organomagnesium (Grignard) Chemistry Mg 0  RX  RMgX reactivity is I > Br > Cl and alkyl > aryl mechanism is poorly understood Sample Reactivity Formation of primary alcohols RMgX  O2  ROOMgX RMgX  2ROMgX 2 HX 2ROH  2MgX2 Formation of substituted alcohols RMgX  HX  MgX2   Organomagnesium Reactivity In general Grignard reagents always react as the carbanion (nucleophile) to attack an electrophile:  Organomagnesium Reactivity In general Grignard reagents always react as the carbanion (nucleophile) to attack an electrophile: