Group 13 (3A) - The Elements - Boron

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1 Group 13 (3A) - The Elements - oron! oron is a hard, crystalline, black, semimetal found in borate ores such as borax, Na 2 4 O 5 (OH) 2 O, found in vast deposits in Death Valley, Nevada & California.! The element boron has powerful abilities to strengthen, toughen and make fire-resistant glasses, metals, wood, and fibers. It is used in approximately three hundred high-tech products. A few of its uses are as soldering flux, in welding rods, as preservatives for wood and fabric, as fire retardant, in insecticides, in pottery glaze, as antiseptics, in hybrid fuels, and in experimental fuel cells. 1! Impure boron is obtained by reduction of the oxide with Mg, followed by washing with alkali, HCl(aq), and HF(aq). 2 O 3 + 3Mg 3MgO + 2 (~95% pure)! High purity boron is obtained with difficulty by pyrolysis or reduction of a halide over a hot Ta, W, or N surface. 2I 3 (g) + 3H 2 (g) Ta oc 2(s) + 6HI(g) 1 Death Valley National Park website:

2 oron Physical and Chemical Properties! oron is very non-reactive and high melting (m.p. = 2300 o C), making it useful for fire resistant and high-temperature applications.! Naturally occurring boron consists of two stable isotopes, 10 (19.6%) and 11 (80.4%).! oron has several crystal forms, all containing 12 icosahedra (I h ). Individual icosahedra are linked by 3c-2e bonds.! oron, boron nitride (N), and carborundum (C), have hardness approaching diamond. Mohs scale: diamond = 10, C = 9.3! N has forms isomorphous with diamond and graphite, but it resists oxidation up to 800 o C.

3 Group 13 (3A) - The Elements - Aluminum! Aluminum is the third most abundant element and the most abundant metal. Samples of it were rare before the Hall-Héroult process.! Charles M. Hall in 1886 developed a technique for electrolyzing poorly conductive fused bauxite (Al 2 O 3 ) by dissolving in molten cryolite (K 3 AlF 6 ) oc Al 2 O 3 K 3 AlF 6 /elect. 4Al + 3O 2 Canada is the principal source of bauxite for American use.! Al is an active metal. 2(Al Al + 3e ) E o = 1.66 V 3(2H 2 O + 2e H 2 + 2OH ) E o = 0.83 V 6H 2 O + 2Al 3H 2 + 6OH + 2Al E o cell = 0.83 V! Aluminum in contact with air immediately forms an amphoteric oxide coating that passivates the metal. In acid or base the coating dissolves and the metal becomes reactive. Al 2 O 3 + 2OH + 3H 2 O 2Al(OH) 4 Al 2 O 3 + 6H + (aq) 2Al (aq) + 3H 2 O With the oxide coating removed, Al shows typical active-metal reactivity with acid. 2Al(s) + 6H + (aq) 2Al (aq) + 3H 2 (g)! The very exothermic heat of formation of Al 2 O 3 (ΔH o f = 1670 kj/mol) is the driving force of the Goldschmidt or thermite reaction. Fe 2 O 3 + 2Al Al 2 O 3 + 2Fe ΔH o = 849 kj/mol

4 Group 13 (3A) - The Elements - Gallium, Indium, Thallium! Ga, In, and Tl are rare elements.! All are soft, white, lustrous, and reactive metals with long liquid ranges. Ga has longest known liquid range: m.p. = 30 o C, b.p. = 2071 o C. Ga(l) wets glass, like H 2 O, and expands below its m.p.! Obtained by electrolysis of aqueous solutions of their salts. Ion Ga In Tl Tl + E o (V) M n+ M E(H 2 O) = 0.42 ph 7 Ga can be reduced from aqueous solution, despite an unfavorable E o, because of a high hydrogen over-voltage on Ga.

5 Group 13 onding Ionic Radii of Group 13 Elements Al Ga In Tl r + (pm) r (pm) ! The group-characteristic oxidation state is +3, but +1 becomes more important down the group. The stable state of thallium is +1. Tl + 2e Tl + E o = V Tl + + e Tl 0 E o =!0.336 V Tl + has an ionic radius intermediate between K + (133 pm) and Rb + (148 pm), resulting in very similar ionic chemistry.! In the +3 state, all have high charge density, so compounds have significant covalent character.! All boron compounds are covalent, although some (e.g., F 3 ) have very polar bonds. The F bond is the strongest known single bond (D = 757 kj/mol). There is no ionic boron chemistry.! Only Al 2 O 3 and AlF 3 are considered ionic among Al compounds.

6 Oxidation State and the "Inert Pair Effect"! Increasing stability of the lower state for heavier group elements in group 13 and succeeding groups is sometimes called the "inert pair effect" for the reluctance to lose the ns 2 pair. Tl [Hg 2+ ]6s 2 6p 1 Tl + [Hg 2+ ]6s 2 Tl [Hg 2+ ]! Increasing stability of the lower state is the result of rapidly declining bond strength and less rapidly declining ionization energy going down the group. Poor shielding by filled (n-1)d 10 subshell makes ionization energies of Ga, In, Tl comparable to Al. Very poor shielding of 4f 14 subshell makes ionization energies of Tl greater than In, despite larger size. Ionization Enthalpies (kj/mol) Al Ga In Tl M M M M Mean ond Enthalpies (kj/mol) 2 H F Cl r I Al Ga In Tl Data from P. Atkins, T. Overton, J. Rourke, M. Weller, and F. Armstrong, Inorganic Chemistry, 4 th ed., Freeman, NY, 2006, p. 289.

7 Ions in Solution! In aqueous solution, all M ions are acidic, although (aq) does not exist. Ion K a [(H 2 O) 3 ] >>10 +3 (?) Al(H 2 O) 6 Ga(H 2 O) 6 In(H 2 O) 6 Tl(H 2 O) x x x 10 4 ~1 x 10 1! Greater acidity of heavier ions is due to poor shielding by underlying d subshells (Ga, In, Tl ) and 4f subshell (Tl ).! If (H 2 O) 3 existed it would immediately hydrolyze to form boric acid, (OH) 3. (H 2 O) 3 + 3H 2 O (OH) 3 + 3H 3 O +

8 oric Acid! Orthoboric acid (boric acid) is unique in its acid hydrolysis, acting as a hydroxide acceptor, rather than a proton donor. (OH) 3 + 2H 2 O º (OH) 4 + H 3 O + pk a = 9.3! Made in vast quantities commercially by acidification of borax.! Individual (OH) 3 molecules are planar, C 3h.! In the solid, molecules are linked together in sheets by asymmetric hydrogen bonds, with large separation between sheets (318 pm), similar to graphite.! (OH) 3 is the expected hydrolysis product of many boron compounds, rather than the hydrated ion as with the other group 13 elements. Cl 3 + 3H 2 O (OH) 3 + 3HCl Al 2 Cl H 2 O 2Al(H 2 O) 6 + 6Cl

9 Group 13 Hydroxides! As charge density declines, the hydroxides go from acidic to amphoteric and then to basic. (OH) 3 Al(OH) 3 Ga(OH) 3 In(OH) 3 Tl(OH) 3 TlOH acidic amphoteric amphoteric basic basic basic! Hydrated Al(OH) 3 is precipitated as a gelatinous solid whenever Al (aq) is treated with a base: Al(H 2 O) 6 + 3OH Al(H 2 O) 3 (OH) 3 + 3H 2 O Al(H 2 O) 6 + 3NH 3 Al(H 2 O) 3 (OH) NH 4 Al(H 2 O) 6 + 3HCO 3 Al(H 2 O) 3 (OH) 3 + 3CO 2 + 3H 2 O! A strong base is required to make Al(OH) 3 behave as an acid: Al(H 2 O) 3 (OH) 3 + OH Al(H 2 O) 2 (OH) 4 + H 2 O Al(H 2 O) 3 (OH) 3 + NH 3 no rxn Al(H 2 O) 3 (OH) 3 + HCO 3 no rxn! Treating solutions of In (aq) or Tl (aq) with base only gives the hydroxide: In(H 2 O) 6 + 3OH In(H 2 O) 3 (OH) 3 + 3H 2 O In(H 2 O) 3 (OH) 3 + OH no rxn! Tl(H 2 O) 6 (D 4h ) is so acidic that the hydrous oxide precipitates even at ph

10 oron Oxygen Compounds! oron has a large and complex chemistry, including boronoxygen compounds.! The oxide is formed by fusing boric acid: 2(OH) 3 2 O 3 + 3H 2 O 2 O 3 is a glass-like substance with random 3 O 3 rings connected by bridging oxygen atoms. Similarity to SiO 2 structure makes it possible to mix 2 O 3 in glass to make borosilicate glass (Pyrex ).! Oxoanions contain O 4 and O 3 units. The simplest oxoanion is (OH) 4, the conjugate base of (OH) 3. In concentrated solutions (OH) 4 polymerizes to form a variety of ions, predominated by 3 O 3 (OH) 4. The anion in borax, Na 2 4 O 5 (OH) 2 O, is 4 O 5 (OH) 4 2 : HO O O O OH OH O O OH 2-

11 oron Trihalides! All trihalides, X 3 (X = F, Cl, r, I), have a trigonal planar structure (D 3h ). The V model has with sp 2 hybrids with an "empty" p z orbital that serves as a site of nucleophilic attack by a Lewis base when X 3 functions as a Lewis acid. In the MO model, the LUMO is π*(a 2 "), which involves overlap of 2p z with the A 2 " SALC formed from np z orbitals on the three X atoms.! Lewis acid strength increases in the order F 3 << Cl 3 < r 3 < I 3. With small amounts of water F 3 forms Lewis acid-base adducts F 2 O and F 2 O, but it does not readily hydrolyze. When small amounts of F 3 (g) are passed through water, a solution of fluoroboric acid results: 4F 3 + 6H 2 O 3H 3 O + + 3F 4 + (OH) 3 The others hydrolyze completely and vigorously (I 3 explodes!). X 3 + 3H 2 O (OH) 3 + 3HX X = Cl, r, I

12 Lewis Acid Strength of X 3 Compounds! The order of the X 3 Lewis acid strengths, F 3 << Cl 3 < r 3 < I 3, is contrary to expectations based on steric or electronegativity arguments.! The "classic" explanation for the order of Lewis acid strengths is the effectiveness of pi bonding as an inhibition to forming CN4 coordination about the boron atom. Calculations suggest that the order of pi-bond strength is F 3 > Cl 3 > r 3 > I 3. Adduct formation of the type X 3 + :Y X 3 Y results in tetrahedral coordination about, which precludes effective pi bonding.! Size alone is not the principal factor, because F 4 is quite stable, but Cl 4 and r 4 can only be stabilized with large cations such as Cs + and N(CH 3 ) 4 +.! Low F 3 acid strength may have more to do with the strength of the F bond and the unfavorable thermodynamics to lengthen it in forming tetrahedrally coordinated in adducts.! There is no single explanation that is completely satisfying!

13 orazines! orazines are N analogues of benzene.! -trichloroborazine, 3 N 3 H 3 Cl 3, can be synthesized by refluxing NH 4 Cl and Cl 3 in chlorobenzene: C 6 H 5 Cl 3NH 4 Cl + 3Cl 3 + 9HCl oc H Cl N Cl N! orazine, 3 N 3 H 6 (b.p. 55 o ), is formed by reaction of 3 N 3 H 3 Cl 3 with NaH N 3 H 3 Cl 3 + 3NaH N 3 H 6 + 3NaCl 4! -trimethylborazine, 3 N 3 H 3 (CH 3 ) 3, is formed from 3 N 3 H 3 Cl 3 by reaction with methyl magnesium bromide. 3 N 3 H 3 Cl 3 + 3CH 3 Mgr 3 N 3 H 3 (CH 3 ) 3 + 3MgrCl! Aminoboranes are ethane analogues: C 6 H 5 Cl CH 3 NH 2 + Cl 3 Cl 3 NH 2 CH 3 m.p o C boil! Polarity of the N bond favors addition over substitution: 3 N 3 H 6 + 3HCl 3 N 3 H 9 Cl 3 Cl H H N H N H H H H Cl Cl N H H H N H Cl -trichlorocycloborazine

14 oranes! oranes are boron hydrides, which were first prepared by Alfred Stock in the period , using acidification of Mg 2 to yield a mixture of boranes. Most boranes are flammable, so Stock developed glass vacuumline apparatus and techniques to do the work. Air flammability decreases with molecular weight, becoming stable at 6 H 10, and 10 H 14 is very stable. Most are liquids, but 2 H 6 is a flammable gas, and 10 H 14 is a white solid (m.p 99.7 o ) stable in air.

15 Synthesis of oranes! The simplest isolable borane is diborane(6), 2 H 6, which can be made in quantitative yield in ether at room temperature in a vacuum line: ether 3NaH 4 + 4F H 6 + 3NaF 4 A convenient laboratory synthesis 3 is diglyme 2NaH 4 + I 2 2 H 6 + 2NaI + H 2 Industrial quantities are prepared by the following reaction: 2F 3 + 6NaH 2 H 6 + 6NaF! It mixes well with air and easily forms explosive mixtures. Diborane will ignite spontaneously in moist air at room temperature. Diborane is used in rocket propellants, as a reducing agent, as a rubber vulcanizer, as a catalyst for hydrocarbon polymerization, as a flame-speed accelerator, and as a doping agent. 4! Thermal decomposition of 2 H 6, resulting in transient H 3, leads to higher boranes. 2 2 H 6 [H 3 ] + 3 H 9 higher boranes 3 Diglyme = diethylene glycol dimethyl ether = 4 U.S. Department of Health and Human Services, Public Health Service Agency for Toxic Substances and Disease Registry,

16 onding in oranes! onding in boranes defies simple V modeling.! The following bond types are used to describe borane structures: Terminal 2c-2e boron-hydrogen bond H 3c-2e Hydrogen bridge bond H 2c-2e boron-boron bond Open 3c-2e boron bridge bond Closed 3c-2e boron bond! Complete description requires an MO approach for each compound.

17 Compounds of Al, Ga, In, Tl! All trihalides are known, but TlI 3 is [Tl + ][I 3 ].! Trihalides of Al, Ga, and In are more stable than those of Tl. Fluorides are ionic [M ][X! ] 3. Other trihalides are dimeric with normal 2c-2e bridge bonds. X X M X X M X X M = Al, Ga, In; X= Cl, r, I! Hydrides are limited to simple tetrahedral species; e.g., AlH 4, GaH 4, R 3 N:AlH 3.! Allane, AlH 3 may exist in the gas phase, but the solid is polymerized (AlH 3 ) n. Failure to form analogues to the boranes results from weaker M M bonds and greater size, which precludes M H M bridge bonds.! Only important organometallic compounds are those of aluminum. With simple R groups, these are dimeric Al 2 R 6, with a C of the R group making 3c-2e bridge bonds. R R M R R M R R R = CH 3, C 6 H 5, cyclo-c 3 H 5, H 2 C=CH 2 onding in Al 2 R 6 is similar to polymeric e(ch 3 ) 2. D 2h D 2h