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Chemistry of Group 13 Elements

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Chemistry of Group 13 Elements Chemistry of Group 13 Elements (Prepared by Prof. S. Bhattacharya as a study material for B.Sc. Sem II students) GENERAL FEATURES Element (symbol) Electronic Configuration Oxidation State Atomic / Metallic Radius (Å) B [He] 2s22p1 III 0.885 Al [Ne] 3s23p1 (I) III 1.43 Ga [Ar] 4s24p1 I III 1.225 In [Kr] 5s25p1 I III 1.67 Tl [Xe] 6s26p1 I III 1.70 • Oxidation state III is common, however, stability of oxidation state (I) increases down the group. (Inert pair Effect) • Anomalous reduction in size from Al to Ga is due to the presence of 10 d electrons in the penultimate shell which shield the nuclear charge less effectively • Boron is a typical non-metal obtained by reduction of B2O3 as an amorphous black solid. All other elements are metals. • Aluminum is the most abundant element in the earth’s crust. Gallium has a very low M.P. (30 oC) though its density (at room temp) more than twice of that of Al. • Oxidation state of (I) is more stable in case of Tl. Tl(I) salts are poisonous. OXIDES OF BORON AND RELATED COMPOUNDS 1. Orthoboric Acid Structure 2 B(OH)3 is trigonal planar (sp hybridization of orbitals of central boron). In solid state B(OH)3 units are hydrogen bonded together into 2D sheets with almost hexagonal symmetry. Distance between any two layers is quite large (3.18 Å) as a result crystals easily break into fine powders H H O O H H H B H O O O B H B H H H O O O O H H H H O O O O H H H B H B O O O H H H B H O O H H Chemical Nature - H3BO3 [or B(OH)3] is a weak acid. It does not give protons but accepts OH ion (Lewis acid) and only partially reacts with water. 1 H O+ + [B(OH) ]- B(OH)3 + 2H2O 3 4 (pK = 9.25) (eq-- i) At higher concentrations polymeric metaborate species are formed. H O+ + [B O (OH) ]- pk = 6.84 (eq-- ii) 3H3BO3 3 3 3 4 HO B O OH O B OH B O HO Thus, B(OH)3 cannot be titrated properly with NaOH as a sharp end point is not observed. In presence of cis-diols the titration can be performed satisfactorily because such compounds shift the above written equilibrium (eq-- i) to the right hand side completely. HO OH HO HO O O B O B + HO O OH B HO OH HO O O OH When heated, orthoboric acid is dehydrated to give metaboric acid and finally to boron sesquioxide 2. Boron Sesquioxide (B2O3) and Borates Boron and all other elements of the group when heated in oxygen give respective sesquioxide. As mentioned already, dehydration of orthoboric acid also gives boron sesquioxide o 100 C red heat B O B(OH)3 HBO2 2 3 orthoboric acid metaboric acid boron sesquioxide Chemical Nature Boron sesquioxide is amphoteric. It reacts with metallic oxides (bases) forming salts called borates or metaborates. In borax bead test boron sesquioxide (or borax) is mixed with a metal oxide and heated on a flame forming a glass like bead. The beads of many transition metals are characterized by their colors. (Borax bead test). This test provided first proof that vitamin B12 contains cobalt. CoO + B2O3 Co(BO2)2 cobalt metaborate (Blue) B2O3 may also behave like a base if reacted with a strongly acidic compound such as phosphorus pentoxide giving boron phosphate. 2BPO B2O3 + P2O5 4 2 3- Orthoborates contain discrete BO3 ions. For example Mg3(BO3)2, LnBO3 (Ln is a lanthanide metal ion). In metaborates simple (BO3 triangles or BO4 tetrahedra) join together to form a variety of polymeric chain and ring structures. Two units join together sharing one corner it is called pyroborate 4- O O B O B O O Three units join together sharing corners may form a ring as in the case of sodium and potassium metaborates O B O O B B O O O Many triangular units may polymerize into an infinite chain O O B B O O O O O B B O O Similarly tetrahedral units may also be present as discrete ions, sharing one corner, sharing two corners forming a ring or a polymeric chain. A few other Compounds: O O B O O B O B O O B B O O O A complex metaborate such as K[B5O6(OH)4] consists of a spiro anion in which there are 4 triangular BO3 units and a central tetrahedral BO4unit. Borax, Na2[B4O5(OH)4].8H2O This is the most common metaborate used as a primary standard for titrating against acids. Na2[B4O5(OH)4].8H2O + 2HCl 4H3BO3 + 2NaCl + 5H2O. Two moles of HCl are consumed because when borax is dissolved in water both B(OH)4 and B(OH)3 are formed but only 3 B(OH)4 reacts with HCl. The end product of this titration is H3BO3 (again an acid but very weak) thus indicator used in this titration should not be affected by it. Methyl orange is one such indicator that changes color in the pH range 3.1-4.4 OH B O O HO B O B OH O O B OH Structure of borax. Sodium Peroxoborate is a brightener in washing powders. 2- HO O O OH B B HO O O OH Peroxoborate ion. Oxides of other elements Alumina (Al2O3) There are 2 crystalline forms and 1 fibrous form: 1. Corundum (- alumina)- Very inert material . Consists of hcp lattice (formed by Oxygen atoms) and two thirds of octahedral holes are occupied by Al3+ ions. Extremely hard material (9 on Moh’s scale); used for cutting glasses. When impurity of iron oxide and silica are present the material is called emery. 2. -alumina is used in chromatography. It dissolves in acids 3. Fibrous alumina: fine silky fibers are commercially made which find application in chemically inert paper, rope and fabric. Al(OH)3, Ga2O3,Ga(OH)3 are amphoteric while Tl2O3 and In2O3 are basic and do not form hydroxides or hydrates. TlOH resembles alkali metal hydoroxides Tetrahydroborates (borohydrides) - Thetrahydroborate (BH4 ) ion is tetrahedral. The alkali metal salts are white ionic solid. The sodium salt being the most common, used as reducing agents in organic reactions. LiBH4 reacts violently with water 4 NaBH4 can be recrystallized from cold water KBH4is quite stable. (Correlate the reactivity with size of cations) Beryllium, aluminum and transition metal borohydrides become increasingly covalent and volatile. The structure of Be(BH4)2 given in J.D Lee’s book is the following H H B H H H H B B H H However, the structure reported by Marynic and Lipscomb is helical. Al(BH4)3 H H B H H H H Al B H H H H B H H Zr(BH4)4 2- 1. Closo-boranes BnHn H H B H H H H H H B Zr B H H H H H H B H H Hydrogen bridges are present in all these compounds. These are (electron deficient) 2-electron-3- center bonds. AlH3 is also an electron deficient compound. It is extensively polymerized by 3-center bonds. Li[AlH4] is a useful reducing agent. Boron Hydrides Diborane B2H6 is a covalent colorless gas which is highly reactive. It reacts with oxygen to give B2O3, with water to give H3BO3 and with ammonia to give products as shown below: 5 B2H6.2NH3 ow H /L ess N 3 Exc e ratur empe (BN)x B2H6 t Excess NH3/Higher Temp NH 3 in 2:1 mol ar rat o/ Hi B N H gher 3 3 6 Temp Other than diborane there are a number of higher Boron hydrides or higher boranes which are broadly classified into three categories: Type Formula Example 2- BnHn H Closo-boranes B H B (Closed structure) B H H B B H B 2 - B6H6 H Nido-boranes BnHn+4 H B H B H H H H B B (Nest like structure) H H B B5H9 H H Arachno-boranes BnHn+6 H B H H H (Spider’s web like structure) B H H H B H B B4H10 H Besides normal 2-electron-2-centre bonds these molecules may also have electron deficient BBB and BHB bonds. Diborane: Structure Bonding H 1. 33 H H Ao o o B 96 B 122 1 .1 o 9 A H H H 6 Important Features: 1. Molecule has a centre of symmetry. 2. Presence of Bridging hydrogen atoms (Note: valency of H is 1 in most of its compounds but the bridging hydrogens here are bonded to 2 different B atoms). 3. Four terminal Hydrogens are different from the two bridging ones as confirmed by Raman spectroscopy and only the terminal hydrogens can be substituted (resulting in B2H2Me4) without breaking the molecule into BMe3. Diborane: Bonding Consider any of the two boron atoms. It has 3 electrons in its valence shell (2s22p1). Classically, these three electrons can participate in three covalent bond formations only, however it is forming 4 bonds. Compounds like BF3 are Lewis acids and can accept an electron pair (lone pair) from a suitable donor like NH3 to form adducts such as BF3NH3. In these adducts B is tetrahedral and has coordination no. 4. However, in diborane all the 4 bonds are with hydrogen atoms only. None of the hydrogens (each having only one electron) can act as a lone pair donor in this molecule.
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