UNIT 6 ELEMENTS OF GROUP 13 structure 6.1 Introduction Objectives I 6.2 Oailcrence, Extraction and Uses Occurrec - Extraction uses i 6.3 General Characteristics

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- 6.5 Halides of Bpron anel Aluminium Halides of Boron Halides of Aluminium 6.6 Oxides of Boron and Aluminium I Boric Oxide Aluminium Oxide 6.7 Oxoacids of Boron and Borates 6.8 Borazine 6.9 Complexation Behaviour 6.10 Anomalous Behaviour of Boron 6.11 Summary 6.12 Terminal Questions 6.13- Answers

' -- 6.1 INTRODUCTION

In the previous two units, you studied the main features of the chemistry of Group 1 and Group 2 elements, i.e. the alkali and the alkaline earth metals. In this unit you - will study the elements of Group 13, namely, boron, aluminium, gallium, indium and, thallium. While studying the alkali and alkaline earth metals, you have seen that all Zhe elements of these two groups are highly reactive metals and the first element of each group shows some differences from the rest. In Group 13, the differences between the first element and the remaining elements become so pronounced that the first member of the group, i.e. boron is a nonmetal wheieas the rest of the elements are distinctly metallic in nature. In a way, this is the first group of the periodic table in which you observe a marked change in the hature of the elements . down the group.

describe the chemistry of hydrides, halides and oxides of boron and aluminium, elucidate the structures of hydrides of boron and aluminium,

6.2 OCCURRENC~,EXTRACTION AND USES

Elements bf Group 13 are sufficiently reactive. Therefore, none of them occurs in the native state. Some of these elements and their compounds find important uses in diverse,areas of modern science and technology and even in every day life. Let us, therefore, first study their occurrence, extraction and raes. 5 6.2.1 Occurrence Both boron and aluminium have a high affinity for oxygen, so neither of them is found native. Boron occurs principally in the earth's crust as boric acid, H3B03and as borates, such as, borax, Na2B407-10H20,kernite, Na2B407.4H20and colemanite, Ca2B,0,,.5H20. Aluminium is the most abundant metal (8.13%) in the earth's crust and is the third most abundant element, next only to oxygen (46.6%) and silicon (27.7%)). It occurs widely in the complex alumino-silicates, such as, clay from whicb, however, it cannot be extracted economically. The im ortant ores of aluminium are bauxite, A1,0,-xH20, where x = 1-3, cryolite, Na3!lF, and corundum, ~1~0;.Gallium (1.9 x lo--'%), indium\(2.1 x lod5%) and thallium (7.0 x lo"%) are much less abundant than aluminium. Gallium and indium are found in aluminium and zinc ores. But even the richest sources contain less than 1% galliuni and still less indium. Thallium is widely distributed in nature and occors in sulphide

ores of zinc, lead, copper and iron. \ 6.2.2 Extraction Boron is-obiamed by the red-uction of B2O3wittimagnesium or sodium. B203is first prepared by strongly katingH,B03 which isobtained by the action of HCI or H2S04- on a concentrated solution of borax:

Pure crystalline boron may be obtained in small quantities by the reduction of BBr, with H2 on a heated tantalum metal filament at 1275-1475 K. ' \ Bauxite approximately contains Aluminium cannot be extrpcted economically from the silicate minerals. Therefore, A~,o,=ss%. Fe203=159~, bauxite is the most important ore for the extraction of alumimum, but this has many Ti02=2%, sio2=39" and problems. A1203is a very stable compound. It is not reduced by heating in hydrogen. . H20=25%. On strong heating with carbon, A120, gives the carbide, AI,C,. As A1203 does not India is fortunate in having large melt below 2300 K, it cannot be electrolysed conveniently. However, electrolysis of deposltsof bauxitein a reasonably a solution of A120, in fused cryolite occurs at a much lower temperature of 1100-1300 pure state. K. Thus, aluminium is extracted by electrolysis of purified alumina in fused cryolite. Pure alumina is obtained from bauxite ore. Powdered bauxite mineral is heated in a concentrated solution of sodium hydroxide under pressure when alumina and silica get dissolved: A1203 + 2NaOH + 3H20 2 NaAI(OH)4 Si02 + 2NaOH NazSi03 + H20 Iron oxide and titanium dioxide do not dissolve in the alkali and are filtered off as a sludge. The solution is cooled and most of the aluminium hydroxide is precipitated either by the passage of carbon dioxide or by seeding with some+reshlyprecipitated - aluminium hydroxide : 2NaAl(OH)4 + C02 ~A~(OH),4 + Na2C03 + H20

NaAl(OH)4 ~46~)~J + NaOH The silicates remain in solution, since silica is a more acidic oxide than alumina. Aluminium hydroxide is filtered, washed and heated to give pure alumina.

'.4 Alumma is dissolved in fused cryolite to which calcium fluonfe is added to lower the vlt~ngpoint. The solution is then electrolysed at 1100-1300 K in an iron cell, lined For each kgof A1 produced. about Mth graphite, which acts as the cathode and carbon rods suspended in the electrolyte kg Of A'2°3, 0.150 kg Of atting as the anode (Fig. 6.1). Electrolysis of the solution gives aluminium at the NaOHpO.SOkgoEC and 6'0 lo' cathode and oxygen at the anode. The discharged aluminium sinks to the bottom of kJ of electricity are consumed the cell and is tapped off. Fresh alumina is added as required. The anode is slowly attacked by liberated oxygen to form carbon monoxide. Therefore, anode has to be continually replaced, adding substantially to the cost of the process. The temperature of the cell is maintained by the passage of electric current. Following reactions take place during electrolysis: 6 - 302- . 3/20, .+ 6e, at anode ~AI~++ 6e 2A1, at cathode Gallium, indium and thallium are usually obtained by electrolysing aqueous solutions of their salts. This method is not applicable in the case of aluminium salts as they are hyal G:, sed considerably by water. -Carbon electrodes ~nnnn~iy:

I=-II - -- olten crvoltte-AL~mixture-: Molten Al

Fig. 6.1 : man^^ d Al 6.2.3 Uses Boron is used to increase the hardness of steels. Crystalline boron is used in transistors. Boron is a good neutron absorber and is used as shields and control rods in nuclear reactors. Boranes are used as high energy fuels, for example, in rockets. Boric acid is used as' an antiseptic. Borax is used to make heat resistant borosilicate glass. It is also used for vitreous enamelling of baths, domestic appliances and for glazing tiles and pottery. Another use of borax is to make peroxyborates, e.g., NaBO2.H2O2-3H20,which are useful cleaning and bleaching agents. In the laboratory, borax is used for standardising acids and in the borax bead test in qualitative analysis. ~lu&%umexhibits useful properties of low density, high thermai and electricat conductivity, good corrosion resistance with non-toxic nature of the metal and its compounds. Due to these properties, it is the most widely used non-ferrous metal. Aluminium is used for making electrical conductors, cooking utensils and wrapping When an alum~niurna91cle is made the anode in the electrolysis materials. You must have seen milk bottles capped with aluminium foil. Aluminium of dil. sulphuric or chromic acid, a is extensively used for structural purposes, either alone or alloyed, in aircraft, ship thick hard film of AI2O3is formed and buildiig industries. Large amounts of aluminium are converted into alloys, such on the surface of the article. This as, duralumin and magnalium containing a few per cent of copper or magnesium. process is called anodising. A1203 These alloys are harder and stronger than pure aluminium but possess almost the layer adsorbs many dyes and takes a very high polish. You must have same properties of lightness and corrosion resistance which can be further increased seen many bright, colourful by anodising process. Aluminium beryllium alloys are harder and Iighter than other articles made of anodised alloys of aluminium and are extensively used in space-craft. However, toxic nature aluminium. of beryllium makes their handling difficult. Due to its strong affinity for oxygen, aluminium is a good reducing agent and is used in aluminothermic process for extraction of metals and in welding. Suspension of finely powdered aluminium in oil is used as paint. Anhydrous is an important catalyst, used in organic synthesis and in the cracking of petroleum. A12(S04)3as such or as potash alum, K2S04.A12(S04)3.24H20 is used for sizing paper, for tanning leather, for waterproofing cloth and as a mordant for dying cotton. It is extensively used for purification of water and in sewage treatment. Because of its extreme hardness, high m.p., non-volatility, chemical inertness and good electrical insulating properties, corundum (A120,) finds many applications in abrasives, refractories and ceramics. Large crystals of a-A1203 when coloured with metal-ion impurities are prized as gemstones, e.g., ruby (cr3+, red), sapphire (Fe2+"+ni4+ , blue), oriental emerald (cr3+/V3+, green), oriental topaz (~e~+, yellow), etc. Aluminates are important constituents of Portland cement. Gallium has the longest liquid range (303-2343 K) of any known substance and so finds use as a high temperature thermometer liquid. Gallium is mainly used in semicondyctor technology. It is used for doping othcr semicondl~ctorsand in solid state devices such as transistors. Compounds of Ga with P and As; such as, Gap and GaAs have semiconductor , properties similar to those of elemental Si and Ge. These are used as light emitting diodes (LEDs) familiar in pocket calculators, wrist watches, etc. They are also used in infrared emitting diodes, infrared detectors, photocathodes and photomultiplier tubes. The most important applications of indium are in protection of bearings against wear and erosion, in low-melting alloys and in electronic Thallous salts, being toxic, are used as fungicides, for the treatment of ring-worms. Thallium compounds find optical glass on account of their high refractive power. We will discuss the general characteristics of these elements in the mxt section but before that try this ~AQ.

SAQ 1 a) Write the namorm"1ae of two important ores of the most abuAdant metallic element in the earth's crust.

b) Match the following properties of A1 with the uses to which the metal or its compounds are put: i) Good thermal conductivity a) Building ships and aircrafts ii) Good electrical conductivity b) Utensils iii) Low density and resistance c) Electric wires to corrosion iv) Non-toxicity d) Adsorbent v) Gel nature of Al(OH)3 e) Food packaging

\ 6.3 GENERAL CHARACTERISTICS All the elements of Group 13 have similar valence shell electronic configuration of m2np1,but the underlying core varies. For B and Al, it is the preceding noble gas core, for Ga and In, it is noble gas plus dl0 and for TI, noble gas plus 4f45d''. This variation in the electronic configuration of the core is also reflected in the ionisation . energies of these elements and has a strong influence on their properties.

6.3.1 Physical Properties - Elements of Group 13 are lesnnetallic than $hose of ~iou~s1 and 2. Within the group, there is a variation in metallic charactey. Boron, the smallest element in this group is a nonmetal. The other elements in this group are fairly reactive metals. This is the first group in which 'change from non-metallic to metallic nature occurs. Some physical properties of Group 13 elements are listed in Table 6.1. The elements of Group 13 have smaller atomic radii and higher electronegativities as compared to s-block elembps of the same period. However, these properties do not vary in a regular way down the group, in contrast to the brapertiesof the element of, - Groups 1 and 2. You know that the size and the ionisat~orpenergies of atoms depend on effective nuclear charge of the-atom. B and Al follow immediately the s-block elements Be and Mg, respectively. Their size and ionisation energies are as expected. But between the s- and p-block elements of the fourth and successive rows, the d-block elements, i.e., the transition elements get inserted. Thk insertion of transition elements results in higher effective nuclear charge of the foutth row elements Ga, Ge, etc., than expekted by simple extrapolation from the second and third row elements. In other words, the nuclei of these fourth row elements attract electrons more strongly than expected and tvs affects their properties. Thus, the atomic size of gallium is smaller; its electronegativity and ionisation energies higher than 8 expected. Atoms with dl0. inner.- shell, in general, are smaller and have higher ionisation energies. In a similiir way, the inc~usidnof fourteeh electrons in 4f orbital further affects the size and ionisation energy of TI, As a result of this, irregulaqies in atomic radii, electronegativit'y and ionisation energies are seen from B to Tl. -* Table 6.1 shows that densities of these elements show a gradual increase while melting and boiling points fall in value. The exceptionally low melting point of gallium (303 K). however. has, so far, no simple explanation.

Table 6.1: Some properties of Croup 13 elenvats

Roperty B Al CP In rn Atomic number 5 13 3 1 49 81 Atomic weight 10.81 26.98 69.72 114.82 204.38 Electronic structure [~e]2r~2p'[~e]3s~3p' [~r]3d"'4~~4p' (~r]4d'"5s~5p' [~e]4f~5d'"fis~6~' Metallic radius (pm) 98 143 141 166 171 Covalent radius (pm) 82 125 125 144 155

Ion~cradius M~+(pm) 20 50 62 ' 81 95 Melting point (K) 2573 933 303 429 576 Boiling point (K) 2823 2740 2343 2273 1730 Density (ld x kg m") 2.34 2.7 5 91 7.3 11.8 Ionisatidn 1st 800 577 579 558 589 energy 2nd 2427 1816 1979 1820 1970 (kJ mol-I) 3rd 3658 2745 2%2 2703 2879 Electronegativity (AIR) 2.0 1.45 1.8 1.5 1.45 Electron affinity - (kJ mol-') -27 -44 -30 -30 -30 Common oxidation state (2)'. 3 3 (1)*,3 1,3 1, (3). Common co-ordination numbers 3, 4 3, 4, 6 3. 6 3. 6 3. 6 Natural abundance (%) 1.0 x 1v3 8.13 1.9 x lc3 2.1 x 10-5 7.0 x 10-5 * Corqparatively less stable.

From Table 6.1 you will observe that these elements form small ions of high charge density and the value of the sum of their first, second and third ionisation energies is very high. These properties lead us to the conclusion that these elements will prefer to form covalent rather than ionic compounds. Boron is always covalent and does not form,^^+ ions, because' the energy required to remove three electrons is very high. Many simple compounds of A1 and Ga like AlC13 and GaC13 are also covalent when anhydrous. However, in solution, the large amount of hydrrttion energy evolved compensates the high ionisation energy and all the'metal ions exist in a hydrated state. This can be explained with the help of Born-Haber cycle as given below. However, this is a simplified appkoach given only on the bas; of enthalpy changes, whereas the direction and extent of any reaction depend on the free energy changes which take into account changes both in enthalpy and entropy.

AH,,, = 326 kJ 4 1 AHd,,=3x122W Al(g) + 3Cl(g) I,+I2+13=5138W 1 1 EA=-3~348k.l AH""," = -295 kJ ~l~+(g)+ 3Cr(g) I I AH^^^ '-46kk~ 1 1 AHhN=-3X38SW AI"(~) + ' XI-(aq) . AH,,, = 704 + 326 + 366 + 5138 -I 1044 - 4630 - 1155

Thus, despite tbqlarge ionisation energies of aluminium, the enthalpy of'solution of AlC13 has a large negative value. Therefore, in aqueous solution, AIC13 exists as AI~+(~~)and Cl-(aq). It might be thought that the same argument would apply to BCI;. But to make the enthalpy of solution of BC13 negafive, the enthalpy of . hydration of B" should be - 6009 liJ which is unlikely for the small B~+cation. Therefore, BC13 hydrolyses in aqueous sohtion to give boric acid and not ~~'(a~): BC13 + 3H20 H3B03 + 3HCI . Unlike the s-block elements which show only one stable oxidation state, the elements of this group show more than one stable oxidation state. While the trivalent state is important for all the five elements, the univalent state becomes progressively more stable on descending the group. Thus, for B, A1 and Ga, the +3 state is more stable than +1 state; for In, both are equally stable, and for thallium the univalent state is more stable than the trivalent state. Oxide, sulphide, carbonate, sulphate and halides, etc. of TI(1) are well characterised, and are more stable in aqueous solution than TI(II1) compounds. This is due to the s electrons in the outer shell of thallium tending to remain paired and not participating in bonding because the energy required to unpair them is rather high. This is called the inert pair effect. This effect is noticeable particularly among the heavier elements in the p-block. In addition to the above, some compounds are known, in which the elements show +2 oxidation state, e.g., B2F4 and B2C14. However, these compounds are less stable.

SAQ 2 Covalent radius, ionisation energy and electronegativity of gallium are different from those expected by simple extrapolation of these properties horn those of boron and aluminium. Discuss this anomaly briefly.

6.3.2 Chemical Properties Elements of Group 13 are comparatively less reactive than the alkali and alkaline earth metals of Groups 1 and 2. However, the +activity of the elements increasts on descending the group. The cry'stalline form of boron is black. very hard and inert. It reacts with other elements only at L.gh temperatures. The more common amorphous form is brown and more reactive than the black crystalline form. Aluminium is a light and hard white metal. It reacts with atmaspheric oxygen with the formation of a protective layer of aluminium oxide which prevents further attack. Amalgamation with mercury removes the oxide coating and a rapid reaction occurs with oxygen, water or dilute acids. Gallium, indium and thallium are relatively soft and more reactive than boron and aluminium. All the elementq except thallium when heated with halogens, oxygen or chalcogens form halides (EXh), oxides (E203), chalcogenides (E2S3, etc.). Thallium on the other hand forms TIX, T120, T12S, etc.

The reaction of aluminium with Aluminium hasa very high affinity for oxygen; this is reflected in high exothermic Fe203produces .temperatures heat of formation of A1203 (-1676 kJ mol-'). This allows aluminium to extract approaching 3300 K - enough to oxygen from other metal oxides, and forms the basis of Goldschmidt's mett the ibnmetal produad. This aluminothermic process for Ca, Sr, Ba, Mn, Cr, Mo, etc.: is called tde (bcraldc nrtiol and is used to weld cricks in iron and 3Mn02 + 4Al + 2AI,03 + 3Mn steel articles. Fe203 + 2A1 Al,O, + 2Fe(l) Bbron and aluminium when heated wlth nitrogen 1016 the nitrides BN and AIN. Nitrides of Ga and In are formed by heating the elements with ammonia. Boron nitride can also be made by the action of ammonia on boron at 1300 K, or by passing nitrogen over a mixture of boron trioxide and carbon at a slightly higher temperature: K 2B + 2NH3 --1300 2BN + 3H2 > 1UW)K B,O, + 3C + N2 ? 2BN + 3C0 BN is isoelectronic with C2, and like carbon, it exists in diamond and graphite forms. The latter is a useful lubricant with additional advantage of being inert. Boron nitride is a white solid. It is chemically rather inert, but is hydrolysed to NH, and B(OH)3 by the action of steam or hot acids: Boron and aluminium on heating react with carborl to form the carbides B,,C, and A14C3, respectively. Aluminium carbide is a colourless, hi& melting ionic solid and is decomposed by water to liberate methane. Therefore, it can be termed as aluminium methanide also: i A14C3 + 12H20 4Al(OH)3 + 3CH4 On the other hand, the isolable form of boron carbide has the.molecular composition B12C3.It is a black, extremely hard, high melting and chemically i+rt covalent compound. It is used as an abrasive for polishing and tool sharpening. I ' Boron on heating reacts with many metals to f&m binary compounds called borides, e.g., MgB2, VB and Fe2B, whereas other elements of Group 13 form alloys. Metal bqrides are extremely hard, chemically inert, non-volatile, refractory materials. They have high melting points and high thermal and electrical conductivities. The diborides of Ti, Zr, Hf, Nb and Ta all have melting points higher than 3200 K. The thermal and electrical conductivities of TiB2 and ZrB, are about ten times greater than those of Ti and Zr metals. Boron does not react with non-oxidising acids. Even hot concentrated oxidising acids react with boron only slowly to form boric acid: 2B + 3H2SO4 --- 2H3B03 + 3S02 B +. 3HN03 H3B03 + 3N02 Al, Ga, In and TI react with warm dilute HCI and H2S04to replace hydrogen and form kf3+ except TI which forms TI+: 2M + 6HCI 2~~++ 6C1- + 3H2 2Tl + 2HCI -- 2TI+ + 2C1- + H2 With h9t and cox. H2S04, SO, is liberated: %I+ 6H2S04 M2(S04), + 3S02 + 6fi0 Hot and conc. HNO, renders A1 and Ga passive. The initial attack of the acid covers the metal with an impervious, coherent, unreactive layer of oxide which prevents further attack. In and TI react with conc. HN03to form metal trinitrates liberating NO,: M + 6HN03 > M(N03), + 3N02 + 3H20 Boron reacts with fused alkalies forming the borates:

2B + 6NaOH 9 2Na3B03 + 3H2 Aluminium and gallimn dissolve in aqueous alkalis to form aluminate and gallate ions, respectively: M + 4NaOH + NaM(OH)4 + 2H2 Thus, you see that Lheelements of Group 13 aie fairly reactive and form many useful compounds such as hydrides, halides, nitrides, oxides, oxoacids and their salts, ttc. Let us now study somk of the compounds of B and A1 in detail. Compounds of Ga, In and TI will be described only in brief, where appropriate, for the sake of cdmparison only.

a) Explain in the space given below why the action of conc. nitric acid renders aluminium passive...... , 1 ...... b) Write the formulae of the compounds you would expect B and A1 to form with halogens, bxygen, sulphur, nitrogen and carbon.

...... 11 Boron forms a series of volatile hydrides which resemble in some respects, e.g., volatility and covalent nature, the hydrides of carbon and silicon. The boron hydrides are generally called boranes by analogy with the alkanes and silanes. In view of its trivalency, boron is expected to form a simple hydride BH,, but it is unstable. The simplest stable hydride of boron is dib.orane, B2H6. There are about a dozen well characterised boranes which correspond to the following two stoichiometries:

i) BnH,+4 : B2H6, B5H9, B6H10, BSHIZ,B10H14, B18H22and iso-B1,H2,

Boranes are usually named by indicating the number of B atoms with a prefix and the number of hydrogen atoms by an arabic number in parenthesis. For example, B,H,, is named as tetraborane(l0) and B,H,, as nonaborane(l5). The hydride, B2H6is simply called diborane as the compound of stoichiometry B2H8is not known. Diborane is of special interest because it is the starting material for the preparation of various other boron hydrides and because of its synthetic uses. So, let us first discuss BZH6in detail. 6.4.1 Diborane (B,H,) The chemistry of diborane has aroused considerable interest because of its usefulness BZH, has only 12 valence in many synthetic reactions and also because the elucidation of its structure helped electrons; for ethane type - to clarify the basic concepts about the structure of electron deficient compounds. We structure 14 valence electrons are shall briefly discuss the chemistry of diborane below. needed ,' a) Preparation: Diborane can be prepared in almost quantitative yields by the reduction of boron trifluoride etherate (BF3,0Et2)with lithium aluminium hydride (LiAIH,) or sodium borohydride (NaBH,):

diglyme 4BF,.0EtZ + '~N~BH,- 2B2H6 + 3NaBF, + 4Et20 where. diglyme is diethyleneglyco! dimethyl ether, (MeOCH,CH2)20. Diborane can also be prepared by treating NaBH, with conc. H2SO4or H3PO4:

2NaBH, + H2S04 ------+ B2H6 + 2H2 + Na,SO, 2NaBH, + 2H3P0, + B2H6 + 2Hz + 2NaH2P0,

b) Properties of diborane: Diborane is a colourless gas (b.p., 183K). It is rapidly , decomposed by water with the formation of H3B03 and HZ: B,H, + 6HZ0 2H,BO, + 6H2 Mixtures of diborane with air & oxygen inflame spontaneously producing large amount of heat. Diborane has a higher heat of combustion per unit weight of fuel than most other fuels. Therefore, it is used as a rdcket fuel. B2H6 + 30, - --B2O3 + 3H20, AH = -2165 kJ mol-I Pyrolysis of B2H,in sealed vessels at temperatures above 375 K is an exceedingly complex process producing a mixture of various boranes, e.g., B4Hlo,B5H9, B5H,,, B6HIo,B4H12 and BloHI+ By careful control of temperature, pressure and reaction time, the yield of various intermediate boranes can be optimised. For example, by storing BZH6under pressure for 10 days. B4HI,,is produced in 15% yield according to the following equation:

~B,H, B4H~o + Diborane undergoes a facile addition reaction with alkenes and alkynes in ether solvents at room temperature to form organoboranes: ' 6RCH=CH2 + B2H, 2B(CH2C'H2R)i This reaction known as hydroboration reaction was discovered by Brown and Elements of Group 13 Subba Rao in 1956. It is regiospecific, boron atom showing preferential attachment to the least substituted carbon atom. You may compare this addition Hydrobration reaction has with polar additions to the double bond, e.g., addition of HX, which obey proved to be of outstanding Markownikoff's rule. Reaction of the resulting organoborane with an anhydrous synthetic utility. H.C. Brown was carboxylic acid yi'elds the alkane corresponding to the initial alkene whereas awarded the 1979 Nobel Pr~zein oxidative hydrolysis with alkaline H202yields the corresponding primary alcohol: chemistry for developing hydrobr'tion reaction. EtC02H B(CH2CH2R)3 3RCH,CH3 NaOHM202 (CH,CH2R)3 3RCH2CH20H c) Structure of diborane: The structure of diborane is of great inierest since it cannot be explained on the basis of simple theories of bonding. As mentioned earlier, there are not enough valence electrons in B2H6 to form the expected number of covalent bonds, in other words, B2H6 is an electron-deficient coppound. Since H H boron atom has three unpaired electrons in the outermost orbit in the excited 1 I state, it can form three covalent bonds. If each of the boron atoms in diborane H-B-B-H I I links itself to three hydrogen atoms, there will be no electrons left to form a bond H H between the two boron atoms. Thus, diborane cannot have an ethane type Not possible structure as shown in the margin. Diborane, B2H6, has been found to possess a bridge structure (Fig. 6.2) in which each B atom iq bonded to two H atoms called terminal H atoms by regular electron-pair bonds. The resulting two BH, fragments are bridged by two H atoms as indicated by electron diffraction studies and by Raman and infrared spectroscopy. The two boron atoms and the four terminal hydrogen atoms lie in the same plane while the two bridging hydrogen atoms lie in a plane perpendicular to this plane as shown in Fig. 6.2. The bridging hydrogen atoms prevent rotation between the two boron atoms.

Fig, 6.2: Bridge structure of diborane

In the biidge structure, diboral~ehas eight bonds but there are 12 electrons available for bonding, three per B and one per H. Hence, all the bonds in the molecule cannot \be electron-pair bonds which would require 16electrons for the structure in Fig. 6.2. The terminal B-H bond lengths are the same as the bond lengths in non-electron deficient compounds. This means that the four terminal B-H bonds are normal electron pair bonds accounting for a total of eight electrons. Thus, electron deficiency must be associated with the bridging B-H-B bonds in which a pair of electrons binds three atoms, viz., B, H and B. Therefore, these bridging B-H-B bonds are called three centre electron pair bonds abbreviated as 3c-2e. Alternatively, we can give a simple molecular orbital description of bonding in B2H6 as follows. Each boron atom is sp3 hybr~disedgiving four ~p"~brid orbitals, one of which is vacant and the other three are singly filled. Two of the sp3 hybrid orbitals on each boron atom are used to form terminal B-H bonds with singly filled 1s orbital of hydrogen. Two BH, units are then brought together so that all six atoms are coplanar. Then one singly filled sp3 hybrid orbital on one B atom and one vacant sp3 hybrid orbital on the other B atom overlap with the singly filled 1s orbital on hydrogen atom to ford a bonding orbital shaped like a banana and covering all three atoms, viz, R, H and R. Similarly the other bonding orbital iz :!so formed (Fig. 6.3). This orbital binding three atoms contains only two electrons; the bonding between the bridging H atom and the B atom is thus only about half as strong as in the conventional two-centre two-electron terminal bonds. The B2H6 molecule contains two such three-centre electron pair bonds. Due to repulsion between the two H nuclei. the delocalised orbitals are bent away from each other giving it the banana'shape. 6.4.2 Borohydrides . ,Borohydrides like NaBH,, Be(BH,), and Al(BH& are the salts of complex tetrahydridoborate anion, BH,. Since in BH;;, boron has a complete octet, the borohydrides are more stable than the boranes. sodium borohydride is obtained by the reaction of NaH and methyl borate: 4NaH + B(OMe)3 NaBH, + 3CH30Na Other borohydrides are prepared from sodium borohydride. The alkali metal borohydrides are white, non-volatile, ionic solids which are stable in dry air. In contrast to the alkali metal borohydrides. Be(BH,), and Al(BH,), are covalent and volatile in nature. The alkali metal borohydrides react wZIh water with varying ease. Thus, LiBH, reacts violently with water, NaBH, can be recrystallised from cold water while KBY, is even more stable. LiBO, + 4H,

. in alcoholic and aqueous solutions make .it a useful reagent in the reduction of aldehydes and ketones to primary and secondary alcohols, respectively. Other functional groups such as >C=C<, -COOH and -N02.are not attacked normally:

RCHO --- 6.,4.3 Hydrides of Aluminium The extensive chemistry of the boron hydrides finds no parallel with the hydrides of heavier elements of Group 13. Out of the four possible binary hydrides of Al, Ga, In and TI, only aluminium hydride, (AlH,),, is known. It is prepared by the action . of either 10O0/~pure H2S04or AlCI, on lithium aluminium hydride in fin ethereal solvent: 2LiAIH4 + H2S04 ' ,' 2/n(AIH3), + L~,SO, + 2H2 3LiAIH4 + AICl, 4/n(AIH3), + 3LiC1 replaceable proton; examples are acids, water, alcohols, etc. 2(A1H3), + 6nH20 2nAI(OH)3 + 9nH2

6.4.4 Lithium '~luminiumHyddde Lithium aluminium hydride is much more useful than aluminium hydride. It is ether L-ts d Group I3 4LiH + AICl, + LiAIH, + 3LiCli - Lithium aluminium hydride is a greyish-hite solid which decomposes into its elements above 400 K. It' is stable in dry air but reacts violently with water: LiAlH, + 4H20 LiOH + AI(OH), + 4H2 LiAlH, is readily soluble in ethers. It is a very important reducing agent for both inorganic and organic compounds. It reduces inorganic halides to hydrides, e.g.: 4BC1, + 3LiAlH4 2B2H, + 3LiCI + 3AlC13 MCI, + LiAlH, -- ---+ MH, + LiCl + AICI, (M = Si, Ge, Sn) LiAlH, is one of the most important reuucing agents in organicchemistry because it reduces as many as 60 functional groups including ethylenic >C=C< double bonds. But, it is less selective than sodium borohydride. Before we discuss the halides of boron and aluminium, you may like to attempt the following SAQ based on the hydrides of boron and aluminium.

SAQ 4 a) Explain briefly the difference between a two centre electron pair bond and a three

, centre electron pair bond.

...... b) Compare NaBH4 and LiAIH, as reducing agents.

...... v ...... c) Which of the above two would ypu usemhe following conversions: i) R-CH=CH-CHO + R-CH=CH-CH20H ii) R-CH2-COOEt R-CH2-CH20H

6.5 HALIDES OF BORON AND ALUMINIUM I

All the elements of Group 13 form binary halides. All the four trihalides of each element are known, with one exception. The compound Tl13 is not thallium(II1) iodide, but rather thallium(1) triiodide T1+(1,)-, which is similar to potassium triiodide, K+I~.You will learn more about triiodides in Unit 10. Thallium(II1) chloride and bromide are also very unstable and decompose into the thallium(1) halides and the free halogen. Thus, the only stable trihalide of thallium in +3 oxidation state is the trifluoride, TIF3, which is an ionic solid. This is in keeping with the general trend that Tl(1) compounds are more stable than TI(I1I) compounds. Let us now briefly discuss the halides of B and Al. 6.5.1 Halides of Boron Boron trihalides of the type BX3 exist for all the four halogens. Boron trifluoride can be prepared on a large scale by the fluorination of boric oxide or borax with fluoaspar and conc. H2S04: B203 + 3CaF2 + 3H2S0, -2BF3 + 3CaS0, + 3H20 Na2B4O7 + 6CaF2 + 8HzS04 -----,4BF3 + 6CaS0, + 6NaHS0, + 7H20 4 ,, In the laboratory, pure BF3 is best prepared by thermal decomposition of Gene diazonium tetrafluoroborate, PhN,BF4: PhN2BF4 PhF + N2 + BF3 BCI, and BBr, are prepared on a large-scale by direct halogenation of B2O3 in the presence of C, e:g.: 775 K B203 + 3C + 3C12 -+ 2BCI3 + 6C0 In the laboratory BC13 and BBr, are prepared by halogen exchange reaction between BF3 and A12X6: / BI3 is prepared in good yield by reacting-LiBH, or NaBH, with I2 at 400#;ina43 K, respectively: LiBH, + 41, B13 + LiI + 4HI The boron trihalides are highly volatile; BF3 (b.p., 173 K) and BCI, (b.p., 260.5 K) are gases, BBr3 a volatile liquid (b.p., 319 K) and B13 a low melting solid (m.p., 323 K). Boron halides are all hydrolysed by water giving H3B03, and hydrohalic acids, HX or hydrofluoboric acid, HBF,: H3B03 + 3HX, (X = CI, Br, I) 4BF3 + 3H20 > ,H3BO3 + 3HBF4 In addition to the trihalides, boron forms lower halidesof formula, B2X4. But only B2F4 and B2C14 have been studied in some detail. B2F4 is a colourless gas whereas 'B2c14is a colourless liquid at room temperature. These halides are much less stable than the corresponding trihalides. B2F4is the most stable of all the B2X4compounds. These arespontaneously inflammable in air and react with Hz, H20, ROH, CI,, etc. Unlike BH3 which is unstable, boron trihalides are monomeric molecular compounds and have no tendency to dimerise. In this respect boron trihalides resemble organoboranes, BR3, but differ from diborane, B,H, and the aluminium halides, AI2X,. Thus, boron trihalides having three electron pair B-X bonds are electron deficient. However, the interatomic distances, B-X, are substantially shorter than those expected for single bonds. For example, the B-F bond length in BF3 is 130 pm which is shorter than the sum of the covalent radii of B (80 pm) and F (72 pm). This shortening of bonds has been explained in terms of appreciable pr-pn bonding between an empty p orbital of the sp2 hybridised boron atom and filled p orbitals of one of the fluorine atoms. All the four trihalides are trigonal planar molecules. This can be explained on the basis of sp2 hybridisation of the boron atom.

>. Electronic configuration of boron atom in state

Electronic configuration of boron atom in excited state

The three hybrid orbitals of boron overlap with singly filled 2p orbitals of three halogen atoms giving rise to three B-X bonds. The empty 2p orbital of boron which is not involved in hybridisation in perpendicular to the plane of the triangle. Its energy is comparable to that of the filled 2p orbitals of halogen atom. Thus it can accept a pair of electrons from a filled 2p orbital of any one of the three halogen atoms, forming a dative n bond. This makes an octet of electrons around the boron atom. The Bxj molecule exists as a resonance hybrid of the following three structures as shown in

16 Fig. 6.4: Structure of boron trihalides Because of lack of efficient overlapping, the extent of rr bonding decreases as the size Elementsof Croup 13 of atoms involved in bonding increases. Thus, the extent of rr bonding in boron halides decreases from BF, to BI?.

6.5.2 Halides of Aluminium All the iour trihalides of aluminium, i.e. AIF,, AICI,, AlBr, and AlI, are known. AIF, is made by treating A1203with HF gas at lOOOK and the other trihalides are prepared by the direct exothermic reaction of the elements, e.g.:

A1203 + 6HF loMK , ~AIF,+ ~H~O 2A1 + 3X2 2A1X3, (X = C1, Br, 1)

AlCl, is also obtained by heating a mixture of alumina and coke in a current of Clz: A1203 + 3C + 3C12 2A1C13 + 3C0 Aluminium trifluoride differs from the other trihalides of A1 in being ionic and Fig. 6.5: Structure of A12C16 nonvolatile in nature. Other halides of Al, as also of Ga and In, are covalent in nature when anhydrous and are relatively more volatile. AlCI,, AlBr, and AlI, exist as dimeric species formed by pairing of two AIXRunits as shown in Fig, 6.5. The pairing occurs by formation of a coordinate covalent bond from the halogen on one AlX3 unit to the A1 atom of another. Thus, for AlCl,, the species Al,CI, is formed. This is similar to the linking together of BeCI, units in solid BeCl, which you have already studied in the preceding unit.

The dimerisation of AlX, occurs because these halides are electron def~cient.By dimerisation, the halides attain' an octet of electrons. You have studied that the . trihalides, BX3 are also electron deficient and attain an octet by prr-prr bonding. This is not possible in case of A1 and other larger elements because-of lack of efficient. rr-overlap, and hence, they dimerise. This dimerisation is retained when the halides dissolve in non-polar solvents such as C6H6 and CCl,. In coordinating solvents, such as, diethyl ether, trimethyl amine and phosphorus oxochloride, AlCl, forms complexes like AlC13.0Et2, AlCI,.NMe, and A1Cl3.OPCl3, e.g.: .. A12C16 + (C2H5)2? 2 (C2H5)20.AlC13

Alkyl and acyl chlorides, RCI and RCOCl, react with AlCl, to form complexes of the type R+AICI, and RCO+AICI,, respectively; these are formed as intermediates in Friedel-Crafts alkylation and acylation reactions.

As explained in Section 6.3, due to high heat of hydration of A],+, the covalent Aqueous solutions of salts of most dimers are broken into [AI(H~o)~]~+and 3X (aq) ions, when the halides dissolve in heavy metals are acidic in nature water. ~queoussolutions bf aluminium halides and othe~aluminiumsalts are acidic because of dissociation of their in nature. This is because the hexa-aquo complex or the aqua ion [AI(H,o),]~+ aqua ions as shown in the case of aluminium. dissociates readily in solution giving hydroxonium ions by a series of changes as shown below:

If a base like NH40H is added to an aqueous solution of salts of aluminium, the Gelatinous nature of A1(Ol$)3 is H30+ ions are neutralised and hydrated aluminium hydroxide is precipitated as a due to its hydrated nature. gelatinous precipitate: Because of its gelatinous nature. AI(OH), is used for purifying water and as a mordant in dyeing

An exdess bf a strong alkali like NaOH, causes the above reaction to continue further 'with the formation of the soluble aluminate anion, [Al(H20)2(OH)4]-:

[A1(H2O),(OH),] + NaOH Na[A1(H20)2(OH)41

The above reactions can be reversed by the addition of an acid. Thus, when a basic solution containing aluminate ion is slowly neutralised, the hydroxide ..-- [AI(H20)3(0H)3]precipitates and then redissolves as more acid is added: [Al(H20)2(OH),]- + H3O+ [A1(H20)3(OH)314 + H2O [AI(H20)3(OH)31 + 3H30f [AI(H~o)~]~++ 3H20 I This explains the amphoteric nature of AI(OH)3.

SAQ 5 / Explain briefly why boron trichloride is a gas and aluminium trichloride is a dimeric solid.

6.6 OXIDES OF BORON AND ALUMINIUM

All the elements of Group 13 form trioxides also known as sesquioxides of the formula, M2O3. Thallium forms stable T120 also. The basic character of oxides increases down the group with increase in the atomic number. Thus, B2O3is acidic, A1203and Ga203are amphoteric whereas In203and Tl2O3are basic. T120is soluble in water and the resulting hydroxide TlOH is, in fact, as strong a base as KOH. Let us now study the oxides of boron and aluminium in a little more detail, this will be followed by a discussion of the acids of boron and the borates. 6.6.1 Boric Oxide Boric oxide, B203,is the principal oxide of boron. It is also known as boron trioxide, boron sesquioxide and boric anhydride. It is prepared by burning boron in oxygen or by heating boric acid to red heat:

2B + 30, p B203 2H3BO3 B203 + 3H20 Boric oxide is a white hygroscopic solid. It is acidic in nature and dissolves in water to form boric acid:

When fused with metal salts, it forms, metaborates known as borate glasses. Metaborates of coloured cations have characteristic colours. This forms the basis of the borax-bead test in qualitative inorganic analysis, e.g.: COO + B2O3 CO(BO~)~,deep blue CuS04 + B203 CU(BO~)~,blue Cr2(S04)3 + Bz03 ~CT(BO~)~,green The bond energy of the B-0 single bond is very high (523 kJ mol-'). Therefore, unlike carbon and nitrogen, boron does not form stable BO double bonds to oxygen. Instead of forming small volatile covalent molecule, B203(g) and small anion, B0:; it forms pol mers having -B-0-B-0-B-0- chains. Thus, boric oxide is a polymeric solid.7 6.6.2 Aluminium Oxide Aluminium oxide, A1203is a highly ionic compound of aluminium. It is also tnbwn as alumina. As you have studied earlier, it occurs in nature not only as bauxite and colourless corundum, but also as coloured gem stones like ruby, emerald, sapphire and topaz, etc. Colours of these gem stonesare due to the presence of transition inetal ions. Anhydrous aluminium oxide can exist in two forms both of which ate white. These are a-A1203and 7-AI2O3. The 7-AI2O3 is formed by dehydrating AI(OH)3 below 750 K: ~AI(OH)~ * 7-Al2O3 + 3H20 The 7-A1203 is quite reactive. It dissolves readily in both acids and bases. If the 7-AI2O3 is heated to 1500 K, it changes to the a-AI2O3, which is called corundum. Corundum has a very high melting point (about 2328 K). It is very hard and inert,- especially towards acids. SAQ 6 Discuss briefly the nature of oxides of Group 13 elements.

6.7 OXOACIDS OF BORON AND BORATES

Orthoboric acid H3BO3 commonly known as boricacid and metaboric acid HB02, are two well-known and important oxoacids of boron. On a large scale, H3BO3 is prepared by the action of HCl or H2S04on a concentrated solution of borax: Na2B,07 + 2HC1 + 5H20 4H3B03 + 2NaC1 Boric acid is a flaky, white crystalline solid. It is moderately soluble in water. Boric acid is a very weak monobasic acid (pK = 9.25), because it acts as an electron pair acceptor (Lewis acid) from OHrather than as a proton donor (Arrhenius acid). H3BO3 + 2H20 7[B(OH),]- + H30+ Its acid strength is considerably enhanced on complex formation with polyhydric alcohols such as glycerol and mannitol. With mannitol K drops to 5.15 indicating an increase in acid strength by a factor of more than 1.0b: .

Fig. 6.6: Complex formation between borle add and a l:Md On heating bonc acid at 375 K, metaboric acid, HB02 is formed. On further heating above 500 K, B2O3 is formed:

In solution metaboric acid changes into orthoboric acid. Boric acid has a two dimensional layer structure in which planar B03 units are linked to each other by unsymmetrical hydrogen bonds as shown in Fig. 6.7. In contrast to the short 0-H....O distance of 272 pm within the plane, the distance between consecutive layers is 318 pm. This is the cause of slippery and waxy feel of boric acid which is also a good lubricant.

Fig. 6.7: Layer structure of HaO, ~-HIUC~~~:~ernrntr-~ Borax Sdlts of boric acids are known as borates. As said earlier, hydrated hor;~tcsoccur naturally, e.g., borax, Na2B,0,~10H20, kernite, Na+0,.4H20. colemanite. Ca2B,0,,.5H20, etc. Anhydrous borates can be made by fusion of horic acid and metal oxides. Sodium tetraborate decahydrate, Na2B+07+10H20,is commonly known as hori~x.It occurs in certain lakes in India, Tibet and U.S.A: It is obtained by ext_ractinqimpure borax with water and concentrating the solution until cry'stals of borax sepnratC out. Borax can also be prepared from the mineral colkdanite by boiling it with Na,COI solution: Ca2B6O1, + 2Na2C03 Na2B407 + 2NaB02 + 2CaC03 4 Borax is crystallised from the filtrate after removing insoluble CaC03, By passing COz, NaB02 present in the mother liquor is converted into borax: ' 4NaB02 + C02 Na2B407 + Na2C03 Borax is a white crystalline solid. It is hydrolysed by water to give an alkaline solution:

On heating, borax loses, water to become anhydrous. Anhydrous borax on strong heating with NH4Cl gives boron nitride and boron trioxitde:

Na2B407 + 2NH4Cl ----i------, 2BN'+ B2O3 + 2NaCl + 4H20 On heating alone, it decomposes to form NaBOz and B203 > 2NaB02 + B203

. SAQ 7. a) Explain why bofic acid behaves as a weak monobasic acid......

b) Boric acid can be estimated by titration with standard alkali solutions, in the presence of glycerol using phenolphthalein as an indicator. What is the function of

- 6.8 BORAZINE

borazine or borazole: Borazine is best prepared by reducing B-trichloroborazme with sodium borohydride, Elements &Croup 13 NaBH4. B-trichloroborazine is first prepared by heating BC13 with NH4CI.

Borazine has many physical properties closely similar to those of isoelectronlc benzene as shown in Table 6.4. Therefore, sometimes it is also called as inorganic benzene.

Table 6.b: Comparison of physical properties of borazlne and benzene

Mol. wt. MP (K) BP (K) Density (10' x kg m-')

Borazine, however, is more reactive than benzene. It readily reacts with H20,MeOH and HX to yield 1:3 adducts which eliminate Hz on being heated to 373 K, e.g.:

~orazinehas a regular plane hexagonal ring structure. The B-N bond distance of 144 pm in borazine molecule is less than the sum of single-bond covalent radii of B (82 pm) and N (70 pm). This indicates the presence of n bonding involving the lone pair of electrons on nitrogen and an empty p orbital on boron. Thus, injyalence bond terminology, the structure of borazine can be written as shown in Fig. 6.9.

Fig. 6.9: Valence bond structures for boraaine

Molecular orbital calculations, which are beyond the scope of discussion in this course, indicate that in borazine the n electrons are only partially delocalised. This is because of the difference in the energy of the n orbitals of B and N. The experimental results of the reactions of water and HC1 with borazine indicate that the reactions proceed by nucleophili~attack on boron atom. This suggests that the Nucleophiles are the electron actual sign of net charges on B and W atoms in borazine should be opposite to that donating molecules or ions that indicated in Fig. 6.9. This apparent paradox is explained by the existence of bse'or share electrons with considerable polarity in the B-N a-bond (electronegativity B, 2.0, N, 3.05) in a another atom or ion. direction opposite to that in the B-N n-bond. In fact the drift in electron density in B-N u-bond outweighs the drift in electron density in the B-N n-bond so that the nitrogen atoms are relatively negative.

SAQ 8 Borazine is isoelectronic with benzene but it is much more reactive than benzene. Explain briefly why this is so and what will be the nature of reagent attacking the borazine molecule.

I ...... Y"" 6.9 COMPLEXATION BEHAVIOUR

As compared to the elements of Groups 1 and 2, elements of Group 13 show a greater tendency of complex formation. Bec?use of lack of d-orbitals, boron is invariably tetrahedrallv coordinated in these compounds. For example, in compounds like NaBH,, NaBF,, NaB(C,H,),, BH3.NMe3, BF3-NH3as well as in chelates such as [B(O-C,H,O~)~]-and [B(o-OC~H,COO)~]-the coordination number of boron atom is four. Due to the presence of d-orbitals, the higher members of the group can expand their coordination number even up to six. Thus, Al, Ga, In and T1 form complexes such as (i) A1Cl3.NMe3,RCO+AI&(X= CI, Br), Et4N+M& (M = Al, Ga, X=Cl, Br) in which the coordination number is four, (ii) AlC13.2NMe3,in which the coordination number is five, and (iii) Na3[AlF6] in which the coordination number is six. With chelating ligands like p-diketones, pyrocatechol, dicarboxylic acids and 8-quinolinol, Al, Ga, In and T1 form anionic or neutral complexes in which the coordination number of the metal is six. Structures of some of these complexes are shown in Fig. 6.10. You know that the formation of AlCl, is important in Friedel-Crafts reaction whereas the 8-quinolinol complex of aluminium is used in gravimetric estimation of aluminium.

acetyl acetone oxalate 8-quinolinol complex complex complex

Fig. 6.10: Some chelates of Al, Ga, In and TI

In addition to coordination complexes, aluminium forms a number of double sulphates of general formula MAI(S04)2.12H20,where M is usually K, Rb, Cs or NH,. These double sulphates are known as alums. For example, potash alum, KA1(S04)2.12H20 and ammonium alum, NH4AI(S04)2.12H20.In alums, A1 can- .also be replaced by a number of cations of the same charge and not too different in size, e.g., Ga, In, Ti, V, Cr, Mn, Fe and Co. For example, chrome alum, KCr(S04)2.12H20and ferric alum, NH4Fe(S04)2.12H20. The alums are isomorphous with each other. It is important to realise that the alums are double salts and not complex salts. In solution they behave simply as a mixture of component sulphates and give reactions of their individual cations.

SAQ 9 a) Explain why boron cannot expand its coordination number beyond four

b) Work out the coordination number of the metal in the chelates given in Fig. 6.10.

6.10 ANOMALOUS BEHAVIOUR OF BORON

Just like lithium and beryllium in Groups Land 2, boron also shows anomalous behaviour. In general, the boron chemistry resembles that of silicon (occupying a diagonal position in the periodic table) more closely than that of Al, Ga, In and TI. This is because of the small size and high electronegativity of boron as compared to those of Al, Ga, In and TI. B~ronresembles silicon and differs from Al, Ga, In and TI in the following manner: '\ Both B and Si are nonmetals whereas Al, Ga, In and TI are distinctly metallic in Ekmene of Group 13 nature. B(OH), and Si(OH)4 are acidic in nature, AI(OH), and Ga(OH), are amphoteric and In(OH), and Tl(OH), basic in nature. The hydrides of B and Si are volatile, spontaneously inflammable liquids. These are readily hydrolysed by water and acids whereas aluminium hydride is a nonvolatile polymeric solid. Hydrides of Ga, In and T1 are not stable. BC13 and SiCI, are monomeric covalent compounds which are readily hydrolysed by water to B(OH), and Si(OH)4. Anhydrous AlCl, is also a covalent compound and exists in the form of a dimer, i.e. A12Cl,. On dissolving in water, it readily gives ~l,+(a~)and C1-(aq) whereas ~,+(a~)does not exist. Both Bz03 and Si02 are acidic in nature and react with metallic oxides on fusion to form borate and silicate glasses. On the other hand, A1203 is amphoteric. Boron reacts with more electropositive elements, i.e. metals to form borides which are very hard substances; Al, Ga, In and TI form alloys with metals. SAQ 10 List four Crdperties in which boron differs from rest of the elements of the group.

Let u: now summarise the main points of the chemistry of Group 13 elements which I you have studied in this unit. Boron, aluminium, gallium, indium and thallium are members of Group 13 of the periodic table. You have studied occurrence, extraction, uses and the general characteristics of these elements and gradations in their properties. Boron the first member of the group exhibits anomalous behaviour showing 1 resemblance to silicon and differing from other members of the group. 1 Hydrides of boron and aluminium are electron deficient compounds and exhibit three centre electron pair bonding in addition to normal electron pair bonding. Complex hydrides of boron and aluminium are important reducing agents. Halides gf boron are monomeric covalent compounds which are hydrolysed by water. Boron halides exhibit pn-pn bonding. Aluminium trffluoride is an ionic solid whereas its other halides when anh drous are dimeric covalent compounds. In aqueous solution, the halides furnish AlK (aq) ions. Boron forms two stable acids, viz., orthoboric acid and metaboric acid.'In solution metaboric acid changes into orthoboric acid which behaves as a w'eak monbbasic acid. Salts of these acids are known as borates. Boron forms borides, boron nitride and borazine. Borides are extremely hard compounds. Boron nitride which b isoelectronic with C2can exist in diamond and graphite forms. Borazine is isoelectronic with benzene and is also known as inorganic benzene. Aluminium oxide is an extremely stable ionic compound which can exist in two different forms: a-A1203 and 7-Al2O3. Boron and aluminium form a large number of addition complexes and chelate complexes with various nitrogen and oxygen donor ligands.

6.12 TERMINAL OUESTIONS

1 Describe various steps involved in the extraction of aluminium from bauxite. Explain why it cannot be prepared by reduction of A1203 with C. 23 2 While Al, Ga, In and TI form both covalent and electrovalent compounds. boron forms mostly only covalent compounds. How would you explain this behaviour? 3 Describe the structure of diborane molecule. What is the nature of bonding in this molecule? 4 Write chemical equations amphoteric nature of AI2O3 and AI(OH),. 5 How would you explain nature of boron? 6 How would you prepare borazine? Compare its.pmperties with those of benzene. 7 Diborane is added to CH~-CH=CH~and the product treated with (a) CH3COOH (b) H202/0H-. What will be the product formed in each case?

6.13 ANSWERS

\ Self Assessment Questions 1 a) Aluminium is the most abundant metallic element in the earth's crust. Its two important ores 'are bauxite, AI20,.xH20 and cryolite, Na3AlF,. b) i) - b) , ii) - c), iii) - a), iv) - e), v) - d) . 2 Due to insertion of ten 3d elements between Ca and Ga, the effective nuclear charge of Ga is higher. Consequently its atomic radius is smaller as well as its ionisation energies and electronegativity are higher than expected. 3 a) By the action of conc. HN03, a thin impervious, coherent, unreactive layer of A1203is deposited over the surface of Al metal. This coating protects Al from further attack and thus A1 becomes passive. b) With halogens, oxygen, sulphur, nitrogen and carbon, boron and aluminium form compounds having the formulae BX3 and AIX,, B203 and A1203, B2S3 and A12S3, Bfd-and AIN, BI2C3 and A14C3, respectively. 4 a) In a two centre electron pair bond, a pair of electrbns binds two atoms, whereas in a three centre electron pair bond a pair of electrons binds three atoms together. b) LiAIH, is a more versatile reducing agent than NaBH,; it reduces as many as sixty functional groups in organic chemistry. On the other hand, NaBHj is more selective. For example, NaBH, reduces >C=O group in aldehyde and ketones to alcohols, but functional groups such as >C=C<, -COOH aRd -NO2 are not normally attacked. c) i) NaBH, ,,~ii) LiAIH4 / 5 In BCI, boron attains an octet of electrons'by means of pn-pn binding between B and C1 atoms. Thus it exists as a monomeric gas. Due to the larger size of

aluminium iq aluminium trichloride, efficient pn-pn donding cannot take place \ between A1 and CI atoms. Consequently, aluminium trichloride dimerises to attpin an octet of electrons around A1 by forming a dative bond between C1 and Al atoms. Thus, it exists as a solid. \ 6 The elements of Group 13 form oxides of the type M203which become more basic 6n moving down the group. Thus. B2O3 is acidic, A1203 and Ga203are amphoteric, whereas In203and T1203 are basic. 7 a) Boric acid behaves as a weak monobasic acid because it ionises in water by accepting a pair 'of electrons from the hydroxyl ion as given'below: H3B03 + 2H20 -. [B(OH),]- + H30- b) As explained above, boric acid is a weak monobasic acid. On addition of glycerol to boric acid, a chelate complex is formed due to which the strength of boric acid increases by a factor of lo4. Thus, in the presence of glycerol, boric acid can be used as a primary standard using phenolphthalein as indicator. 8 In benzene, the n electrons are completely delocalised over all the c&b,on atoms whereas in borazine these are only partially delocalised. Due to the difference in electronegativities of B and N atoms, there is a drift in the electron density in both q- and a- B-N bonds. The drift in B-N u- bond is from B to N and it outweighs Elements of Group the drift in B-N a- bond which is from N to B. Thus, nitrogen acquires a net negative charge whereas boron acquires a net positive charge. This results in a nucleophilic attack on boron in borazine. 9 a) As boron can accommodate at the most eight electrons in its outermost shell (n = 2), it cannot form more than four electron pair bonds and expand its coordination number beyond four. b) The coordination number of the metal in all the chelates lshownin Fig. 6.10 is six. * 10 i) Boron is a nonmetal, whereas Al, Ga, In and T1 are metals. ii) ~,+(aq)does not exist, whereas Al, Ga, In and TI easily furnish M3+(aq). iii) B(OH), is acidic whereas Al(OH), and Ga(OH), are amphoteric and In(OH), and TI(OH)3 are basic in nature. iv) The halides of boron exhibit pa -pa bonding whereas those of Al, Ga, In and TI do not. Also see other differences in Section 6.10. Terminal Questions 1 Purification of bauxite and electrblysis of fused A1203in cryolite are the tw&steps involved in extraction of Al. ~lumihiurncannot be prepared by reduction of A1203 with C, because on heating A1203 with C, AI4C3 is formed. 2 Because of their high ionisation energies, all the elements of Group 13 form covalent compounds. But due to their high enthalpies of hydration, compounds of Al, Ga, In and. TI ionise in aqueous solution to furnish M3'(aq)'ions, also. 3 See sub-section 6.4.l(c). 4 + 3H2SO4 A12(S04)3 + 3H20 \ AI2O3 + 2NaOH + 3H20 2NaAl(OH), Al(OH), + 3HC1 AICI, + ~H,O Al(OH), + NaOH NaAI(OH)4 5 Because of its extremely small size and high electronegativity boron exhibits anomalous behaviour. 6 See sub-sec. 6.8.1. , UNIT ELEMENTS OF GROUP 14 I 4 Structure 7.1 Introduction Objectives 7.2 Occurrence, Extraction and Uses Occurrence Extraction Uses 7.3 General Characteristics Physical Properties Multiple Bonding Catenation Chemical Properties Complex Formation 7.4 Anomalous Behaviour of Carbon 7.5 Silica and Silicates Silica Silicates Sillcones 7.6 Chemistry of Divalent Silicon, Germanium, Tin and Lead Compounds 7.7 Summary 7.8 Terminal Questions 7.9 Answers 9

7.1 INTRODUCTION

In Units 4 and 5 you have studied the general characteristics of the elements of . ,s-block, as well as the periodicity in their properties. You have also studied in Unit 6 these aspects of the chemistry of elements of Group 13, which belongs top-block. Yo? would have noticed that while s-block elements show a regular gradation in properties down the group, the elements of Group 13 stlow some irregularities. It

- was also pointed out that the first element in each group shows some anomalous behaviour . Now we extend our study to another group of p-block elements, namely, Group 14, which consists of carbon, silicon, germanium, tin and lead. This is the first group in which the transition from non-metals, C and Si through typical metalloid, Ge, to weakly electropositive metals, Sn and Pb, can be clearly seeri. However, this does not imply that the properties of carbon are completely non-metallic; its crystalline forms are lustrous, one allotrope (graphite) conducts electricity. Tin and lead, on the other hand, form amphoteric oxides and volatile chlorides. We will discuss these properties in this unit. In the next unit you will study the chemistry of Group 15 elements. -

I Objectives After studying this unit you should be able to : explain the occurrence, extraction and uses of the elements of carbon family, explain allotropy and describe different forms in which these elements exist, compare the general characteristics of the elements of carbon family, explain catenation with special reference to carbon, describe internal n-bonding and the concept of complex formation by elements of this group, explain the nature of bonding in carbides, describe the chemistry of halides, hydrides and oxides of these elements, describe the structure and uses of silicates and silicones, and 26 describe the chemistry of divalent compounds of these elements. \~'\