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Isolation of boron

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General properties

  1. Melting points, boiling points, and structures: The melting points of group III elements do not show a regular trend as did the metals of groups I and II. The group III values are not strictly comparable because B and Ga have unusual crystal structures.
    The melting point of these elements decreases in the group but irregularities occur. B has very high melting point because of its unique covalent structure. Ga has extremely low melting point again because of its unique structure.
    The boiling point for B is unusually high, but the values for Ga, In, and Tl decrease on descending the group as expected. Note that the boiling point for Ga is in line with the others, whereas its melting point is not. The very low melting point is due to the unusual crystal structure, but the structure no longer exists in the liquid.
  2. Size of atoms and ions: The metallic radii of the atoms do not increase regularly on descending the group. However, the values are not strictly comparable because of their unique structures.
    Metallic radius (Å) Ionic radius Pauling electronegativity
    M3+ (Å) M+ (Å)
    B (0.885) (0.27) 2
    Al 1.43 0.535 1.5
    Ga (1.225) 0.620 1.2 1.6
    In 1.67 0.800 1.4 1.7
    TI 1.70 0.885 1.5 1.8
    1. There is no evidence for the existence of B3+ under normal condition, and the value of ionic radius is an estimate.The ionic radii for M3+ increase down the group, though not in the regular way as observed in groups I and II. There are two reasons for this:
    2. The electronic structures of the elements are different. Ga and In have a d10 inner shell which is poorly screening and so have higher ionization energies than would otherwise be expected. This contraction in size is sometimes called the d-block contraction.
      In a similar way Tl follows immediately after 14 f-block elements. The size and ionization energy of Tl are affected even more by the presence of 14 f-electrons, which shield the nuclear charge even less effectively. The contraction in size from these f-block elements is called the lanthanide contraction.
  3. Electropositive character: The electropositive or metallic nature of the elements increases from B to Al, but then decreases from Al to Tl.
  4. Ionization energy: The ionization energies increase as expected (first ionization energy < second ionization energy < third ionization energy). The sum of the first three ionization energies for each of the elements is very high. Thus, boron has no tendency to form ions, and always forms covalent bonds. The other elements normally form covalent compounds except in solution where the high hydration energy compensates the high ionization energy.
    The ionization energy values do not decrease smoothly down the group. These value decrease from B to Al is the usual trend on descending a group associated with increased size. The poor shielding by d-electrons and the resulting d-bock contraction affect the values for the latter elements.

Chemical properties of boron

Crystalline B is not active, while amorphous B reacts. The reactions are as follows:
  1. Reaction with air:
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    The above reaction accompanies red flame.
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  2. Action of alkali and acid are given as follows:
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  3. Boron reacts with Mg and consequent hydrolysis gives diborane:
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  4. Boron reduces SiO2, CO2:
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Compounds of boron

  1. Boron trifluoride (BF3)
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    (colorless liquid)
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    BF3 undergoes hydrolysis as
    4BF3 + 3H2O B(OH)3 + 3HBF4
    B(OH)3 + 4HF HBF4 + 3H2O
    Two-stage hydrolysis of BF3 is as follows:
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  2. Boron trichloride (BCl3)
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    When B2O3 is heated with PCl5 at 150°C, it forms BCl3.
    B2O3 + 3PCl5 2BCl3 + 3POCl3
    BCl3 hydrolyzes as follows:
    BCl3 + 3H2O B(OH)3 + 3HCl
    H[BCl4] does not exist, but H[BF3] exists.
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  3. Orthoboric acid (H3BO3)
    Structure of orthoboric acid: Orthoboric acid contains triangular Description: 51068.png units. In the solid, the B(OH)3 units are hydrogen bonded together into two-dimensional sheets with almost hexagonal symmetry. The layers are quite a large distance apart, and thus the crystal breaks quite easily into very fine particles.
    Na2B4O7 + 2HCl + 5H2O 2NaCl + 4H3BO3
    2CaO + 3B2O3 + 2SO2 + 9H2O 2CaSO3 + 6H3BO3
    2CaSO3 + 2H2O + 2SO22Ca(HSO3)2
    H3BO3 + H2O [B(OH4)] + H+(aq)
    Acidic properties of H3BO3 or B(OH)3: Since B(OH)3 only partially reacts with water to form H3O+ and [B(OH)4], it behaves as a weak acid. Thus, H3BO3 or [B(OH)3] cannot be titrated satisfactorily with NaOH, as a sharp end point is not obtained. If certain organic polyhydroxy compounds such as glycerol, mannitol, or sugars are added to the titration mixture, then B(OH)3 behaves as a strong monobasic acid. It can now be titrated with NaOH and the end point is detected using phenolphthalein as indicator (indicator changes at pH 8.3–10.0).
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    The added compound must be cis-diol to enhance the acidic properties. This means that it has OH groups on adjacent carbon atoms in the cis configuration.
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    B2O3 + H2O H3BO3 (silky, pearly white crystal)
    BF3 is a strong Lewis acid and forms adduct with NH3.
  4. Borax (Na2B4O7 10H2O)
    Structure of borax: In borax, two boron atoms are in triangular geometry and two boron atoms are in tetrahedral geometry. The ion is [B4O5(OH)4]2– and the remaining eight water molecules are associated with the two sodium ions. Hence, the borax contains tetranuclear units [B4O5(OH)4]2– and therefore is formulated as Na2[B4O5(OH)4 8H2O.
    Borax can also be prepared from certain other minerals such as boracite, colemanite, and boranatrocalcite. The minerals are powdered and boiled with sodium carbonate solution.
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    Borax is crystallized from the filtrate. Sodium metaborate, present in the mother liquor, is converted into borax by passing carbon dioxide through it:
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  5. Boron nitride (inorganic graphite)
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  6. BoranesGASG
    Structures of the boranes: In diborane, there are 12 valency electrons—three from each B atom and six from the H atoms. Electron diffraction results indicate the structure shown in the adjacent figure.
    The two bridging H atoms are in a plane perpendicular to the rest of the molecule and prevent rotation between the two B atoms. Specific heat measurements confirm that rotation is hindered. Four of the H atoms are in a different environment from the other two. This is confirmed that diborane cannot be methylated beyond Me4B2H2 without breaking the molecule into BMe3.
    It contains two 3-center 2-electron banana bonds (B…H…B). Two electrons from two H atom and two from two boron atoms consist of four electrons. An overlap of sp3 hybrid orbital of B and 1s hydrogen orbital gives the delocalized molecular orbitals of a B…H…B bridge.

Compounds of aluminium

Aluminium halides: The fluorides of Al, Ga, In, and TI are ionic and have high melting points. The other halides are largely covalent when anhydrous AlCl3, AlBr3, and GaCl3 exist as dimers, thus attaining an octet of electrons. The dimeric formula is retained when the halides dissolve in non-polar solvents such as benzene.
When the halides are dissolved in water, the high enthalpy of hydration is sufficient to break the covalent dimer into [M 6H2O]3+ and 3X ions. At low temperatures, AlCl3 exists as a close packed lattice of Cl with Al3+ occupying octahedral holes. On heating, Al2Cl6 species are formed and the volume of the solid greatly increases. This illustrates how close the bonding in this compound is to the ionic/covalent border.

Oxides and hydroxides of group III

On moving down the group, there is a gradual change from acidic through amphoteric to basic character of oxides and hydroxides.
Al(OH)3 is amphoteric. It reacts principally as a base, i.e., it reacts with acids to form salts that contain the [Al(H2O)6]3+ ion. However, Al(OH)3 show some acidic properties when it dissolves in NaOH, forming sodium aluminate. However, Al(OH)3 is reprecipitated by the addition of CO2, showing that the acidic properties are very weak.
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Ga2O3 and Ga(OH)3 are both amphoteric like the corresponding Al compounds. Ga(OH)3 is white and gelatinous and dissolves in alkali, forming gallates. Tl2O3 and In2O3 are completely basic and form neither hydrates nor hydroxides.
TlOH is a strong base and is soluble in water. Thus, TlOH differs from the trivalent hydroxides and resembles the group I hydroxides. Where an element can exist in more than one valency state, there is general tendency for the lowest valency state to be the most basic.

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