Group 13

Question 1

Why are ionic compounds contains B3+ ion not known?

Answer 1

First three ionisation enthalpies for boron are very high - so boron compounds contain covalent bonds

Question 2

How do group 13 overcomes their lack of valence electrons?

Answer 2

By forming 3 centre bonds - often leads to formation of dimers and polymers containing bridging groups

Question 3

Group 13 elements?

Answer 3

Boron no metal

Aluminium, gallium, indium and thallium are all metals

Question 4

How is boron obtained?

Answer 4

Elemental boron extracted from deposits of borax several different allotropes of boron nd these all have structures B12 icosahedra interlinked into 3D networks

Question 5

How is aluminium extracted?

Answer 5

From the ore bauxite AlO(OH) or Al(OH)3, Bayer process dissolves bauxite in aqueous sodium hydroxide to convert it into aluminate [Al(OH)4]- ions, aluminate is cooled to precipitate and solid hydroxide is heated to covert it into oxide
In the second stage aluminium metal is obtained from the purified aluminium oxide by electrolysis where Al2O3 is dissolved in cryolite to reduce the melting point, carried out at 940 -980 degrees C
AlO(OH) + OH- + H2O –> [Al(OH)4]-
[Al(OH)4]- –> Al(OH)3 + OH-
This reaction is the most important
2Al(OH)3 –> Al2O3 + 3H2O

Question 6

Uses of aluminium?

Answer 6

High tensile strength low density, used in construction and manufacturing, low density and high electrical conductivity used in power lines, pure aluminium readily oxidised but in reality resistant to corrosion because of thin coating of oxide that forms on the surface acts as a physical barrier to reagents

Question 7

Group 13 oxides?

Answer 7

All group 13 burn in air to form oxides

eg 4Al + 3O2 –> 2Al2O3

Question 8

What are aluminium and gallium?

Answer 8

Both amphoteric and react with both acids and alkalis to liberate hydrogen

Question 9

Metallic character of indium and thallium?

Answer 9

Both dissolve in dilute acids but no alkalis reflecting the increase in metallic character going down the group

Question 10

Boron oxides?

Answer 10

Boron oxide B2O3 is acidic reacts with water to give boric acid
B2O3 + 3H2O –> 2B(OH)3
B-O bonds are strong, anions containing B-O bonds are called borates and there are many examples of these, their structures include cyclic and linear polymers containing planar BO3 and/or tetrahedral BO4 units that are linked by bridging oxygen atoms

Question 11

Structure of boric acid?

Answer 11

Layer structure molecules with the layers are linked by hydrogen bonds, weak acid but in contract to most oxyacids the acidity is not due to loss of a proton from an O-H bonds, boron atom acts as lewis acid interacting with a water molecule to form B(OH)3(OH2) which loses a proton to form the tetrahedral anion

Question 12

Heating boric acid?

Answer 12

Loses water to form B2O3 vis metabolic acid
2B(OH)3 –> 2HBO2 –> B2O3

Metabolic acid exists in the most common form of cyclic trimers

Question 13

Aluminium oxide?

Answer 13

Forms number of polymorphs which differ in reactivity

Question 14

Group 13 metal cations in solution?

Answer 14

From hexagonal aqua complexes with general formula [M(H2O)6]3+ high charge density on the metal ion polarises the O-H bonds of the coordinated water molecules, ions undergo hydrolysis as a results aqueous solutions are acidic

Question 15

Boron halides?

Answer 15

BF3, BCl3 gases under standard conditions
BBr3 liquid
BI3 solid
All are monomeric and have trigonal planar structures constant with sp2 hybridisation of the central boron atom, compounds are all lewis acids and form adducts with lewis bases, B-X bond length increases form fluoride to iodide

Question 16

Boron halides reactivity?

Answer 16

Lewis acidity increases in the order

BF3 < BCl3 < BBr3 < BI3

Question 17

p-pi-p-pi bonding?

Answer 17

pi bonding is distance sensitive so the p-pi-p-pi bonding is strongest in BF3 weakest in BI3 decreasing in strength as the size of the halogen atom increases. This bonding can only occur with sp2 hybridised boron as there is no available orbital of the correct symmetry to interact with the halogen p orbitals on a sp3 hybridised boron atom. The p-pi-p-pi bonding is therefore lost when the tetrahedral lewis acid lewis base adduct forms (bond formation is exothermic but the previous reorganisation is endothermic), since p-pi-p-pi bonding is weaker in the halides with large halogen atoms the adducts form more readily for these

Question 18

What about the boron halides structure?

Answer 18

Do not dimerise, two factors in the stability of the BX3 monomers are the presence of p-pi-p-pi bonding which would be lost in B2X6 dimers and the greater steric bulk that occurs with an increase in coordination number

Question 19

BF3?

Answer 19

Reacts with metal fluorides to give salts that contain the tetrafluoroborate anion BF4-, all B-F bonds have the same length longer than the bonds in BF3
BF3 synthesis:
B2O3 + 3CaF2 + 3H2SO4 —> 2BF3 + 3CaSO4 + 3H2O
excess water leads to hydrolysis of BF3:
4BF3 + 6H2O –> 3[H3O]+ + 3[BF4]- + B(OH)3
Forms complexes with ethers eg diethyl ether

Question 20

Bonds in 3 coordinate boron compounds?

Answer 20

Shorter than would be expected on the basis of the atomic radii, in addition to possible p-pi-p-pi bonding the other factors contributing to this are the large electronegativity difference between boron and other atoms which leads to a high ionic component to the bonding and the low amount of electron electron repulsion as there are only six valence electrons around the boron atom

Question 21

Aluminium fluoride?

Answer 21

AlF3 colourless unreactive, has an extended octahedral structure with 6F atoms each linked to 2Al, with a high degree of ionic character.
Prepared by reacting oxide with HF
Al2O3 + 6HF —> 2AlF3 + 3H2O
The other aluminium halides have considerably lower melting point and they hydrolyse rapidly in moist air to give the hydrogen halide eg
AlCl3 + 3H2O –> Al(OH)3 + 3HCl
Hydrolysis can occur with more water
AlCl3 + 6H2O –> [Al(H2O)6]3+ 3Cl-
In coordinating solvents adducts are formed AlCl3.Et2O

Question 22

Aluminium chloride?

Answer 22

Layer structure in solid but consists of covalently bonded Al2Cl6 dimers in the liquid and gas phases. These dimers are also present when aluminium chloride is dissolved in a non polar solvent such as benzene

Question 23

Aluminium bromide/iodide?

Answer 23

Both dimeric as solids as well as in the liquid and gas phases and in solution in non polar solvents. The dimers contain bridging halides. Each Al-Br-Al nit is held together by 3 centre 4 electron interaction with each bridging bromide donating three electrons are in a normal covalent bond and the other two in a dative bond. Formation of dimers is driven by the high lewis acidity of the aluminium centre in the AlX3 and made possible by the larger size of aluminium compared to boron

Question 24

Borides?

Answer 24

Elements with a lower or similar electronegativity form binary compounds with boron, wide range of borides ‘boron poor’ such as Mn4B, ‘boron rich’ CaB6 and boron carbide B12C3. Boron poor borides contain boron-boron bonds

Question 25

Reaction with group 15 elements?

Answer 25

Form 1:1 compounds, boron nitride BN several polymeoprhs and room temperature form has a layer structure similar to that of graphite with the hexagonal layers containing 6 membered rings with alternating boron and nitrogen bonds. Unlike graphite boron nitride does not conduct electricity - similar 6 membered rings are present in borazine B3N3H6

Question 26

Inert pair effect?

Answer 26

+3 oxidation state most stable at top of group but +1 most stable at the bottom due to the inert pair effect, increasing atomic size down the group makes weaker bonds so bond enthalpy decreases, higher promotions energies are required to involve the electron to involve the ns2 electrons for the +3 oxidation state is not offset by the energy gain when two new bonds are formed

Question 27

Group 13 halides?

Answer 27

AX3 all Lewis acids have an empty p orbitals, electron poor has 6 valence electrons so can accept electrons form a Lewis base to achieve a stable octet

Question 28

HBF4?

Answer 28

Commoercially available strong acid Et2O non isoluble, another method of synthesis:
B(OH)3 + 4HF –> H3O+ + BF4- + 2H2O
[BF4]- encountered in coordination chemistry ‘innocent’ weakly coordinating compared to strong acid, used to precipitate ions

Question 29

Boron halide ions?

Answer 29

BCl4-, BBr4- BI4- only stabilised by larked cations such as nBu4N+

Question 30

Halides of Al, Ga, In, Tl?

Answer 30

Unlike boron halides these tend to form dimer/oligomers/infinite networks
MX3, dimeric in solution formed from the elements 3 centre 4 electron bonding

Question 31

How is boric acid formed?

Answer 31

From hydrolysis of B2O3

Na2B4O7 —> B2O3 + 3H2O + Na2SO4 —> 2B(OH)3

Question 32

Most common soluble salt of aluminium?

Answer 32

Aluminium sulfate octadecahydrate is the most common soluble salt of Al, it is acidic in aqueous solutions, in water it from a strong hydration complex Al(H2O)6 3+ and this in turn hydrolyses
Al(H2O)6 3+ + H2O —> [Al(H2O)5OH]2+ +H3O+
Hydrolysis can continue until aluminium hydroxide forms
[Al(H2O)5OH]2+ + H2O —> [Al(H2O)4(OH2)]+ + H3O+
[Al(H2O)4(OH)2]+ + H2O —> [Al(H2O)3(OH)3] + H3O+
this product is a while precipitate

Question 33

Aluminium hydroxide?

Answer 33

Amphoteric, with acid it acts as a base in a neutralisation reaction
Al(OH)3 + 3H3O+ —> Al3+ + 6H2O
With bases a hydroxy ions forms (aluminate ion) this acidic behaviour of the hydroxide is typical of a non metal hydroxide
Al(OH)3 + OH- —> Al(OH)4-
This amphoteric character is a reflection of the partial non metal character of aluminium

Question 34

Organometallic chemistry of Al?

Answer 34

Al2Me6 volatile reaction liquid flammable in air, bonding similar to AlCl3, it no lone pairs, filled orbitals to donate electron density into the Al empty p orbital, 3 centre 2 electron bonding comparable to B2H6