Izod L4-6

Question 1

Describe group 13

Answer 1

Ground state electron configuration ns2np1

Question 2

What are the common oxidation states for group 13 metals

Answer 2

+1 and +3

Question 3

When is the +3 oxidation states for for group 13 observed

Answer 3

+3 observed for all group 13

Question 4

When is the +1 oxidation states for the group 13 observed

Answer 4

+1 observed for In, Tl
Lower oxidation state favoured for heavier elements

Question 5

Why is the +1 oxidation states for group 13 due to

Answer 5

The inert lone pair effect

Question 6

Describe bonding for boron

Answer 6

Valence electrons of boron experience high effective nuclear charge. Therefore the bonding is covalent

Question 7

Why are boron trihalides good Lewis acids

Answer 7

Boron has only 6 electrons in its valence shell in these molecules.
It has an empty 2p orbital and can accept a lone pair from a Lewis base. Therefore, BX₃ compounds can form adducts with electron-pair donors.

Question 8

Describe the order of trihalides in terms of Lewis acidity

Answer 8

BBr3>BCl3>BF3

Question 9

What happens when BF3 reacts with a Lewis base

Answer 9

BF3 reacts with Lewis bases to give adducts. During reaction lone pair attacks lumo (pi) populates pi. Breaks pi interaction forming new sigma bonds.

Question 10

Why is BF3 a poorer Lewis acid

Answer 10

Strongest more stable pi-bond formed in BF3. Lower energy homo (pi), higher energy lumo (pi*) due to good pi overlap therefore poorer Lewis acid

Question 11

Why is BBr3 a stronger Lewis acid

Answer 11

Poor B-Br pi overlap. Higher energy homo (pi), lower energy lumo (pi). Pi is more accessible therefore stronger Lewis acid.

Question 12

What is B(OH)3

Answer 12

Boric acid

Question 13

Describe the shape and reactivity of B(OH)3

Answer 13

B(OH)3 planar with delocalised pi-orbital across BO3 core.
B(OH)3 Lewis acidic electron deficient.
Also B3+ is very polarising as it so withdraw electron density and therefore is a bronsted acid .

Question 14

Describe the structure of B(OH)3

Answer 14

Since O-H polar -> O-H - - - - O hydrogen bonding. B(OH)3 solid with sheet structure

Question 15

Describe the reaction when B(OH)3 with OH-

Answer 15

B(OH)3 + OH- -> [B(OH)4]-

Question 16

Describe the boron hydrides BH3

Answer 16

BH3 actually B2H6. Dimerises through donation of B-H electron density into vacant p orbital on adjacent B.

Question 17

Describe B2H6

Answer 17

B2H6 is Lewis acid and forms abducts

Question 18

Which group 13 element is the most electropositive and why

Answer 18

Aluminium is the most electropositive element in the group due to 3d/lanthanide contractions. More covalent character

Question 19

Describe aqueous solutions of aluminium and write the reaction

Answer 19

Aqueous solutions of aluminium are acidic.
[Al(OH2)6]+3 + H2O -> [Al(OH2)5(OH)]+2 + H3O+
Very polarising Al +3 centre

Question 20

Describe the group 13 trifluorides

Answer 20

MF3 poorly soluble high m.p network materials

Question 21

Describe the group 13 trichlorides and tribromides

Answer 21

MCl3/MBr3 dimeric molecules

Question 22

Describe the group 13 trihalides

Answer 22

All MX3 are Lewis acidic and forms aducts

Question 23

What is alane

Answer 23

AlH3

Question 24

Describe the chemistry of alane/AlH3

Answer 24

High melting point solid
Reacts with H2O
Readily forms aducts, the adducts is a very useful hydride transfer reagent

Question 25

Write the reaction with AlH3 and H2O

Answer 25

AlH3 + 3H2O -> Al(OH)3 + 3H2 Very vigorous

Question 26

Describe GaH3

Answer 26

Thermally unstable liquid

Question 27

Describe InH3/TlH3

Answer 27

Cannot be isolate

Question 28

Electron configurations group 14 elements

Answer 28

Ground state electronic configuration ns2np2.

Question 29

Describe the common oxidation states for group 14 elements

Answer 29

+4 all element less favoured for Pb +2 for heavier elements especially Sn and Pb - due to inert lone pair effect

Question 30

Compare the reactivity of CCl4 and SiCl4 with water

Answer 30

CCl4 is immiscible with H2O SiCl4 highly reactive towards H2O

Question 31

Write the reaction when CCl4 reacts with H2O

Answer 31

CCl4 + 2H2O -> CO2 + 4HCl But reactions do not proceed

Question 32

Write the reactions when SiCl4 react with H2O

Answer 32

SiCl4 + 2H2O -> SiO2 + 4HCl Very vigorous instantaneous reactions

Question 33

Why does CCl4 not react with water

Answer 33

In CCl4 C uses all 4 valence orbital (2s/2p) to form 4 sigma bonds. There is good C-Cl orbital overlap. Low energy homo (σ), high energy lumo σ*

Question 34

Why does SiCl4 react vigorously with water

Answer 34

In SiCl4 Si uses all 4 valence orbital (3s/3p) to form 4 sigma bonds. But the silicon valence orbital large/diffuse. Si-Cl orbitals overlap poor. High energy homo σ, low energy lumo σ*. Low lying Si-Cl σ* orbitals are susceptible to attack by nucleophile therefore rapid hydrolysis. C-Cl σ* orbitals to high in energy so not energetically accessible

Question 35

How is a Si-X stronger than C-X but Si-X σ overlap poorer than C-X σ overlap?

Answer 35

Si valence orbitals are large/diffuse, poor Si-X σ overlap, low energy σ*. Lone pair on X overlaps with Si-X σ*. Therefore hyperconjugation with Si-X stronger than just σ bond. There is no hyperconjugation for C-X as the σ* too high in energy for good overlap with X lone pair. C-X purely σ bonding

Question 36

What does the effect of hyperconjugation decrease with

Answer 36

Effect of hyperconjugation decreases with increasing atomic number of X so the difference between C-X and Si-X descreases

Question 37

Describe the reactions of organosilicon halides of SiCl4 with RMgX

Answer 37

SiCl4 + 2RMgX -> R2SiCl2 + 2MgXCl