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CHEM2007 - General Inorganic Chemistry > s-Block Chemistry > Flashcards

s-Block Chemistry Flashcards

1
Q

What are the features of group 1 electronic transitions?

A

Electronic transitions within metal atoms/ions in the flames fall in the visible part of the spectrum.

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2
Q

What are some trends in group 1 elements?

A

First IE is low for all but decreases down the group. This is due to an increase in atomic radius.

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3
Q

How do group 1 elements react with water?

A

2M + 2H2O → 2MOH + H2

These reactions are very exothermic and increase in violence from the lightest to the heaviest elements in the group.

They must be stored under hydrocarbon oil to prevent reaction with atmospheric oxygen.

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4
Q

What can happen in the bonding of lithium?

A

Li+ can show a high degree of covalent character in its bonding to the polarising power of the Li+ ions (high charge density).

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5
Q

What are the physical properties of group one elements?

A

They are very soft and have low melting points which decrease down the group - metallic bonding is weak as each atom only contributes one electron to the MO band.

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6
Q

Why are the group 1 elements strong reducing agents?

A

They have low ionisation energies and are easily oxidised. This is due to a larger radius of the atom and the electron is far from the nucleus.

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7
Q

Why are the group 1 elements poor complexing agents?

A

They have a low charge with a relatively large ionic radius. ΔHhyd is low.

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8
Q

What are the features of group 1 nitrides?

A

Lithium is the only alkali metal to form a nitride from the element and N2 gas.

However, sodium has been shown to form the nitride at liquid nitrogen temperatures.

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9
Q

What are the features of group 1 azides?

A

The azides can be synthesised through:

2 NaNH2 + N2O → NaN3 + NaOH + NH3.

NaN3 can then be used to form organic azides.

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10
Q

What are the features of group 1 hydrides?

A

All form ionic hydrides when reacted with hydrogen; these salts react vigorously with water:

NaH(s) + H2O(l) → NaOH(aq) + H2 (g)

They are used as reagents for the synthesis of other compounds.

They are also useful as non-nucleophilic bases and reductants.

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11
Q

What are the features of group 1 halides?

A

They are colourless crystalline solids, high melting temperature.

Enthalpies of formation are large and negative; for fluorides enthalpy of formation is less negative down the group; for other halides the enthalpies of formation are more negative down the group.

All halides (except LiF) are soluble in water to give ionic solutions. LiF is only sparingly soluble in water due to high UL.

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12
Q

How do group 1 elements react with oxygen?

A

All group 1 elements react vigorously with oxygen - however, the products differ.

4Li + O2 → 2Li2O

2Na + O2 → Na2O2

K + O2 → KO2 (Rb and Cs too).

All can be prepared for all alkali metals but LiO2 and Li2O2 are unstable with regards to decomposition.

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13
Q

What are the features of group 1 oxides?

A

They are basic and react with water to form MOH and other products depending on whether the peroxide or the superoxide is used.

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14
Q

How are group 1 oxides formed?

A

By heating the metal with limited O2 or by thermal decomposition of peroxide or superoxide.

Na2O2 →Na2O + 0.5O2.

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15
Q

What are the trends in group 1 metal oxides?

A

Stability of peroxides and superoxides increases down the group.

This is linked to lattice energy strength. On descending the group M+ radius increases and UL of the oxide decreases. Difference between the two UL decrease leading to a lower tendency to decompose.

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16
Q

How are group 1 ozonides formed?

A

By burning M, or heating the peroxide or superoxide with ozone.

These are unstable and explode violently.

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17
Q

What are the features of group 1 coordination chemistry?

A

Group 1 cations (Li to K) are hard Lewis acids and therefore form complexes from Coulombic interactions with hard donors such as O and N.

They all coordinate to six water ligands apart from lithium which forms four coordinate bonds.

In aqueous solution they readily exchange (slower for harder Li+ and faster for softer Rb+ and Cs+)

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18
Q

What are the features of crown ethers?

A

Macrocyles form very stable complexes with alkali metals.

Crown ethers are cyclic polyethers with repeating -OCH2CH2- units.

The cavities compare in size well to the group 1 metals.

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19
Q

What is the macrocyclic effect?

A

A combination of an entropic effect as seen in the chelate effect, together with an additional energetic contribution from the preorganised nature of the ligands.

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20
Q

Which metal is the most stable when binding to crown ethers?

A

Potassium. This is attributed to the formation of five-membered chelate rings when the crown ether binds. The size of K+ is ideally suited to this.

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21
Q

What are the features of cryptands?

A

Bicyclic cryptands form more stable complexes with alkali metals than crown ethers.

Unlike crown ether, the cavity size is more rigid and therefore cannot contract to fit small cations and cannot expand to fit large cations. This results in more selectivity.

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22
Q

How can alkalides (alkali metal with a negative charge) be formed?

A

By adding cryptands to solutions of the metal in amines.

Na- is very similar in size to I-.

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23
Q

How can electrides (compounds with solvated electrons) be formed?

A

Additions of sodium metal with ammonia and a cryptand or crown ether.

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24
Q

What are the features of group 1 organometallic chemistry?

A

The alkali metals form a number of organometallic compounds which are unstable to water and are pyrophoric.

The ionic character of the M-C bond increases down the group as the metals become larger and less polarising.

Organometallic chemistry with heavier alkali metals is limited.

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25
Q

How are organolithium compounds formed?

A

Direct synthesis: CH3Br + 2Li –Et2O at 20 degrees → LiCH3 + LiBr

Metallation: C5Me5H + nBuLi –THF at -78 degrees→ LiC5Me5 + nBuH.

Metallations are actually an equilibrium which is shifted to the right with increasing CH acidity of R’-H.

26
Q

What are some widely used organolithium deprotonating agents?

A

MeLi, nBuLi and tBuLi.

27
Q

What are the features of organolithium deprotonating agents?

A

They react rapidly with water and are pyrophoric.

Their basicity increases with the number of alkyl groups.

28
Q

How are nBuLi and tBuLi important in lithium/halogen exchange reactions?

A

They can be used to form halogenoalkanes.

nBu-Li + R-X ↔ nBu-X + R-Li.

The position of the equilibrium varies with the stabilities of the carbanion intermediates involved.

29
Q

What do organolithium compounds form in solution and solid state?

A

They form clusters due to electron deficiency by the formation of multicentre bonds.

These consist of an Li skeleton with 4 Li sp3 hybrids per Li atom:
1 x axial.
3 x tangential pointing out of the triangular faces.

These 3 form group orbitals.

30
Q

Why do Be and Mg not have a characteristic flame test colour?

A

They are smaller and have a higher Zeff. This means a greater amount of energy is needed for excitation.

31
Q

Why are group 2 metals harder and have higher melting points than group 1 metals?

A

The metallic bonding is stronger as 2+ charges are formed with 2 electrons per ion.

32
Q

What happens to IE, reactivity and electronegativity down group 2?

A

IE decreases.

They become more reactive.

Electronegativity decreases.

33
Q

How do group 2 metals react with water?

A

M + 2H2O → M(OH)2 + H2.

Ca, Sr, Ba and Ra will react readily with cold water whilst Mg will only react with warm water or steam.

34
Q

How do group 2 metals react in air?

A

It is covered by a thin layer of BeO.

Mg and Ca also form an oxide layer in air.

Sr and Ba ignite in air.

35
Q

What are the features of Be2+?

A

It is very small and has a high charge density and polarising power. Therefore it forms compounds that are largely covalent and Be2+ is a strong Lewis acid.

36
Q

How do group 2 metals react with oxygen?

A

All of them form oxides with the formula MO by direct combination with oxygen.

However Ba forms BaO2.

37
Q

How do group 2 peroxides react?

A

They decompose to form the oxide.

MO2 → MO + 1/2 O2.

38
Q

What happens to thermal stability of peroxides down group 2?

A

Thermal stability increases.

MgO2 is the least stable peroxide due to differences between the lattice enthalpies of O2- and O22-.

39
Q

How do group 2 metals form nitrides?

A

The metal is heated in a N2 atmosphere.

3M + N2 → M3N2

Nitrides will form the hydroxide and ammonia upon reaction with water:

M3N2 + 6H2O → 3M(OH)2 + 2NH3

40
Q

How are group 2 hydrides formed?

A

They are formed from the direct reaction of the metal with H2.

With the exception of Be, they all form ionic saline hydrides that contain the H- ion. These all react vigorously with water.

MgH2 + 2H2O → Mg(OH)2 + 2H2

41
Q

How does the reactivity of metal hydrides in water vary between group 1 and group 2?

A

The reaction for group 1 metals is much more violent.

42
Q

Why is Mg suitable for hydrogen storage?

A

Hydrogen storage requires reversible uptake of H2 near room temperature.

MgH2 reversibly loses H2 above 250 °C.

43
Q

What are the features of BeH2?

A

BeH2 is covalent and has a network structure with bridging H atoms.

44
Q

What are the features of Be halides (specifically BeCl2)?

A

They are covalent.

Solid state: polymeric chain with bonding based on sp3 hybridisation.

Vapour phase: forms dimer (sp2).

Above 900 °C: linear monomers are formed (sp).

It is a Lewis acid and forms adducts with the electron pair donors.

45
Q

What is quasilinear?

A

Refers to a species which the calculated energy difference between linear and bent structures (with a change of >20°) is less than 4 kJ / mol.

This means the molecule will resonate between linear and bent.

46
Q

What are the shapes of group 2 metal halides?

A

There is a preference for bent structures for the heavier metals with F, Cl or Br and this can be explained in terms of inverse polarisation. This occurs when the metal ion is polarisable and is polarised by F-, Cl- or Br-.

An alternative explanation is the participation of d orbitals bonding. This is because the d orbitals can only overlap efficiently if the molecule is bent.

47
Q

What metal aqua ion does Be form?

A

[Be(H2O)4]2+.

The rest form the octahedral metal aqua ion.

48
Q

How do macrocyclic ligands such as crown ethers and cryptands form complexes with group 2 metals?

A

They form stable complexes with Mg2+ to Ba2+.

Like group 1 metals, there is selectivity matching cation and ligand cavity sizes.

49
Q

What are the features of Be organometallic chemistry?

A

BeR2 compounds are Lewis acids and coordinate to solvent.

Be(CH3)2 is a polymeric solid (analogous to BeCl2), however the bonding bridges are 3c-2e.

In the gas phase it exists as monomeric and linear (sp hybrids).

50
Q

What are the features of other group 2 organometallics?

A

Alkyl and arylmagnesium halides (Grignard reagents) are well known.

Heavier organometallics pose significant challenges due to their higher reactivity.

51
Q

How are group 2 organometallics synthesised?

A

Metathesis: MX2 + 2KR → MR2 + 2KX

Transmetallation: HgR2 + M → MR2 + Hg.

Schlenk equilibirum: 2RMX ↔ MR2 + MX2

52
Q

What are the features of Beryllocene?

A

In this solid state has a slipped sandwich structure: (η1-C5H5)(η5-C5H5)Be

The bonding is essentially ionic and the η1 ring is bound through a C (pz) orbital of the Cp π-system.

53
Q

How does the bonding in BeCp*2 differ to BeCp2?

A

It forms a normal η5, η5 sandwich structure.

54
Q

What are the features of other group 2 metallocenes?

A

MgCp2 adopts a normal linear η5, η5 sandwich structure but the bonding is largely ionic.

Heavier metallocenes are challenging to make as they are very sensitive to air and water.

They form extended structures such as polymeric chains in the solid state.

55
Q

What are the features of the shapes of heavier (Ca onwards) metallocenes?

A

They adopt a bent sandwich structure and the bent angle gets smaller as the element gets heavier.

This is caused by a number of effects:

inverse polarisation
d-orbital involvement
increases van der Waals attraction between the ligands.
Small differences in energy between linear and bent geometries (floppy organometallics).

56
Q

How are Grignard reagents made?

A

By inserting Mg into a C-X bond.

Mg + RX –Et2O→ RMgX(Et2O)n

where X = I, Br or Cl

57
Q

How does a Grignard reaction take place?

A

Via an electron transfer (ET) process.

R-X + Mg –ET→ RX.- + Mg+ → R. + XMg. → RMgX

The Grignard reagent functions as a nucleophile and attacks the electrophilic groups.

58
Q

What are general considerations for a Grignard reaction?

A

Grignard reactions involve the use of finely divided Mg which can get coated with a passivating layer of MgO which stops the reaction. Using Rieke magnesium or adding I2 or dibromoethane can help to initiate the reaction.

Air and moisture must be excluded to avoid hydrolysis or oxidation.

Coordinating solvents such as Et2O or THF must be used.

Organic bromides or iodides are generally used although chlorides are also seen. Fluorides are generally unreactive but can be made reactive with Rieke Mg.

59
Q

What is an alternative preparation of Grignard reagents?

A

A transfer of Mg from a Grignard reagent to an organic halide. This has the advantage that the Mg transfer tolerates many functional groups.

60
Q

What is the Schlenk equilibrium?

A

A chemical equilibrium in solutions of Grignards and Hauser bases (R2NMgX).

The position of the equilibrium is influenced by solvent, temperature and the nature of R and X.

1,4 dioxane pushes the equilibrium to the RHS as it precipitates.

61
Q

What is ether cleavage?

A

In Group 1 and 2 chemistry, the organometallic complex can react with ethers.

62
Q

What happens with heavier group 2 Grignard complexes (Ca down)?

A

Ether cleavage and Schlenk equilibrium issues play a huge role in their chemistry and therefore, these compounds are very rare.

CHEM2007 - General Inorganic Chemistry (6 decks)

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