Main Group Chemistry 2

Front 1

Why is the chemistry of Si-O compounds dominated by polymeric structures?

Back 1

Because unlike carbon, 2(Si-O) bonds are stronger than 1 (Si=O). Combined they have a higher bond enthalpy.

Front 2

What is a siloxane?

Back 2

A functional group with an Si-O-Si linkage

Front 3

The addition of H2O to (OH)2C(R)2 gives a ketone. What does the addition of H2O to (OH)2Si(R)2 give?

Back 3

Cyclic siloxanes

Front 4

What is the most common siloxane polymer?

Back 4

Polydimethylsiloxanes (PDMS)

Front 5

Explain why the general formula for siloxane formula is: [(R2SiO)2(R2SiO3/2)]n

Back 5

Composed of linker stoichiometry and bridger sotichiometry. Linker stoichiometry involves a linear chain where each Si is bound to two R groups and two linking oxygens (difunctional). Each oxygen is shared with another Si giving 2*.5O = O. The layers are linked with a further bridging oxygen which is shared between Si in the linear chains giving 3/2 O. These Si only have one r group and 3 O so are known as trifunctional

Front 6

What is one method for preparing siloxanes? Give an example

Back 6

Hydrolysis-condensation of silyl chloride precursors (Me)nSiCl(4-n).

Front 7

What siloxanes will result from the hydrolysis-condensation of (Me)nSiCl(4-n) when:
n=3
n=2
n=1

Back 7

i) Me3Si-O-SiMe3
ii) -O-(Me2Si)-O- 1Dchains and rings
iii) (MeSi)O3 which forms 2D sheets and double chains

Front 8

Siloxane properties can be tailored to give Si specific roles in the polymer. How can the following be prepared:

Back 8

Using cyclic [Me2SiO]n in the presence of a catalyst gives open chain polymer. The acid catalyses cleavage and reformation of Si-O bonds randomsising cross-linking. End groups are provided by the addition of Me3Si-O-SiMe3.

Front 9

Compare physical properties of siloxane polymers to hydrocarbons

Back 9

Siloxane polymers are less congested and more flexible.

There are more compact random coils and so are less easily crystalised.

They readiy switch between spatial arrangements and so are more rubbery.

Front 10

Explain the reason for the difference in the physical properties between hydrocarbons and siloxanes outlined above.

Back 10

Siloxanes have longer bonds with wider bond angles and are less substituted.

Front 11

What are some useful properties of siloxanes?

Back 11

Water repellent

Flexible at low temperatures

Low toxicity

Resistant to damage by oxygen and UV light

Front 12

What are phosphazines?

Back 12

A group of P(V)/N(III) compounds featuring a chain of cyclic structures and are oligomers of hypothetical N≡PR2.

Front 13

Why is -N=PR2- related to -O-SiR2-?

Back 13

They are isoelectronic

Front 14

How are phosphazines synthesised?

Back 14

n(PCl5) + n(NH4Cl) ---> n(NPCl2) + 4n(HCl)

Front 15

What is important to note about cyclic phosphazine oligomers?

Back 15

There is delocalised pi-bonding

Front 16

How are linear phosphazine chains with [-N=PCl2-] repeating units formed and separated?

Back 16

Formed by ring-opening polymerization of cyclic oligomers at 200-300 degrees and separated using distillation.

Front 17

What 4 features does the bonding in phosphazine need to account for the fact that its bonding cannot be described by either the standard aromatice description or alternating single and double -P-N=P-N= bonds?

Back 17

1) The ring is very stable

2) P-N distances are the same around the ring

3) P-N bond length is shorter than for a single bond

4) Its spectroscopic and redox properties do not show aromatic behaviour

Front 18

Describe the orbitals available for bonding in a phosphazine repeating unit for P and N respectively:

Back 18

P: 4 σ orbitals and a single π electron available for bonding

N: Sp2 hybridized with lone pair in the plane and a single electron in Pz

Front 19

One theory (although flawed) for phosphazine bonding is that there is overlap between the empty P(3d) orbital and the singly occupied N(2p) orbital. Explain how this would account fopr the phosphazine observations above?

Back 19

P(3d) and N(2P) orbitals overlap to form pi bonds with 3 different combinations of orbitals. One involves the unused lone pair in N to bond to the in-plane P d-orbitals. One gives full delocalisation around whole ring and one gives islands of delocalisation with nodes at P atoms. This is consistent with the lack of aromatic behaviour but flawed as P(3d) orbitals are too large and diffuse to interact with N(2p)

Front 20

Why is the P(3d) N(2p) orbital overlap theory for phosphazine bonding flawed?

Back 20

Because the phosporus 3d orbitals will be too high in energy to ovlerlap well with the N(2P) orbitals

Front 21

Why is the simple ionic bonding theory for phosphozine bond formation a possibility? What does it assume?

Back 21

It is consistent with the lack of aromatic character, and consistent with highly polar bond character.

However, it does assume that there is no electron delocalisation.

Front 22

What is the accepted model for R3P=O bond formation?

Back 22

Backbonding occurs from oxygen (2p) lone pair. It does not go into P3d as too high in energy, but into P-C σ* bond which are at a lower energy.

Front 23

How can the model for R3P=O bond formation be applied to phosphazines? Why would this result in a double bond?

Back 23

N is sp2 hybridized. It has 2 lone pairs one of which is an sp2 lone pair and the other in a p(z) orbital. N(2p) forms a single bond with phosphazine whilst its p(z) lone pair can form dative bond by donating electrons into σ* P-Cl orbital. Overall gives a double bond.

Front 24

How do the three previously mentioned theories for phosphazine bond formation account for its character?

Back 24

Overlap of P(3d) with N(2p) accounts for lack of aromaticity (althought P(3d) too large and diffuse)

Ionic bonding model accounts for bond polarity

N(2p) overlap with (P-Cl)σ* has similarities with accepted model for R3P=O bonding

Front 25

Compare physical properties of phosphazine polymers to hydrocarbon-based polymers

Back 25

Polymer chains are less congested and more flexible.

More compact random coils (less easily crystalised)

Readily switch between spatial arrangements (more rubbery)

Front 26

Explain the difference in physical properties between phosphazine polymers and hydrocarbon-based polymers

Back 26

Less rigid, wider bond angles for PNP and NPN bonds compared to CCC.

Less substitution -C(H2)-C(H2)-C(H2)- compared to -N=PR2-N=PR2-

Front 27

What is meant by saying the phosphazines are unusually versatile?

Back 27

R groups can be changed easily.

Chemistry performed can be on the side group, changing the operties of the compound, e.g. elastomers to glass or bio-inert to bio-active

Front 28

What are the two methods of obtaining phosphazines othetr than (NPCl2)n?

Back 28

1) Alter the reactant - i.e. e.g. use PBr5 or Me2PCl2 instead of PCl5 to react with NH4Cl to give (NPBr2)n and (NPMe2)n respectively

2) Substitute for Cl in (NPCl2)n to give a range of products. Substitution of Cl- with NaOR or HNR2 groups can give a wide range of products with varying useful properties e.g. water resistance, fire resistance, water solubility etc.

Front 29

Give some practical applications of Cl- substitution in (NPCl2)n complexes

Back 29

Biomaterials

Fuel lines

Fire retardant coatings

Front 30

For a B-N compound, formally N should be positively charged as it is forming a dative bond with B as shown. What is the actual model for N-B bonding, why does this occur and what evidence is there for it?

Back 30

N is more elecronegative than C which is more electronegative than B. Evidence in MO calculations and experimental evidence.

Front 31

What are amine-borane adducts H3N-BH3 useful for?

Back 31

Hydrogen storage

Front 32

H3N-BH3 is analgous to H3C-CH3. Why is the former solid whilst the latter is a gas?

Back 32

H3N-BH3 more solid as more stable due to large dipole moment and strong intermolecular interactions. H3C-CH3 has no dipole moment and only weak intermolecular reactions.

Front 33

How can amine-borane adducts be synthesised?

Back 33

NH4Cl + LiBH4 --> NH3BH3 + H2 + LiCl

Front 34

Aminoboranes H2N=BH2 are isoelectronic with H2C=CH2. What can be noted about their stability?

Back 34

Stable but reactive. Very short-lived, condense to form cyclic square dimers and cyclo-hexane-like trimers with chemically equivalent B-N bonds.

Front 35

How can B=N bonds be stabilised?

Back 35

Using bulky groups attached to the Nitrogen

Front 36

Borazine is iso-electornic and iso-structural to benzene. Explain why the borazine compound fits with the electronegativities of boron and nitrogen:

Back 36

Front 37

What often drives the formation of borazine?

Back 37

The production of large amount of H2

Front 38

Why can borazines undergo addition reactions so easily with HX compounds e.g. HCl?

Back 38

Because X will attack electrophillic Boron(δ+) and H+ will be attacked by nucleophillic Nitrogen (δ-).

Front 39

How can the product of a borazine addition reaction be oxidised to lose 3H2? Why might you want to do this?

Back 39

Heat. Can then undergo nucleophillic susbstitution to produce other compounds that cannot undergo addition reactions.

Front 40

How are B≡N compounds synthesised?

Back 40

B2O3(l) + 2NH3(g) ---> 2BN(s) + 3H2O(l)

Front 41

What is chemically inert B≡N an analogue to? What forms can it therefore be in?

Back 41

Analagoue to elemental Carbon. Two forms are stacked layer of hexagoanal rings (e.g. graphite) and cubic structure (e.g. diamond)

Front 42

What is the difference between graphite and B≡N hexagonal structures?

Back 42

Graphite has staggered layers to minimise steric interactions whilst BN compounds have their hexagonal layers directly on top of each other to maximise intermolecular reactions.

Front 43

How can Hexagonal C and BN structures can be converted into cubic structures?

Back 43

With heat and a P catalyst and an alkali/alkali earth metal catalyst respectively.

Front 44

Briefly explain why properties such as molecular weight and density are almost identical for BN compounds and CC compounds, but properties such as melting and bopiling points are different?

Back 44

BN compounds show similar physical and structural similarities to CC analogues because BN is isolectronic with CC unit. However, they show different chemical behaviour due to polarity of B-N bond.

Front 45

Why can NO esist but NS cannot?

Back 45

NO is a stable radical but NS is not

Front 46

What is the easiest SN compound to prepare and use as a starting point for the synthesis of other SN compounds?

Back 46

S4N4, it is kinetically stable but thermodynamically unstable with respect to its elements S8 and N2.

Front 47

How can you synthesise S4N4?

Back 47

6S2Cl2 + 16 NH3 + CCl4 ---> S4N4 + 8S + 12NH4Cl

Front 48

What is the structure of S4N4? What properties does this give rise to?

Back 48

A cradle, held together by weak S----S interactions. Longer S--S bonds than S-S bonds, short S-N bond distances due to multiple bonding and equal S-N distances due to delocalised bonding.

Front 49

How many resonance forms account for the structure of an S4N4 cradle?

Back 49

6, including a variety of ions, multiple double bonds and S-S cross ring interactions.

Front 50

How can S2N2 be synthesised? What structure does it form with how many resonance forms?

Back 50

S4N4 + Hot Ag wool ---> S2N2 cyclic dimer, 4 resonance forms.

Front 51

How is polymeric (SN)x formed?

Back 51

Decomposition of S2N2

Front 52

What is significant about (SN)x polymers?

Back 52

They have a high metallic conductivity along chains. Act as a one dimensional metal.

Front 53

Describe the bonding in (SN)x polymers in terms contribution from the N and S valence electrons respectively.

Back 53

Localised and delocalised pi bonding. S has 6 valence electrons, 2 are used in sigma bonds to N, 4 make up two external lone pairs and 2 are available for pi bonding. N has 5 valence electrons, two are used in sigma bonds, 2 are in an external lone pair. This leaves only 1 electron for pi bondingm gives delocalised band as only half filed above/below the plane.

Front 54

What properties of S-N rings make them similar to aromatic hydrocarbons?

Back 54

S-N bond distances very similar and the rings are planar.

Front 55

Explain the localised and delocalised bonding found in SN rings in terms of electrons.

Back 55

S has 6 valence electrons. 2 are used to form σ bonds with adjacent N atoms. This leaves 2 lone pairs. One lone pair is externally-directed in the plane of the ring. The other lone pair (2 electrons) is available for pi bonding above/below the plane. This would lead to localised bonding, however, N has 5 valence electrons. Only has 1 electron available for pi bonding. Delocalised character.

Front 56

In most planar SN rings the pi-electron count is consistent with Huckels theory. Give an example of one that is and one that isnt.

Back 56

Huckel's rule for aromaticity: 4n+2 pi electrons
S2N2 = 2(2) + 2(1) = 6 pi electrons
S3N2+ = 3(2) + 2(1) = 7 pi electrons, not consistent

Front 57

What can be deduced about the aromatic behaviour of SN rings compared to hydrocarbons?

Back 57

SN rings are aromatic but with reduced pi-bond order compared to hydrocarbons.

Front 58

Compare C8H8 and S4N4 rings.

Back 58

Neither can exist planar as it would result in a triplet ground state with 2 unpaired electrons. C8H8 distorts to give buckled boat structure with all electrons paired up. S4N4 distorts into highly buckled cradle with all e- paired up.
C8H8 can be reduced to [C8H8]2- to become aromatic and planar, S4N4 can be oxidised to [S4N4]2+ to become aromatic and planar.

Front 59

Summarise features of S4N4 compounds

Back 59

It is explosive

Has an unusual cradle structure

Precursor for many other SN compounds

Front 60

Summarise the electronic properties of SN compounds

Back 60

Each SN unit has 3 pi electrons

Accounts for aromaticity and planarity in some rings

Accounts for conductivity of polythiazyl (SN)x

Front 61

What are boranes and borates?

Back 61

Boranes: neutral boron hydrides B(n)H(m), borate anions B(n)H(m)x-

Front 62

Why can BH3 not be isolated?

Back 62

It is highly electron defficient, only has a total of 6 valence electron. Condenses into B2H6.

Front 63

Describe the bonding in B2H6. What is the consequence of this on BH bond length?

Back 63

Formed from 3-centre-2-electron bonds. There are 2 electrons in each B-H-B linkage. The two electrons in a BH bond act as a nucleophile and attack the empty pz orbital other electron efficient B. Bridging bonds are longer than terminal bonds as 2 electrons shared between 3 centres.

Front 64

Explain the MO rationalisation of the bonding in B2H6

Back 64

Consists of a bonding, non-bonding and anti-bonding level for interaction between B and H. Only 2 electrons required to maximise bonding.

Front 65

Why can BF3 theoretically exist?

Back 65

Because the electron defficient B is stabilised by electron rich F forming backbonds with empty B pz orbital

Front 66

What is the nomenclature for a neutral borane?

Back 66

[Non-H's]-[Number of borons]-BORANE-[Number of substituents]

E.g. B2H6 = diborane(6)

B10H14 = Decaborane (14)

Front 67

What is the nomenclature for a borane anions?

Give an example for [B10H14]2-

Back 67

Substituents-number of borons-borate-charge

[B10H14]2- = tetradecahydrodecaborate 2-

Front 68

How are larger boranes synthesised? What is produced?

Back 68

From di-borane addition to larger borane clusters, they build up. Release H2.

Front 69

What 5 reactions are boranes typically used for?

Back 69

Proton abstraction and substitution

Cluster isomerisation

Cleavage

Cluster degradation

Cluster expansion and coupling

Front 70

Why do terminal BH stretches have a higher wavenumber than bridging?

Back 70

Because terminal BH strethces are shorter stronger bonds as 2-centre-2electron as opposed to 3-centre-3-electron

Front 71

What can be noted about the NMR spectra of boranes?

Back 71

There is coupling between B11 and H1. Number of environments can be deduced but chemical shift data hard to interpret

Front 72

Borane strucures are typically clusers. How is the type of cluster generally denoted?

Back 72

By a prefix before the formula.

Front 73

Name this type of cluster and give the general formula

Back 73

CLOSO-

[BnHn]2-

Front 74

Name this type of cluster and give the general formula

Back 74

NIDO-

[B(n)H(n+4)]

Based on closo with one missing vertex

Front 75

Name this type of cluster and give the general formula

Back 75

Arachno-

[B(n)H(n+6)]

Closo with two missing vertices

Front 76

What is a deltahedron?

Back 76

A polyhedron whos faces are all equilateral triangles

Front 77

How are [B6H6]2-, [B5H9] and [B4H10] related?

Back 77

Clusters are classified according to the deltahedron (CLOSO-parents) on which they are based. CLOSO, loses a vertex to become NIDO, loses a vertex to become ARACHNO

Front 78

What is the main principle of polyhedral skeletal electron pair theory (PSEPT)?

Back 78

The cluster structure is based on an n-vertex deltahedron with n+1 electron pairs used in cluster bonding.

Front 79

Explain the lack of aromatic behaviour for C8H8 compounds and how it can be gained by reduction in diagrams

Back 79

Front 80

Explain the lack of aromatic behaviour for S4N4 compounds and how it can be gained by oxidation in diagrams and deduce its bond order

Back 80

Bond order 3

Front 81

Discuss the behaviour benzene

Back 81

Fully delocalised aromatic system with pi bond order 3. Undergoes electrophillic addition.

Front 82

Discuss the behaviour of borazine compared to benzene:

Back 82

Iso-electronic with benzene but due to electronegativity differences between B and N is succeptible to addition of HX accross double bonds and nucleophillic attack at B. Does not undergo electrophillic substitution.

Front 83

Discuss the behaviour of phosphazine compared to benzene:

Back 83

Not fully aromatic/delocalized , bonding unclear and may involve P d-orbitals. Undergoes nucleophillic attack at P and ring polymerisation (benzene does not as would lose aromaticity).

Front 84

Discuss the behaviour of the following compound compared to benzene:

Back 84

Aromatic as contains 10 pi electrons but only has a bond order of 1 spread over the 6 atoms