G15

Question 1

What are the 4 main group 15 elements?

Answer 1

P (Phosphorus) = Non-metal
As (Arsenic) = Metalloid
Sb (Antimony) = Metalloid
Bi (Bismith) = Metal

Question 2

What are features of G15?

Answer 2

1. The valance configuration for these elements is ns2 np3, so there common oxidation states will be +3 and +5
2. The +5 oxidation state is more common for the lighter elements
3. And the +3 oxidation state is more common for the heavier elements – the inert pair effect

Question 3

What are characteristics of white phosphorus?

Answer 3

1. A waxy yellow solid, exists as molecules made up of four atoms in a tetrahedral structure
2. This tetrahedral arrangement results in ring strain and instability
3. Therefore, P4 is highly flammable and pyrophoric (self-igniting) upon contact with air (O2)
4. But is stable in water and is best stored fully submerged in water

Question 4

What are characteristics of red phosphorus?

Answer 4

1. Red phosphorus has a polymeric structure
2. viewed as a derivative of P4 wherein one P-P bond is broken, and one additional bond is formed with the neighbouring tetrahedron resulting in a chain-like structure
3. Much more stable
4. Only ignites at 300 °C

Question 5

Draw reactions of silylene and germylene with P4

Answer 5

Question 6

How to synthesise RP=PR (diphospene)

Answer 6

• P=P distance = 2.015 – 2.003 Å
• 31P NMR shift typically 500 – 600 ppm!

Question 7

What type of R groups do we need to protect RP=PR (diphospene) bond

Answer 7

Need sterically bulky ligands to protect the double bond

Question 8

How to obtain non-symmertrical disphosphenes

Answer 8

1. Can be obtained from dehydrochlorination reaction of R-PH2 and R’-PCl2 in the presence of a
   organic base such as DBU
2. If mixtures of dichlorophosphines used, all 3 possible diphosphenes are generated

Question 9

How to synthesise non-symmetrical diphosphenes

Draw the proposed reactions

Answer 9

1. Transition metal catalysed metathesis reaction
2. The non-symmetrical diphosphene was detected in solution
   However:
3. Attempted isolation only gave the two symmetrical species
4. Suggested a dynamic exchange of phosphinidene (‘M=P’ double bond) intermediates
   * catalysis ‘killed’ by addition
   of benzaldehyde and formation of metal oxide

Question 10

Why are the majority of diphosphenes trans?

Answer 10

Bulky sterics

Question 11

Draw a diagram of UNSTABLE cis phosphenes

Answer 11

Question 12

Draw disphospne + activated sulphur (S8 in NMe3) reaction

Answer 12

31P NMR used to easily distinguish between the 2 products

Question 13

Draw disphospne + (non-activated) sulphur reaction

Answer 13

Question 14

Draw disphospne + dienes reaction

Answer 14

Question 15

What are the 3 products of diphosphene and M (metal) reactions

Answer 15

1. End on
2. Side on
3. Both

Question 16

Draw a reaction of disphosphene and metal showing photoisomerism

Answer 16

Question 17

Draw a reaction of disphosphene and metal showing loss of Cr fragment

Answer 17

Question 18

What are Diarsenes, Distibenes, Dibismuthenes?

Answer 18

Molecules that are derived from the extenstion of the procedures for phosphorus
* Less well explored due to lower stability

Question 19

What are features of Phospha-alkynes (RC≡P)?

Answer 19

1. Tervalent P(III) compounds that contain a ‘P≡C’ TRIPLE bond
2. Once thought to be incapable of existence on theoretical grounds (p-bonding involving heavier elements too weak to sustain monomeric compounds)
3. Therefore thought that formation of rings and cages would be favoured
4. HCl formed during the reaction can removed using base)
5. May be stored at temperatures below –124 °C but polymerises above this temperature
6. Both monomer and polymer are pyrophoric in air

Question 20

What were the breakthroughs in Phospha-alkynes (RC≡P)?

Draw reaction scheme

Answer 20

1. Elimination of hexamethyldisiloxane from a suitable phosphaalkene
2. Known for a range of different R-groups and are stable at room temperature

Question 21

Recall key points of RC≡P bond

Answer 21

1. Short P≡C bond length 1.55 Å compared with P–C single (1.82 Å) and P=C double (1.67 Å)
2. Electronic ionisation energies much lower than corresponding nitriles
3. HOMO is the P≡C triple bond (suggests chemistry similar to alkynes)
4. Phospha-alkynes RC≡P and nitriles RC≡N differ significantly in the polarity of the P≡X bond
5. Therefore despite the presence of a lone-pair at the P-atom, protonation occurs exclusively at the carbon centre of the P≡C bond

Question 22

How to obtain Arsaalkynes?

Answer 22

Elimination of hexamethyldisiloxane

Question 23

Draw addition reactions for phospha-alkyes with RSIR and RGeR

Answer 23

Question 24

Draw the 5 different coordination chemistries for phospha-alkynes

Answer 24

Question 25

Draw reaction for side on coordination for phospha-alkyenes

Answer 25