G15 Flashcards
What are the 4 main group 15 elements?
P (Phosphorus) = Non-metal
As (Arsenic) = Metalloid
Sb (Antimony) = Metalloid
Bi (Bismith) = Metal
What are features of G15?
- The valance configuration for these elements is ns2 np3, so there common oxidation states will be +3 and +5
- The +5 oxidation state is more common for the lighter elements
- And the +3 oxidation state is more common for the heavier elements – the inert pair effect
What are characteristics of white phosphorus?
- A waxy yellow solid, exists as molecules made up of four atoms in a tetrahedral structure
- This tetrahedral arrangement results in ring strain and instability
- Therefore, P4 is highly flammable and pyrophoric (self-igniting) upon contact with air (O2)
- But is stable in water and is best stored fully submerged in water
What are characteristics of red phosphorus?
- Red phosphorus has a polymeric structure
- 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
- Much more stable
- Only ignites at 300 °C
Draw reactions of silylene and germylene with P4
How to synthesise RP=PR (diphospene)
- P=P distance = 2.015 – 2.003 Å
- 31P NMR shift typically 500 – 600 ppm!
What type of R groups do we need to protect RP=PR (diphospene) bond
Need sterically bulky ligands to protect the double bond
How to obtain non-symmertrical disphosphenes
- Can be obtained from dehydrochlorination reaction of R-PH2 and R’-PCl2 in the presence of a
organic base such as DBU - If mixtures of dichlorophosphines used, all 3 possible diphosphenes are generated
How to synthesise non-symmetrical diphosphenes
Draw the proposed reactions
- Transition metal catalysed metathesis reaction
- The non-symmetrical diphosphene was detected in solution
However: - Attempted isolation only gave the two symmetrical species
- Suggested a dynamic exchange of phosphinidene (‘M=P’ double bond) intermediates
* catalysis ‘killed’ by addition
of benzaldehyde and formation of metal oxide
Why are the majority of diphosphenes trans?
Bulky sterics
Draw a diagram of UNSTABLE cis phosphenes
Draw disphospne + activated sulphur (S8 in NMe3) reaction
31P NMR used to easily distinguish between the 2 products
Draw disphospne + (non-activated) sulphur reaction
Draw disphospne + dienes reaction
What are the 3 products of diphosphene and M (metal) reactions
- End on
- Side on
- Both
Draw a reaction of disphosphene and metal showing photoisomerism
Draw a reaction of disphosphene and metal showing loss of Cr fragment
What are Diarsenes, Distibenes, Dibismuthenes?
Molecules that are derived from the extenstion of the procedures for phosphorus
* Less well explored due to lower stability
What are features of Phospha-alkynes (RC≡P)?
- Tervalent P(III) compounds that contain a ‘P≡C’ TRIPLE bond
- Once thought to be incapable of existence on theoretical grounds (p-bonding involving heavier elements too weak to sustain monomeric compounds)
- Therefore thought that formation of rings and cages would be favoured
- HCl formed during the reaction can removed using base)
- May be stored at temperatures below –124 °C but polymerises above this temperature
- Both monomer and polymer are pyrophoric in air
What were the breakthroughs in Phospha-alkynes (RC≡P)?
Draw reaction scheme
- Elimination of hexamethyldisiloxane from a suitable phosphaalkene
- Known for a range of different R-groups and are stable at room temperature
Recall key points of RC≡P bond
- Short P≡C bond length 1.55 Å compared with P–C single (1.82 Å) and P=C double (1.67 Å)
- Electronic ionisation energies much lower than corresponding nitriles
- HOMO is the P≡C triple bond (suggests chemistry similar to alkynes)
- Phospha-alkynes RC≡P and nitriles RC≡N differ significantly in the polarity of the P≡X bond
- Therefore despite the presence of a lone-pair at the P-atom, protonation occurs exclusively at the carbon centre of the P≡C bond
How to obtain Arsaalkynes?
Elimination of hexamethyldisiloxane
Draw addition reactions for phospha-alkyes with RSIR and RGeR
Draw the 5 different coordination chemistries for phospha-alkynes
Draw reaction for side on coordination for phospha-alkyenes
