lecture 6 Flashcards
valence e- and hybridisation seen in b
3
sp2
valence e- and hybridisation seen in N
5 e-
sp2
explain where the valence e- are in both N and B if theyre sp2 hybrids
sp2 = 3 hybrid orbitals
both n and b use 3 e- here.
b thereofre has an empty p orbital
n therefore has a filled p orbital which is its lone pair
what is the p orbital used for
its used for pi bonding
explain the sigma framework of borazine
sigma = sp2 bonds
think of borazine from above so u can only see the sp2 hybrids (3 on each atom)
each atom uses 2 orbitals to overlap with its neighbours
and uses the other sp2 orbital to bond with s which has a circle orbital
explain the pi framework in borazine
think of it from above and ignore the sigma framework made up of sp2.
the b’s have an empty p orbital and the n’s have a full p orbital with a lone pair in it.
in this example theyre all in phase. aka the bottom of the p orbital is coloured in while the top bit isnt
how many pi electrons does borazine have
it has 6 pi electrons which all come from the N
3 N’s and they all bring 2e- from their lone pair i nthe p orbital.
B-N bond both ways explained
resonance: (-)B-N(+)
bc the N gave B its lone pair.
partial charges: (+)B-N(-)
bc N is more electronegative than B.
explain the pi framework for benzene
each C has a p orbital with 1e- in it,, they all contribute 1e- to the pi system.
1e- in the upper p orbital
in borazine,, what contributes more to the bonding MO
the N
bc it has a larger Zeff,, more close e- density
also more electronegative
therefore more stable and lower in energy
remember bonding mos are lower in energy than antibonding
how many MO are in borazine and benzene seperately
they each have6 MO
bc 6AO
3/6 = bonding
3/6= antibonding
the filled mo in borazine have what character
the filled mo in borazine have more N character bc the lone pair came from N
N sp2 is also more electronegative than Np and B – so the sigma bond is more poalrised to the N
does benzene or borazine have more aromatic stabilisation energy and why
benzene has a greater degree of aromatic stabilisaion energy
this is bc in borazine,, the e- in the pi system come from N,, N also has a larger Zeff so it has a large e- density surrounding it.
this means the e- are less delocalised in the pi system and are found closer to the N. due to the pi bond polarisation
reules for aromaticity
huckels rule of pi e-: 4n+2
flat
conjugated
cyclic
rules for antiaromaticity
when there is 4n pi e-
if theres 6 p atomic orbitals thennnn
theres gona be 6 molecular orbitals
what are the 6 molecular orb itals for both benzene and borazine
3 bonding
- no nodes - all in phase
- one node x2
3 antibonding
- 2 nodes x2
- 3 nodes - all out of phase
what do we need to thibk about when drawing the borazine MO
we need to remmeber that N has a larger coefficient aka more e- density around it which means the N contributes for to the bonding MO’s.
this means that when we draw it,, we need a bigger circle around the N when draeing the bonding MO. and a smaller circle than B when drawing the antibonding orbitals.
all in phase
half half
2 nodes
3 nodes and all out of phase
in benzene all the coefficients are the same as they all have the same Zeff and the same contribution to the pi system.
why does N get a larger circle in the bonding MO diagrams
N has a greater Z effective
meaning its orbitals are lower in energy
so they have a larger coefficient in the BMO
this gives a lumpy pi system as the p orbitals are different sizes.
are all the bonding MOs filled for benzene and borazine
yesss
all the bonding mos are filled.
the sigma and pi’s are filled with a total of 6e-. thats why theyre both so stable + aromatic.
whats the difference between the bonding and antibonding mos of borazine
larger coefficient on N for the bonding MOs
larger coefficient for B on the antibonding MOs
shown by the larger circle // orbital size.
when theres a nodal plane // node in a moelcule shown by like a ninja fruit slice type of line,, what happens
every time u pass across the node,, the phase of the orbitsals change.
bc theres a change in the wave function.
borazine molecular formula
B3N3H6
physical properties of borazine are similar to benzene bc theyreee
isoelectronic
same amount of valence e- in both of them
physical similarities between borazine and benzene
both colourless liquids
similar densities
similar enthalpies of evaporation
whats more reactive,, borazine or benzene and why
borazine is more reactive
lower pi stabilisation energy due to less e- delocalisation (due to N having a greater Zeff)
and bc the bonds are polar: 2 different polarity possibilities depending on if we think of resonance or partial charges.
the pi stabilisation energy is lower for borazine
N and B being lewis acids or bases
lewis acid accepts so B is the lewis acid
N is the lewis base bc it donates its lone pair.
if theres an EWG attached to the B,, what happens
B is a lewis acid meaning it accepts e-
by having an EWG next to it,, the complexes formed will no longer be 1:1 .
this is bc the B already accepts e- from N,, making it,, the B,, a weaker acid. and the N is a weaker base bc its already donated its lone pair to the B.
can borazine undergo addition reactions
yessss
with 3Br2,, then reflux to remove 3HBr,, leaving 1H on each N.
name a simple borazine derivative that can react with a Grignard reagent (RMgX, RLi, R2Hg) to form b substituted derivative
B3N3H3Cl3
or (HBNCl)3
what does the borazine derivative (HBNCl)3 forming b substituted derivatives mean
it means the things it react with,, normally grignard reagents,, get added onto the B
the driving force of the reaction is salt formation,, LiCl // XCl
the borazine derivative (HBNCl)3 and 3PhLi reaction
b substituted derivative
Ph goes onto the B’s,, Cl is removed from the B’s
3LiCl is made,, salt formation which is the driving force of the reaction.
N’s still have a H attached.
descrive the borazine derivative (HBNCl)3
borazine but with 1 H on every N
and 1 Cl on every B
thermolysis and self condensation of borazine gives what products
loss of H2 // HX
polycyclic borazines (2 borazines bonded together)
or further condensation to give boron nitride (isoelectronic to graphite but the layers are eclipsed) eg hexagonal layers eclipsed
hexagonal boron nitride is isoelectronic to what
isoelectronic to graphite,, but the graphite layers are staggered and the hexagonal boron nitride layers are eclipsed
cubic boron nitride is isoelectronic to what
its isoelectronic to diamond
theyre both hard.
what is a cyclophosphazene
inorganic,, cyclic pi delocalised ring made up of N and P and a halogen
(NPX2)n
n = number
X = halogen
in a cyclophosphazene,, what coordinate geometry does N and P have
N has 2 coordinate geometry as its bonded to 2 diff things
P has 4 coordinate geometry as its bonded to the N’s and 2 X’s
X’s being halogens
in cyclophosphazenes,, how many e- does each element provide
the N and the P both provide 1 e- each!! for pi bonding.
the starting materials for most cyclophosphazene reactions areeee
the Cl derivatives of cyclophosphazenes.
how can cyclophosphazenes be made
condensation route
azide route
cyclophosphazene condensation route
high boiling solvents
150*C heat for 5 hours
cyclic oligomers and polymers are produced and then these are normally separated by fractionation.
4n NH4Cl + nPCl5 –> (NPCl2)n + 4nHCl
azide route for cyclophosphazenes
sometimes used to form disubstituted cyclophosphazenes!!1
R2PCl is dissolved in MeCN
LiN3 or NaN3 (salt formation) is added to form the azide (N3)
azide is heated to generate intermediate R2PN –> oligomerises.
R2PCl + LiN3 – loss of LiCl —> [R2PN3] —loss of N2–> 1/n (NPR2)n
trimeric meaning
polymer made up of 3 monomer units
most cyclic trimeric cyclophosphazenes areeee
planarrrrr
n = 3 so its trimeric!! bc theres 3x PN
(NPCl2)n
is (NPF2)3 perfectly planar?? and are the other trimeric rings also planar // with other halogens
the (NPF2)3 is perfectly planar!!!
but the other trimeric rings are more//less planar.
when n = 4 for a cyclophosphazene,, (NPC2)4,, whats it called
tetramer!! tetrameric instead of trimeric
are tetrameric cyclophosphazenes planar,, if not,, what shape do they have
non planar – they have a large variety of structures including chair, boat, saddle and crown derivatives.
chair looks like a chair,, just keep doing P-N
boat is also like a boat,, just keep writing P-N down
bonding in cyclophosphazes must take what into account
- very strong + stable P-N bonds
- P-N bond lengths are all the same + short
- N-P-N has 120* angle
- N is weakly basic,, it can be protonated// involved in pi bonding.
- hard to add e- to the structure,, its therefore difficult to electrochemically reduce the phosphazene skeleteon
(r = gain of h // e- and loss of o)
describe the tetramic rings
boat and chair
single PN bonds only
just keep writing p - n
flip one side to get boat from chair.
P and N hybrids and other exp for cyclophosphazes
N:
sp2 hybrid
so 120* angle
1e- for pi bonding
1 lone pair in the ring plane
P:
sp3 hybrid
120* but due to bond pair bond pair repulsions
1e- for pi bonding
6 pi electrons in total: 1 from each N and P…
explain N in cyclophosphazes
N has 5 valence e-!!
and is sp2 hybridised
lone pair on the hybrid
1 e- in the other hybrids
1e- in the p orbital!! for pi bonding. this is different to borazine where its lone pair is in the p orbital.
this is bc P has 1e- in the p orbital whilst B had an empty p orbital.
