lecture 8 Flashcards
what are the 2 types of bonding in cyclophosphazines
theres in plane pi bonding (N: is in the sp2 hybrid and it donates into the next PN sigma*)
out of plane pi bonding:
( N: is in the pz orbital and is donated into the PX sigma* where X is the substituent on P,, if this is EWG,, its preferred as this lowers the energy of the sigma* orbital) also not truley aromatic
makes PN bonds very strong,, so reactivity is low,, theres a ring retention (no cleavage)
why isnt cyclophosphazine that reactive
its not aromatic bc theres not complete delocalisstion of e-.
due to there being 2 types of pi bonding,, the PN bonds are very strong,, meaning theres ring retention as it doesnt want to cleave.
the N: is also involved in P back bonding and therefore isnt rlly used to react with stuff,, no reaction at N
most reactions are at P!!
N: areeee,,,, and why
theyre weakly basic
aka they are weakly donated
bc theyre backbonded to P
can N: still react even tho theyrinvolved in backbonding to P
yesss!!
but usually only with strong lewis acids or bronzed acids!!!
H+ or HCl H2SO4
or strong electrophile SO3
the N: in cyclophosphazines with Cl on the P is weakly basic,, what do we use to react with it
we use SO3 - a very strong acceptor
u get an SO3 bonded at every N
can use H+ with HCl
u get a H on 1 nitrogen only!! unless u add more H+.
the ability of the N: to react,, depends on whatttttt
it depends on the R group on P
bc N: in plane back donation
back donation depends on how delta (+) the P is
EWG on P = P more positive = more back donation of N: –> P
more EWG on P in cyclophosphazines
more EWG = P is more + (CF3)
means the N: is more likely to back donate in in plane pi bonding into the PN sigma*
N: is less basiccc + less reactive
more EDG on P in cyclophosphazines
P is less + and more - (amine // alkyl group)
less in plane pi bonding back donation of N: to NP sigma*
N: are less basic + more reactive.
most reactivity in cyclophosphazes occurs where
at the P
normally substitution reactions on halides on the P.
P SUBSTITUTION ON CYCLOPHOSPHAZINES IS SIMILAR TO WHAT REACTION
sn2 reaction
one step
switch the halide for the nucleophile
cyclophosphazine with Cl and excess nucleophile (KF)
F- as nuc
each Cl is substituted for F.
-F attacks the P!! and kicks off the other halide// substituent.
all the halides –> F
+ 6KCl
cyclophosphazines with Cl and amines (12NH3)
each Cl is substituted for an aminne ligand so NH2 is attached
so u get 6HCl as the product!!
HCl (an acid) reacts with base (NH3) to form a salt
6HCl reacts with NH3 to form 6NH4Cl!! acid is mopped up by the base
so if u see 12NH3,, u only add 6 to the ring!!!
if u see 4NH3 u add 2 to the ring,, bc the rest mops up each HCl formed to give 2NH4Cl (ammonium chloride salt)
cyclophsophazine with Cl reacting with excess azides (sodium azides)
substitute each Cl for an azide (N3)
each Cl–> N3
explosive.
lots of N atoms = explosive.
cyclophosphazines with Cl withhh limited nucleophiles: do they attack the same P or nope:
1st nuc attacks any P bc theyre all idensiical if theyre all bonded to Cl.
so now 1 is bonded to the nuc.
2nd nuc goes to the same P: geminal product!!
2nd nuc goes to other P: non geminal product!!
whats special about non geminal substitution
the P is sp3 hybridised,, meaning theres stereochemistry
the nuc can either go on the dashed or wedged lines
so u can get isomers!!
both dashed = cis
both wedged = cis
one wedge one dash = cis
bulky ligands when substituted on cyclophosphazine in a nongeminal way
they before being far away and therefore being transsss
what determines if the nuc attacks the same P or different,, when nuc attacks cyclophosphazine.
- if nuc bonded activated the P: we add the second nuc at the same P GEMINAL (F-, RO- ,, hard electronegative ligands)
- if nuc bonded deactivates the P,, we add the second nuc on a different P
hard nucleophiles
small
high charge
soft nucs
large
easily polarisable
cyclophosphazines with hard nucs + soft nucss giveee
geminal products
same P due to it being activated
what does the P activation on cyclophosphazine when doing nucleophilic substitution depend on
electrostatic : hard nucs (F,O)
polarisation: soft nucleophiles (RS-)
polarisation reasons contributing to site activation on P when doing nucleophilic substitution.
look at the transition state: SN2 type transition state,, where Lg and incoming nuc comes in at the opposite side.
P dashed with Nuc and LG to show bonds forming and breaking.
we need to rehybridise + reorganise orbitals in the TS and this is easier when we have polarisable ligands. also S is more similar and energy and size with P. they lower the energy barrier of the reaction!!
soft ligands lower the EA of the reaction from SM –> products. the reaction is more favourable. due to easier rehybridisation of the TS. gives geminal substitution
electrostatic reasons contributing to site activation on Pwhen doing nucleophilic substitution
when incoming nuc is more electronegative than the LG // old X substituent on P.
when u add an F,, the P becomes more deltaP than the other P’s (with the old X substituent)
so the next nuc attacks it as its more + and reactive.
gives geminal product
whats needed for geminal subs
highly electronegative nucs (F,RO-)
softer and more polarisable nucssss (sulfur)
when does non geminal substitution occur
when incoming nuc is less electronegative than the previous substituent on P.
P will have more electrondensity than the other ones so the nucs wouldnt be attracted to that one anymore, theyll attack a different P.
also if the nuc isnt soft enough enough to allow reorganisation and rehybridisation of nucleophiles.
what nucleophile DEACTIVATES P
nitrogen nucleophiles!!
ammonia NH3
amines RNH2
bc they have a lone paie that donates to the P and deactivates it. it increases e- density at P so nucs wont be attracted to it anymore. bc its deactivated.
remember that when u have the Cl cyclophosphazine and N nuc,, u only add 1/2 of them as the other nucs react with the HCl to give a salt!!! ammonium chloride salt
the NH2 go at different P’s - nongeminal
what do we need to rmemeber when using N nucs
remmeber that u only add half of them to the cyclophosphazine!!!
the other half will mop up the HCl to give ammonium salts!!
also remember that N donates its lone pair to the P which deactivates it,, so u get nongeminal products. aka different N nucs will attack different P’s.
non geminal isss
when nucs attack different P’s
u can get isomers depending on if theyre both dashed// wedged to give cis.. or opposites to get trans
geminal isss
when nucs attack the same P
think geminal where thr C had 2 OH’s
du to activation of P,, due to hard // soft nucs due to electrostatic // polarisation strategies which make the P more attractive to NUCs.
either by removing electron density and making it more +
or by lowering the Ea of the reaction making it more favourable due to making the rehybridisation // reorganisation of orbitals easier.
if the P is sp3 hybridised
X —- P —— NUC (breaking and making bonds)
when amines are the nucs to the cyclophosphazine with the Cl’s how should we add them to be most stable
try to get 2 cis and 1 trans.
aka 2 wedged and 1 dashed sp theyre not all cis to one another
what hybridisation is seen in RRN ligand
sp2 pi donor
lone pair in the P orbital
donates to the cyclophosphazine P (either the d orbital or the sigma* of the PN ring - backdonation of N” into one of the PN sigma* orbitals )
remember u only use halfffffff of the number of nucs.,, the rest mop up the HCl to give ammonium chloride salts and the nongeminal product!.
u get trans for less steric hinderance.
think about which one is chiral!! think about flipping it and seeing if the things are the same or not - redo that organic workshop we love
what can we use to see if smt is geminal or not geminal
we can use P NMR to see if we will get geminal or nongeminal substitution!!
spin of P
i = 1/2
is Cl nmr active
nopee so we dont see in P nmr
spin of F
i = 1/2
the cyclophosphazine where u have the Cl’s attached is calleddd
chlorophosphazine!!
geminal substitution of 2F on chlorophosphazine what is seen in P nmr
- look at P bc P NMr
- 2 diff P environments
- assume no coupling through the ring (no coupling to P)
- singlet for PCl2
- triplet for PF2
chlorophosphazine boned to 2F,, what will we see in F NMr
- look at F bc F nmr
- F = 1 env
- coupled to 1P
- so doublet
can cyclophosphazines with the Cl: chlorophosphazines readily react with 6water
yesss
-6HCl
and u put an OH at every P where there was F,,
6 OH’s in the new molecule
this tautomerises: H moves from P to N,, N: attacks H,, OH e- form double bond from P to O,, P=N breaks to give P-N.
the cycle is now made up of single bonds.
p=o and OH
N-H
this occurs bc the P=O bond is very very strong : so this is the driving force of the reaction.
RING IS NO LONGER PLANAR - MORE OF A CHAIR CONFORMATION!!
