lecture 5 Flashcards
what is borazine and whats so special about itt
its isoelectronic with benzene
its called inorganic benzene
its a pi system aka conjugated heterocyclic ring made up of B and N with H’s on both the B and the N’s.
old way of preparing borazine - B3N3H6
B2H6(amoroso lecture) + excess NH3 (low temp)
heat to 200*C - salt loses H2 and cyclises to form (HNBH)3
how is borazine made now
heat NH4Cl + BCl3 -> (HBNCl)3
(bubble BCl3 gas through NH4Cl in chlorobenzene at 130*C - this drives off HCl formed)
(HNBCl)3 can be converted to (HNBCl)3 using NaBH4 in Et2O and a base to mop up BH3 made.
(HNBCl)3 is used to make (HNBH)3
u need as many NaBH4 as the amount of H’s u need to convert the Cl –> H.
how else can borazines be made
the azide route
R2BCl + LiN3 –> R2BN3
in MeCN at room temp.
pyradine adduct is filtered and dried before pyrolysis (heating in the presence of O2) gives of the py and N2(g)
gives R2B=N very unstable!!
R moves to be with the N to give R-B=N-R this is then repeated 3 times!! to give borazine
the pi electrons in borazine come from
the Nitrogen
in borazine and benzene,, are all the bonding mos filled
yesss
physical properties of benzene and borazine areee
similar
bc theyre isoelectronic
mp
bp
etc
whats more reactive,, borazine or benzene
borazine is more reactive bc benzene is aromatic - it has more pi stabilisation energy.
also maybe bc theres ionic character and therfore a difference in polarity.
how is the pi system in borazine generated
the B normally has an empty P orbital
the N has a lone pair and donates an e- pair to the B’s empty p orbital
when borazine is made,, what changes about N and B
B is normally a good lewis acid bc it can accept e- in its empty p orbital.
but due to already accepting from N,, its less accepting now - a weaker acid.
N donated to the B,, so its now a weaker lewis base.
lewis bases can attack when on borazine
the borons,,
theyre less acidic tho bc the lone pair from N has already been accepted.
benzene and bromine product
no reaction bc benzene is aromatic + too stable
borazine + bromine product
extensive addition when reacted with excess bromine
when can benzene react with bromine
when the bromine is activated with FeBr3!!!
borazine + 3Br2 explanation
Br bonds to every B and N
theyre also bonded to a H
the double bonds are broken between the BN into single bonds
borazine + 3Br2 + then refluxxxx
- 3HBr
u have borazine with the double bonds between the BN but now u also have Br bonded to the B, not the Ns.
the N’s are bonded to Hs.
benzene + HCl
no reaction bc benzene is aromatic and therefore very stable.
borazine and HCl
every atom has a H,, B has Cl aswell,, N has 2 H’s now!!
double bonds are removed
N was protonated basically.
borazine + HCl + reflux afterrrrrr
u have the normal borazine
then u add the HCl –> removes the double bonds,, protonates the N and adds Cl to the B’s.
reflux causes the removal of 3H2,,, regenerates the double bonds.
now the B is only bonded to a Cl and the N’s are bonded to 1 H only. ,, a H was removed from every element tbh.
borazine + H2O,, first explain the polarity of B-N
N is more electronegative so gets a pertailly negative charge
B is less electronegative and gets a partial positive sign.
the OH- attacks the B as opposites attract
the H+ attacks the N as opposites attract.
B is bonded to OH and H from before
N is bonded to 2 H’s (only single bonds now aswell)
reflux removes 3H2 -> removes a H from every atom so B is only bonded to OH and N is only bonded to H. conjugation occurs again
