lecture 5

Question 1

what is borazine and whats so special about itt

Answer 1

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.

Question 1

old way of preparing borazine - B3N3H6

Answer 1

B2H6(amoroso lecture) + excess NH3 (low temp)

heat to 200*C - salt loses H2 and cyclises to form (HNBH)3

Question 2

how is borazine made now

Answer 2

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.

Question 3

how else can borazines be made

Answer 3

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

Question 4

the pi electrons in borazine come from

Answer 4

the Nitrogen

Question 5

in borazine and benzene,, are all the bonding mos filled

Answer 5

yesss

Question 6

physical properties of benzene and borazine areee

Answer 6

similar
bc theyre isoelectronic

mp
bp
etc

Question 8

whats more reactive,, borazine or benzene

Answer 8

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.

Question 9

how is the pi system in borazine generated

Answer 9

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

Question 10

when borazine is made,, what changes about N and B

Answer 10

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.

Question 11

lewis bases can attack when on borazine

Answer 11

the borons,,

theyre less acidic tho bc the lone pair from N has already been accepted.

Question 12

benzene and bromine product

Answer 12

no reaction bc benzene is aromatic + too stable

Question 13

borazine + bromine product

Answer 13

extensive addition when reacted with excess bromine

Question 14

when can benzene react with bromine

Answer 14

when the bromine is activated with FeBr3!!!

Question 15

borazine + 3Br2 explanation

Answer 15

Br bonds to every B and N
theyre also bonded to a H

the double bonds are broken between the BN into single bonds

Question 16

borazine + 3Br2 + then refluxxxx

Answer 16

• 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.

Question 17

benzene + HCl

Answer 17

no reaction bc benzene is aromatic and therefore very stable.

Question 18

borazine and HCl

Answer 18

every atom has a H,, B has Cl aswell,, N has 2 H’s now!!
double bonds are removed

N was protonated basically.

Question 19

borazine + HCl + reflux afterrrrrr

Answer 19

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.

Question 20

borazine + H2O,, first explain the polarity of B-N

Answer 20

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