lecture 5 Flashcards
benzene + normal bromine
no reaction
benzene and mcPBA
no reaction
why can’t bromine and mCPBA react with benzene
bc benzene is aromatic, more stable
- cyclic
- conjugated
- flat
- huckels rule
what happens to e- when they are conjugated
they are shared between the double bonds that are conjugated
they are shared between the system
this can be shown in resonance forms
what must happen to bromine in order to react with benzene
it must be activated
explain how bromine is activated
Br2 must react with a lewis acid
what is a lewis acid
electron deficient
accepts e-
what lewis acid is used to activate a bromine
AlCl3
aluminium tri chloride
how is bromine activated using AlCl3
Bromine lone pair attacks the Al in AlCl3
product of bromine activation with AlCl3 (lewis acid)
Br-Br- Al-Cl3
+ -
how does AlCl3 activate bromine
the lewis acid weakens the Br-Br bond by removing some e- density
allowing it to react with aromatics
draw the reaction between aromatics and an activated bromine
- aromatic arrow to bromine
- bromine arrow to the + Bromine
- the 2 products are a CC+ cyclohexene with a bromine attached and a Br-Al-Cl3 molecule
what can occur in the product when aromatics react with an activated bromine
the CC+ cyclohexene bromine can have resonance forms in the molecule. (it is no longer aromatic)
the Br-AlCl3 has a - charge on the Al
what is needed in order for the cc+ cyclohexene with bromine attached to regain its aromaticity
it needs to have a proton removed
how does cc+ cyclohexene with bromine regain its aromaticity
it loses a Hydrogen
the e- in the Br-Al bond in Br-Al-Cl3 attack the H which gives its electrons to the ring, regaining its aromaticity
okay so steps of the reaction of bromine with an aromatic
- bromine + Al-Cl3 gives Br-Br(+)-Al(-) -Cl3
- the double bond then attacks one br giving Br-Al(-)-Cl3
- the Br-Al then attacks the H giving HBr, AlCl3 (-) and an aromatic
what is needed for a normal nitration reaction
HNO3 (nitric acid)
can nitric acid HNO3 react with an aromatic
nope!!
it must first be activated
how is nitric acid, HNO3 activated
it uses H2SO4 to protonate itself
lone pair on one O attacks the H of H2SO4
H e- are moved onto the O
- protonated HNO3 now has a H2O group (good LG)
- O- electrons kick off the H2O
draw a nitric acid molecule
HNO3
+
-:O: - N = O
|
:O:H
(this one attacks the H in H2SO4)
and is later forced to leave
when HNO3 reacts with H2SO4 it is activated,, what does this give
a very reactive electrophile
nitronium ion
+
O=N=O
what is used to nitrate aromatics
activated HNO3 (nitric acid)
aka a nitronium ion ( O=+N=O)
reaction between an aromatic and a nitronium ion step 1
double bond attacks the + charged N
e- are taken to the double bonded O
reaction between an aromatic and a nitronium ion product of step 1
aromatic is bonded to the NO2
the N is +
the O is -
step 3 of the reaction between an aromatic and a nitronium ion
the aromatic wants to regain the aromaticity lost when bonded to the NO2 so it loses a H and it’s e- are donated to the ring system, regenerating it’s aromaticity
what is used to remove the H in the reaction of an aromatic and a nitronium ion in order for it to regain its aromaticity
a HSO4 (-) charged
what is the final product of nitric acid // nitronium and a benzene aromatic
a nitrobenzene product
benzene with NO2 bound to it
benzene and nitric acid is called aaaaa
nitration reaction
what is needed in a sulfonation reaction
2 H2SO4 in order to activate themselves
how is H2SO4 activated in a sulfonation reaction
it uses another H2SO4 in order for one of the single bonded Os to gain a H by attacking it, this will give a H2O which can then be lost as a good LG (one step of attacking the H and the H2O leaving) the lone pairs on the other single O donate : to form a double bond with the S and neutralise it as it was +
what is the final product when H2SO4 is activated
a protonated SO3 // HSO3
O(+) charged, S is neutral, other Os are neutral
a strong electrophile which can react with aromatic
how to remember the products
remember they have to be electrophiles so will probably have a + charge
what does the aromatic attack in a sulfonation
it attacks the S in the middle
the double bond e- then go to the newly made O(+)
this O will have a + charge
what is used to remove a H from the sulfonation product
the HSO4 (-)
this helps it regain its aromaticity
what is the product of the sulfonation of benzene
benzene
sulfonic
acid
what is the RDS when the aromatic and the electrophile react
the rds is the breaking of aromaticity
when the aromatic attacks the activated electrophile (SO3, NO2, Br-AlCl3)
what is the fast step when an aromatic attacks an activated electrophile
the fast step is the regeneration of an aromatic
when the H is removed
rearomatisation
electrophilic aromatic substitution reactions
bromination
nitration
sulfonation
what is needed in a bromination reaction with an aromatic
Br2
AlCl3
combine
what is needed in a nitration reaction with an aromatic
HNO3
H2SO4
nitronium ion
what is needed in a sulfonation reaction
H2SO4
H2SO4
when an aromatic attacks an electrophile,, what is made
a cyclohexene attached to the electrophile
it has resonance forms as it is a CC+ intermediate
what then occurs to the benzene x electrophile x CC+ intermediate
a hydrogen is removed in order to regain its aromaticity
steps in electrophilic aromatic substitution
- activation of electrophile
- attack of electrophile + CC+ intermediate formation
- removal of H to regain aromaticity
