3c Flashcards
what is pyridine,, aka describe it
6 membered ring with N as one of the members
it has 3 double bonds and the N isnt bound to any H’s
lone pair is in an sp2 orbital aka resonance cannot occur,, making pyridine and e- poor heterocycle
for pyridine to react with an electrophile for electrophilic substitution,, what needs to happen
the pyridine needs to be activated with an EDG to increase the amount of e- in the e- poor ring
describe pyrrole
5 membered heterocyclic ring
lone pair is in a p orbital meaning it can be included in resonance etc!!
its e- rich
N is bonded to a H and has a lone pair
describe the resonance structures of pyrrole
the lone pair can form a (-) on the C2 or C3 position
and bc the moelcule is symmetric,, basically all the C’s are e- rich and can have a (-) it just depnds on if u wanna move the N’s lone pair clockwise or anticlockwise
pyrrole + excess Br2
u can have Br at every C.
every C can have a (-) resonance form meaning it can attck Br.
pyridine and Br2
only the lone pair on N,, that is orthogonal tp the pi system can attack the Br
so it Br will bond to the N of pyridine.
describe what an indole is
its a benzene fused down a bond with pyrrole
aka they share 2C’s
describe indole in terms of e- and everything
it has a pyrrole so its e- rich as the lone pair can be resonanted throughout the structure.
sp2 C with 1e- in the p making a pi system
sp2 N with 2 e- in the p orbital making a pi system
10e- pi system : so its aromatic and planarrrrr
in indole,, what C can be nucleophilic
the one with the (-)
normally C3 or C2
BUT MAJORITY OF THE TIME C3 BC AROMATICITY IS NOT BROKEN THIS WAYYYYYY
is loss of aromaticity favourable
nopeeee
thats why if a resonance form breaks aromaticity,, trust we dont want to attack using that C.
NH and single bonds
pyrrole
e- rich
sp2 N with lone pair in p orbital so resonance can occur
N with double bond
pyridine
e- poor
lone pair cannot be used in resonance
whats cool about imidazole and pyrazole
they have the pyrrole type N and the pyridine type N
when u have a pyrrole and pyradine type N in one compound,, why is tautomerisation a problem
bc u have one starting material and if u add a substituent
u get 2 different products due to tautomerisation occuring.
the group can add on either N.
what do electrophiles like
they like e- rich aromatics
aka pyrrole
what do nucleophiles like
e- poor aromatics
aka pyridine
how can an electrophile react with a e- poor aromatic aka pyridine
if we add an EDG to it to activate it.
how can a nucleophile react with an e- rich aromatic
aka pyrrole
u add an EWG to it
how can pyridine be activated for electrophilic substitution
we add an EDG
aka MeO or NH2
is MeO or NH2 a bvetter EDG
MeO
bc lone pair and resonance
and bc NH2 will probs be protonated itself,, lowering its EDG abilities
the better EDG does what
have power over where we direct the electrophile
MeO is what directing
para position to itself
the N in pyradine is what directing
its meta directing
bc we dont want a positive charge by an EW N.
bc pyrrole is e- rich,, what reactions occur easily
electophilic subs reactions occur easily as the C’s can easily have a (-) by moving the N’s lone pairs around
pyrrole: what if we want to only add 1 subs,, where would we put it out of the 4 diff carbonds but tbh only 2 different positions bc the rest are equivalent due to symmetry,, do we put it on C2 or C3
C2!!!
bc C2 has more resonance forms meaning the (+),, yes the (+) once u add the E- ,, can be shared out between more Carbonds,, making it more stable..
so we add E- onto C2 when we have pyrrole!!!
the alpha position
C3 woul be the beta position!!
what is halogenation
where we add a electrophilic halogen to smt
name some electrophilic hydrogens
F
Cl
Br
I
whats like a cool structure that we use to add electrophilic haogens to heterocycles
NIS
NBS
NCS
NFSI
imagine a 5 memberes ring with N and 2 carbonyls on the alpha position from N,, then the N is bonded to either CL, Br or I.
why do NCS NBS NIS NFSI work as electrophilic halogens
bc the pyrrole can attack the halogen and kick the other e- onto the N which is an electrophilic atom and can stabilise an (-).
how can 1,2 di bromo ethane be used as an electophile and what governs the reaction
BR C C BR
pyrrole N attacks the Br and this moves e- all the way up to the other Br.
this gives u the substituted product,,, an alkene and a Br-
there is an entropic driving force to the reaction!!
bc ur going from 2 things to 3 !!!
5 membered ring with S // N
what position do we add electrophiles to
we would add it to C2
the alpha position bc it has more resonance structures!!!
the (+) formed once the E- has been added can be shared among more C’s which lowers its energy and stabilises it… and this is what we want.
