3.1 Heterocycles Flashcards
how is Nitrogen heterocycles aromatic?
a lone pair of electrons occupies the space of the C–H bond in benzene. This makes N trivalent
Pyridine is not as symmetrical as benzene but…..
the three types of proton all resonate in the same region
pyrrole
has a five member ring and is bonded to a H. It is still planar and aromatic
The 1H NMR spectrum of pyrrole
the proton on the ring resonate at higher field (6.5 and 6.2 ppm) than those of benzene or pyridine but they still fall in the aromatic rather than the alkene region
pyrrole is also
more reactive towards electrophiles than benzene or pyridine.
but it does the usual aromatic substitution reactions (Friedel–Crafts, nitration, halogenation) rather than addition reactions
Pyridine is
a unreactive aromatic imine
why is the imine stable in pyridine?
because of its aromaticity.
basicity of pyridine?
All imines are more weakly basic than saturated amines and pyridine is a weak base with a pKa (for its conjugate acid) of 5.5. This means that the pyridinium ion is about as strong an acid as a carboxylic acid.
can pyridine be a nucleophile
yes.
because the lone pair of electrons on nitrogen cannot be delocalized around the ring.
why cannot the lone pair on the Nitrogen delocalise?
They are in an sp2 orbital orthogonal to the p orbitals in the ring and there is no interaction between orthogonal orbitals.
Pyridine does not undergo
electrophilic substitution
why does pyridine not do electrophilic subsititution?
electronegativity of nitrogen, pyridine is much less reactive than benzene in ES. this makes the ring it electron poor
why are pyridines good at nucleophilic subsititution?
especially at the 2- and 4-positions, by lowering the LUMO energy of the π system of pyridine.
think of cyanide c attack by Nu N takes it all.
Electron donating groups
electrophilic substitutions occur only on pyridines having electron-donating substituents such as NH2 or OMe.
N-oxides
Pyridine N-oxides are reactive towards both electrophilic and nucleophilic substitution
why do N-oxided have their reactivity?
Theyare stable dipolar species with the electrons on oxygen delocalized round the pyridine ring, raising the HOMO of the molecule. Reaction with electrophiles occurs at the 2- (ortho) and 4- (para) positions, chiefly at the 4-position to keep away from positively charged nitrogen.
how to remove the oxide once you are finished?
with trivalent phosphorus compounds such as (MeO)3P or PCl3
where does each part react in pyridines?
The reactivity of pyridine derives from its aromaticity (slow electrophilic substitution in C3) and from its cyclic
imine character (fast nucleophilic substitution in C2 or C4)
how to makes N-oxide
h2o2
heat hcoh
where does n oxide like to react?
c2, c4
how does px3 react
accepts lone pair in the d orbital
donates lone pair of electrons too.
Pyridine with Meo/edg C1 to it
electrophile will be attack para
nitration of pyridine catalyst
hno3
h2so4
Chichibabin Reaction
NaNH2/NH3 (liq)
H+ H2O
-H2
amination will attack from c1 from N
