3.1 Heterocycles

Question 1

how is Nitrogen heterocycles aromatic?

Answer 1

a lone pair of electrons occupies the space of the C–H bond in benzene. This makes N trivalent

Question 2

Pyridine is not as symmetrical as benzene but…..

Answer 2

the three types of proton all resonate in the same region

Question 3

pyrrole

Answer 3

has a five member ring and is bonded to a H. It is still planar and aromatic

Question 4

The 1H NMR spectrum of pyrrole

Answer 4

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

Question 5

pyrrole is also

Answer 5

more reactive towards electrophiles than benzene or pyridine.

but it does the usual aromatic substitution reactions (Friedel–Crafts, nitration, halogenation) rather than addition reactions

Question 6

Pyridine is

Answer 6

a unreactive aromatic imine

Question 7

why is the imine stable in pyridine?

Answer 7

because of its aromaticity.

Question 8

basicity of pyridine?

Answer 8

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.

Question 9

can pyridine be a nucleophile

Answer 9

yes.
because the lone pair of electrons on nitrogen cannot be delocalized around the ring.

Question 10

why cannot the lone pair on the Nitrogen delocalise?

Answer 10

They are in an sp2 orbital orthogonal to the p orbitals in the ring and there is no interaction between orthogonal orbitals.

Question 11

Pyridine does not undergo

Answer 11

electrophilic substitution

Question 12

why does pyridine not do electrophilic subsititution?

Answer 12

electronegativity of nitrogen, pyridine is much less reactive than benzene in ES. this makes the ring it electron poor

Question 13

why are pyridines good at nucleophilic subsititution?

Answer 13

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.

Question 14

Electron donating groups

Answer 14

electrophilic substitutions occur only on pyridines having electron-donating substituents such as NH2 or OMe.

Question 15

N-oxides

Answer 15

Pyridine N-oxides are reactive towards both electrophilic and nucleophilic substitution

Question 16

why do N-oxided have their reactivity?

Answer 16

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.

Question 17

how to remove the oxide once you are finished?

Answer 17

with trivalent phosphorus compounds such as (MeO)3P or PCl3

Question 18

where does each part react in pyridines?

Answer 18

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)

Question 19

how to makes N-oxide

Answer 19

h2o2
heat hcoh

Question 20

where does n oxide like to react?

Answer 20

c2, c4

Question 21

how does px3 react

Answer 21

accepts lone pair in the d orbital
donates lone pair of electrons too.

Question 22

Pyridine with Meo/edg C1 to it

Answer 22

electrophile will be attack para

Question 23

nitration of pyridine catalyst

Answer 23

hno3
h2so4

Question 24

Chichibabin Reaction

Answer 24

NaNH2/NH3 (liq)
H+ H2O

-H2

amination will attack from c1 from N

Question 25

halogen

Answer 25

can be replace in nucleophilic sub

Question 26

pyridine with NR2

Answer 26

HNR2

Question 27

Pyridine with SR

Answer 27

SRH
NaoH