1. Contents:
1) Introduction to aromatic Heterocycles
2) Aromaticity
3) Structure, Synthesis and reactions of
some common aromatic heterocyclic
compounds
2. Aromatic Heterocycles
Heterocycles that have aromatic character or that fulfil basic rules
of aromaticity are known as Aromatic Heterocycles.
Probably about two-third of organic compounds belong to this
class!
They number among some most significant compounds for human
beings especially in medicinal field.
Page no 1147, Ch 43 Clayden, Greeves, Warren and Wothers Organic Chemistry
3. AROMATICITY:
RULE 1: An aromatic compound is cyclic and planar
RULE 2: compounds should have parallel p-orbitals
for continuous overlapping around the ring
RULE 3: It must follow Hückel rule
http://www.chem.ucla.edu/harding/tutorials/aromaticity.pdf
4. PYRIDINE ______Structure, Synthesis And Some Reactions
Lone pair of nitrogen does not take part in delocalization
because it is perpendicular to the plane
Pyridine have 5 carbon atoms and one nitrogen atom.
All are sp² hybridized.
The p-orbital of nitrogen and all carbon atoms lie in the
same plane.
Overlapping of p-orbitals result in delocalization of six
π-electrons in the cyclic ring, following Hückel rule, imparts
aromatic character to pyridine.
Page no: 912 Organic Chemistry By Mehta Mehta
5. Page no:74 and 27 Heterocyclic Chemistry, 2nd Edition, By Joule And Smith
Synthesis Of Pyridine:
6. Mechanism of that reaction is shown on next slide
Page no: 912. Organic Chemistry By Mehta Mehta
15. In most cases electrophilic substitution of pyridine occurs very much less
readily than in the case of a correspondingly substituted benzene.
some of typical electrophilic substitution reactions do not occur at all
Friedal- Crafts acylation and alkylation
Reaction with nitrobenzene
Reaction with acetophenone
Milder electrophilic reagents such as diazonium salts, nitrous acid etc, which
do not react with benzene, likewise fail to effect
C-substitution.
Usual nitration and sulphonation can not occur but may occur if and only if
providing some special conditions.
ELECTROPHILIC SUBSTITUTION REACTIONS OF PYRIDINE:
16.
17.
18. Reactions with Oxidizing Agents:
This pyridine N-Oxide is a very useful synthetic intermediate, yields about 95% from this
reaction
Page no:1153 Clayden, Greeves, Warren and Wothers Organic Chemistry
Page no:53 Heterocyclic Chemistry, 2nd Edition, By Joule And Smith
19. As per given on previous slide
NUCLEOPHILIC SUBSTITUTION REACTION OF PYRIDINE
21. PYRROLE _____ Structure, Synthesis And Some Reactions
Pyrrole have 4 carbon atoms and one nitrogen atom.
All are sp² hybridized.
The p-orbital of nitrogen and all carbon atoms lie in the
same plane.
Overlapping of p-orbitals result in delocalization of 6π-
electrons in the cyclic ring, following Hückel rule, imparts
aromatic character to pyrrole.
Page no: 912 Organic Chemistry By Mehta Mehta
25. Orientation of Electrophilic Substitution in Pyrrole
Electrophilic substitution normally occurs at a carbon
atoms instead of at the nitrogen.
Also it occurs preferentially at C-2 (the position next to
the heteroatom) rather than at C-3 (if position 2- is
occupied it occurs at position 3).
This is due to attack at C-2 gives more stable
intermediate (it is stabilized by three resonance
structure) than the intermediate resulted from C-3
attack (it is stabilized by two resonance structure) .
SOME REACTIONS OF PYRROLE:
26. Pyrrole is sensitive to strong acids.
This is due to protonation occurs at one of C-3 and
the resulting protonated molecule will add to another
unprotonated pyrrole molecule this continues to give
pyrrole trimer.
This reaction is considered as electrophilic addition to
pyrrole
http://www.copharm.uobaghdad.edu.iq/uploads/%D9%85%D8%AD%D8%A7%D8%B6%D8%B1%D8%A7%D8%AA/%D9
%85%D8%AD%D9%85%D8%AF%20%D8%AD%D8%B3%D9%86/2nd%202s/H.C.4.pdf
28. REFERENCES:
Clayden, Greeves, Warren and Wothers Organic Chemistry
Hawley’s Condensed Chemical Dictionary, 11th Edition, Revised By Irving And
Richard
Organic Chemistry By Mehta Mehta
Heterocyclic Chemistry, 2nd Edition, By Joule And Smith
Hydrogen Storage Technology: Materials and Applications
www.chem.ucla.edu/harding/tutorials/aromaticity.pdf
jpkc.zju.edu.cn/k/146/Organic_Chemistry_Level_1/Presentations/Hantzsch_Dihydropyridin
e_Synthesis.pdf
www.copharm.uobaghdad.edu.iq/uploads/%D9%85%D8%AD%D8%A7%D8%B6%D8
%B1%D8%A7%D8%AA/%D9%85%D8%AD%D9%85%D8%AF%20%D8%AD%D8%B3
%D9%86/2nd%202s/H.C.4.pdf