6 – membered hetercyclic compounds, Preparation of Pyridine, Molecular Orbital Structure / Picture / M.O. diagram of Pyridine

1. 1
B.Sc. Final year
MEB Students
for
Today’s
Chemistry Lecture
by
Dr Pramod R Padole

2. 2

4. LOGO
A) Heterocyclic Compounds:

5. Classification:
Heterocyclic Compounds
5-membered
heterocyclic compounds
6-membered
heterocyclic compounds
Condensed
heterocyclic compounds

6. LOGO
6 – membered hetercyclic compounds:
 These types of compounds contain five carbon
atoms and one hetero atom in a ring, i.e., total
6-atoms in a ring, are called as 6-membered
heterocyclic compounds.
 Example:
 The nitrogen containing six membered heterocycle’s names usually
ends with –ine.
Q.1) Pyridine is a 6 membered hererocyclic compound. (S-19 & W-19 , ½ Mark)
N
1 2
3
4
5
6
Pyridine

7. LOGO
b) 6-membered Heterocyclic compounds
(ii) Pyridine or AZABENZENE:
Molecular Formula of Pyridine is C5H5N

8. LOGO
(ii) PYRIDINE or AZABENZENE:
Natural products like
Vitamin B6, nucleotides, coramine,
isoniazid etc. and
alkaloids like nicotine, piperine etc. contain
pyridine ring in their molecules.
N
1 2
3
4
5
6
Molecular Formula = C5H5N & Structural Formula is

9. LOGO
Preparation of
Pyridine

10. Methods of Synthesis
or Preparation of Pyridine:
From
Acetylene
Preparation of Pyridine
From
Penta-methylene
diamine hydrochloride
Reactions that produce a particular functional group are called preparations
2 acetylene and HCN
Pentamethylene diamine hydrochloride
Piperidine
heated heated with
conc. H2SO4
at 300oC
?

11. www.themegallery.com
Prepared by Dr Pramod R. Padole
Preparation of Pyridine From Acetylene:
 From Acetylene:
When passing a mixture of acetylene (2 equivalents)
and hydrogen cyanide (H-CN) through red hot tube; to
form pyridine.
Q.1) How will you obtain: Pyridine from acetylene? (W-11, W-15 & S-17, 2 Mark)
Q.2) How will you convert: acetylene to pyridine? (W-14, 2 Mark)
Q.3) Give the synthesis of Pyridine from acetylene. (W-18,2 Mark)
Q.4) What happen when: Mixture of acetylene and hydrogen cyanide is passing through
red hot tube?
C
C
C
C
N
C
H
H
H
Red hot tube
N
H
H
Acetylene
Acetylene
(H-CN, Hydrogen cyanide)
Pyridine

12. Prepared by Dr Pramod R Padole
From Penta-methylene diamine hydrochloride:
Or From Penta-methylene diamine:
 When pentamethylene diamine hydrochloride on
heating; to form piperidine,
which is further heated with conc. H2SO4 at 300oC (573K)
or with nitrobenzene at 260oC (533K); undergoes
dehydrogenation(-H2); to from pyridine, as a final product.
Q.1) How will you obtain / prepare: Pyridine from pentamethylene diamine hydrochloride?
(S-18 & W-19, 2 Mark)
- HCl
N
H
N
- NH4Cl or 2) Nitrobenzene,
Pentamethylene
diamine hydrochloride
Piperidine
Pyridine
H2C
H2C H2C
CH2
H2
C
NH2.HCl
NH2.HCl
3 H2
Pentamethylene
diamine hydrochloride
H2C
H2C H2C
CH2
H2
C
NH3Cl
NH3Cl
or
at 300 C
at 260 C
o
o
On heating
Conc. H2SO4
1)

13. LOGO
Q.1) Discuss the molecular orbital picture/structure of pyridine.
(W-11, S-13, W-14 & S-15, 4 Mark)
Q.2) In pyridine, nitrogen atom is in a state of sp2 hybridisation.
(W-12, ½ Mark)
Molecular Orbital Structure /
Picture / M.O. diagram of
Pyridine:

14. L o g o
Molecular orbital structure of Pyridine:
pramodpadole@gmail.com By Dr Pramod R. Padole
M.O. of
Pyridine
1
4
2
3
5Representation
of Pyridine
Molecular Formula
E.C. &
Hybridization of
C & N
E.C. of H
Structure
Formation of
delocalized pi (π)
bonds M.O. diagram
Formation of sigma (σ)
bonds M.O. diagram

15. LOGOMolecular orbital structure of
Pyridine:
Molecular formula of pyridine is C5H5N
 According to MOT, the sigma (σ) bonds and pi (π) bonds
molecular orbital diagram / structure of pyridine are as,
E.C. of H1
1s
&
sp2
hybridization
E.C. of C6
V.S.E.C.
2s2
, 2p2
2s 2px 2py 2pz
G.S.
E.C. of C6
E.S.
E.C. of C6
sp2
H.S.
unhybridised
orbital
3 sp2
H.Os.
sp2
hybridization
E.C. of N7
V.S.E.C.
2s2
, 2p3
2s 2px 2py 2pz
G.S.
E.C. of N7
E.S.
E.C. of N7
sp2
H.S.
unhybridised
orbital
3 sp2
H.Os.
Electronic configuration of Carbon: Electronic configuration of Nitrogen:

16. LOGO
Structure of Pyridine:
In pyridine, all carbon and nitrogen undergoes sp2-hybridisation.1
All ring atoms in pyridine ( Five carbon & one nitrogen) contains
three sp2 H.O’s2
sp2 hybrid orbital of each C-atom is half-filled (singly filled), whereas
out of three sp2 hybrid orbital of nitrogen, one sp2 hybrid orbital is full-filled
(completely filled) having a lone pair of electrons.
3
The unhybridised p-orbital (i.e., 2-pz) of each carbon atom &
nitrogen atom is half-filled (singly filled)4
According to molecular orbital theory (MOT);
Pyridine ring is planar due to sp2-hybridisation
(All C, N & H atoms are in one plane).5

17. LOGO
E. C. of Carbon:

18. LOGO
Hybridization:
C

19. LOGO
Hybridization:
N

20. LOGO
Formation of sigma (σ ) bonds M.O. diagram:
 The sp2 H.O’s overlap with each other and with s-atomic
orbital of the five hydrogen forming four carbon – carbon,
two carbon – nitrogen (by sp2-sp2 overlap) and five carbon -
hydrogen (by sp2-s overlap ) sigma bonds.
 So, total eleven sigma (σ) bonds are formed in pyridine.
 All these sigma bonds lie in one plane and are at 1200 from each
other.

21. LOGO
Formation of sigma (σ ) bonds M.O. diagram:
C
C
N
C
C
C
1s
sp2
sp2
sp2
sp2
sp2
Completelly fillled
i.e., lone pair

22. LOGO
Formation of sigma (σ ) bonds M.O. diagram:
 The sp2 H.O’s overlap with each other and with s-atomic
orbital of the five hydrogen forming four carbon – carbon,
two carbon – nitrogen (by sp2-sp2 overlap) and five carbon -
hydrogen (by sp2-s overlap ) sigma bonds.
 So, total eleven sigma (σ) bonds are formed in pyridine.
 All these sigma bonds lie in one plane and are at 1200 from each
other.

23. LOGO
Formation of pi (π) bonds M.O. diagram:
 Each ring atom in pyridine also has an unhybridised
p-orbital and these are perpendicular to the plane of
sigma bonds and are parallel to each other.

24. LOGO
Formation of pi (π) bonds M.O. diagram:
The p-orbitals on carbons and nitrogen
contain one electron (2pz
1) each.

25. LOGO
Formation of pi (π) bonds M.O. diagram:
 Lateral overlap of these six p-orbitals (five from C-atoms
and one from N-atom) gives rise to the formation of
delocalized π-molecular orbital (or π-electron clouds)
above and below the plane of ring.
 This π-molecular orbital contains a total of six electrons,
due to this pyridine shows aromatic properties or
aromaticity by satisfying Huckel’s (4n+2)π e-s, rule.

26. LOGO
Formation of pi (π) bonds M.O. diagram:
 Thus, pyridine shows aromatic character (nature) as it contains
aromatic sextet.
 This aromatic sextet opposes addition reaction and favours
electrophilic as well as nucleophilic substitution reaction.

27. LOGO
Representation of Pyridine:
A common / general representation of
pyridine is simply hexagon with a circle
inside.
The circle represents the π-molecular
orbital.
N..
Common Shorthand Representation
Delocalised -M.O.
N1
2
3
4
5
6
=

28. LOGO

29. LOGO
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Heterocyclic Compounds by Dr Pramod R. Padole
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