Heterocyclic compounds part-I (Pyridine) by dr pramod r. padole

Introduction of Pyridine, Preparation of Pyridine & Molecular orbital diagram of Pyridine

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

2. Our Inspiration 2

3. A) Heterocyclic Compounds:

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

5. 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)

6. b) 6-membered Heterocyclic compounds (ii) Pyridine or AZABENZENE : Molecular Formula of Pyridine is C 5 H 5 N

7. (ii) PYRIDINE or AZABENZENE: Natural products like Vitamin B 6 , nucleotides, coramine , isoniazid etc. and alkaloids like nicotine, piperine etc. contain pyridine ring in their molecules.

8. Preparation of Pyridine

9. 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. H 2 SO 4 at 300 o C ?

10. 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?

11. 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. H 2 SO 4 at 300 o C (573K) or with nitrobenzene at 260 o C (533K); undergoes dehydrogenation(-H 2 ); 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)

12. 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 sp 2 hybridisation . (W-12, ½ Mark) Molecular Orbital Structure / Picture / M.O. diagram of Pyridine:

13. Molecular orbital structure of Pyridine: [email protected] By Dr Pramod R. Padole M.O. of Pyridine 1 4 2 3 5 Representation 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

14. Molecular orbital structure of Pyridine: Molecular formula of pyridine is C 5 H 5 N According to MOT, the sigma (σ) bonds and pi (π) bonds molecular orbital diagram / structure of pyridine are as,

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

16. E. C. of Carbon:

17. Hybridization: C

18. Hybridization: N

19. Formation of sigma (σ ) bonds M.O. diagram: The sp 2 H.O’s overlap with each other and with s-atomic orbital of the five hydrogen forming four carbon – carbon, two carbon – nitrogen (by sp 2 -sp 2 overlap) and five carbon - hydrogen (by sp 2 -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 120 from each other.

20. 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.

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

22. 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.

23. 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 .

24. Representation of Pyridine: A common / general representation of pyridine is simply hexagon with a circle inside . The circle represents the π-molecular orbital.

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