Dr. Alaa J. Mahrath hcc lectures /2nd course college of Pharmacy /BioOrgChem



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Dr.Alaa J.Mahrath HCC lectures /2nd course college of Pharmacy /BioOrgChem


Syllabus for Heterocyclic compounds second course for pharmacy colleage شعار الجامعة

Subject

Hours

Heterocyclic system: Classes of heterocyclic systems; general structures; properties; Occurrence in nature and in medicinal products.

5

Five-membered ring heterocyclic compounds: pyrrole; furan and thiophen.

3

Source of pyrrole, furan and thiophen.

2

Electrophilic substitution in pyrrole, furan and thiophen: Reactivity and orientation.

5

Six-membered ring heterocyclic compounds: Structure & reactions of pyridine.

4

Saturated five-membered heterocyclic compounds.

6

Heterocyclic of five & six member rings with two & three heteroatoms.

5

Recommended Reading :

Text Book

Heterocyclic Chemistry, Morrison &Boyd 7th edition
Other Books and References

* Heterocyclic Chemistry, R. Gupta, M. Kumar, V. Gupta

* Heterocyclic Chemistry, J. A. Joule, G. F. Smith

Five-membered Heterocycles L.3https://encrypted-tbn3.gstatic.com/images?q=tbn:and9gcthzmeeoo2jmexvgy7art86sqyjyzapwn9lfsvmvehcoq0pmba_

Furan_and_Thiophene__Objectives'>Pyrrole, Furan and Thiophene

Objectives:

1-To identify types of five membered heterocyclic compounds

2- To know their sources .

3-To know syntheses methods of five membered heterocyclic compounds

4-To understand their chemical reactions .

A. Five-membered Rings with one Heteroatom


  1. The main reason for the study of pyrrole came from the work on the structure of heme; the blood respiratory pigment, and the chlorophyll; the green photosynthetic pigment of plants.

  2. Thiophene and its derivatives occurs in petroleum.

  3. Furan occurs widely in secondary plant metabolites, especially in terpenoids.

Unsubstituted pyrrole, furan, and thiophene are usually obtained from petroleum.



  1. Pyrrole, furan and thiophene are colorless liquids of boiling points 126o, 32o, and 84oC respectively.

  2. Pyrrole has a relatively high boiling point as compared to furan and thiophene, this is due to the presence of intermolecular hydrogen bonding in pyrrole.



Structure and Aromaticity

  • Pyrrole furan and thiophene are aromatic because:

  1. they fulfill the criteria for aromaticity, the extent of delocalization of the nonbonding electron pair is decisive for the aromaticity.

thus the grading of aromaticity is in the order of: fg15_01-14-1un

Furan < Pyrrole < Thiophene < Benzene

this order is consistent with the order of electronegativity values for oxygen (3.44), nitrogen (3.04) and thiophene (2.56).



  1. They tend to react by electrophilic substitution due appearance of –ve charge on carbon atoms due to delocalization as shown in the following resonance structures



3- Electrons not available for protonation—hence not basic

4- 6  electrons over 5 ring atoms ….. Electron rich… so more reactive than benzene towards electrophilic substitution.

The order of reactivity is:

Pyrrole > Furan > Thiophene > Benzene

5- The pattern of reactivity with Electrophilic reagents.

Aromatic compounds : By substitution addition followed by proton loss



[ onium intermediate ]

Order of reactivity : Pyrrole > Furan > Thiophene > Benzene



6-The order of aromaticity

Benzene > Thiophene > Pyrrole > Furan

In case of Thiophene [S] donate & accept electrons ., so delocalization

as complete as benzene.

In case of Pyrrole [N] -Diene-like character CH2=CH-CH=CH2



In case of Furan [O] electronegativity more …. Diene-like character CH2=CH-CH=CH2

Question : prove pyrrole have aromatic characters ?

The Evidences of aromatic character in pyrrole are :

It tends to react by electrophilic substitution





  • Its exceptional lack of basicity and strong acidity as a secondary amine compared to the aliphatic analog (pyrrolidine). This can be explained on the basis of participation of N lone pair in aromatic sextet (see the resonance structures)




So Its weak acid not basic as the secondary amines .
Lone pair of N is involved in  cloud and not available for sharing with acids

thus the dipole moment of pyrrole compared with pyrolidine is reverted and thus protonation occurs at carbons not at N







Sources & Synthesis:

A) Sources

Pyrrole & Thiophene …. Coal Tar



Pyrrole ring …. Porphyrin system….. Chlorophyll & Hemoglobin

chlorophyll-hemoglobin

Furan ….. Decarboxylation of Furfuraldehyde ……. Oat hulls, corn cobs or rice hulls


rice hulls

corn cobs

Oat hulls



B) Synthesis




Pyrrole

Thiophene

Furan


Reactions of pyrrole



Sulfonation

Acetylation

Vilsmier Rex

Reduction

Nitration



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