Chemical Properties of pyrole.pptx......

Organic chemistry

1. Chemical Properties of Pyrrole , Furan and Thiophene

2. Electrophilic substitution reactions General reactivity: In five-membered heterocyclic compounds, the heteroatoms donate their electron pair to aromatic sextet. Thus, the heterocyclic rings are highly activated and their reactivity towards electrophilic substitution is more compared to benzene . The overall order of reactivity of five membered heterocyclic compounds, as compared to benzene, towards electrophilic substitution reactions is as follows :

3. Attack of electrophile (E+) is preferred at C2 The electrophile can attack either at C2 or at C3 of the heterocyclic ring. In general, the electrophilic substitution occurs preferably at C2. (i) Also, C2 is relatively electron rich as evident ( ii) The intermediate carbocation formed by the attack of electrophile at C2 , is more resonance stabilized (three contributing structures) compared to carbocation formed by attack of electrophile at C3 ( two contributing structures).

4. (1) Nitration

5. (2) Sulfonation

6. (3) Acetylation

7. (4) Halogenation

8. With organometallic compounds

9. Reaction with diazonium salts

10. Salt formation and Kolbes —Schmitt reaction in pyrrole Pyrrole is a weak base. The lone pair of electrons on nitrogen is not available for reaction as it is a part of the aromatic sextet Pyrrole shows a weakly acidic character and reacts with potassium hydroxide to form a potassium salt. The loss of proton from nitrogen is favoured as pyrrolyl anion is resonance stabilized. Due to this acidic character, its potassium salt readily undergoes Kolbes –Schmitt reaction (similar to phenol) to form pyrrole -2-carboxylic acid.

11. Diels–Alder reaction of furan. Furan is the only five membered heterocyclic compound which undergoes Diels–Alder reaction. Furan being least aromatic in character behaves as a diene and reacts with maleic anhydride (a dienophile ) to form an adduct.

12. The end