Heterocycles | Organic Chemistry 3

Heterocycles:

Ring compounds containing at least 1 non-carbon atom in the ring
 

Nomenclature:

A prefix depending on the heteroatom is used in front of the corresponding cycloalkane name: aza- (nitrogen), oxa- (oxygen), thia- (sulfur), phospha- (phosphorus). Atoms in the ring are numbered starting with the heteroatom
 

Other names are widely used for common molecules, especially for aromatic compounds:
 

Heterocyclopropanes and heterocyclobutanes: high strain ⇒ very reactive (ring opening). Heterocyclobutanes are less reactive than heterocyclopropanes ⇒ heating generally required:

Heterocyclopentanes and heterocyclohexanes: no strain ⇒ less reactive than their smaller-ring counterparts

Synthesis of heterocyclopentadienes (Paal-Knorr Synthesis):
 

The reactivity of aromatic heterocyclopentadienes is largely based on the chemistry of benzene (due to the aromaticity)
 

Electrophilic substitution:

Mechanism:

Electrophilic aromatic substitution frequently favored at C2

Ring opening:

Cycloaddition:

Mechanism: Diels-Alder reaction

Synthesis of Pyridine (Hantzsch Synthesis):

Steps of the synthesis:

The reactivity of pyridine derives from its aromaticity (slow electrophilic substitution in C3) and from its cyclic imine character (fast nucleophilic substitution in C2 or C4)

Electrophilic substitution:

Nucleophilic substitution:
 

• Chichibabin Reaction:

• Mechanism: addition-elimination

• Other types of nucleophilic substitution: