HETEROCYCLIC AROMATIC COMPOUNDS:

BS(IV)

PYRROLE

Pyrrole:INTRODUCTION:PYRROLE is a heterocyclic

aromatic organic compound, a five-membered ring with the formula ( C4H4NH ). It is a colourless volatile liquid that darkens readily upon exposure to air. Substituted derivatives are also called pyrroles.

Resonance structure :The lone pair on nitrogen is in the p

orbital so it is involved in the 6 pi-electron aromatic system. Hence pyrrole is not very nucleophilic and is only weakly basic at nitrogen. Looking at the HOMO of pyrrole the lobes are much bigger at the 2- and 5- positions, this indicates that the reactions are most likely to take place at these positions

The resonance contributors of pyrrole provide insight to the reactivity of the compound. Like furan and thiophene, pyrrole is more reactive than benzene towards electrophilic aromatic substitution because it is able to stabilize the positive charge of the intermediate carbocation.

SYNTHESIS OF PYRROLE:Pyrrole could be obtained

through the following reaction: It could be achieved by treating Furan with Ammonia with the exsitence of solid acid catalysts.

2.Synthesis :PYRROLE can e obtained by

heating acetylene and ammonia over red hot tube.

Acidic property of pyrrole:Due to participation of N’s lone

pair in resonance/aromaticity pyrrole has exceptional strong acidic property.

(that is,it can loss the hydrogen attached with NITROGEN when reacting with any base)

Acidic property of pyrrole:PYRROLE can react with Strong base, Grignard reagent, potassium metal etc to give salt

like compounds.

Sensitivity towards strong acids:Pyrrole is sensitive towards strong

acids.This is due to protonation occur at one

of the carbon and the resulting protonated molecule will add to another unprotonated pyrrole molecule this continues until a pyrrole trimer is formed.

The reaction is considered as electrophilic addition of pyrrole.

ELECTROPHILIC SUBSITUTION IN PYRROLE:Pyrrole is reactive towards electrophilic

substitution reaction. It is more reactive than benzene because of

the resonances that pushes away the electron density from nitrogen towards carbons ,thus making the ring electron rich.

The substitution is easier and mild reagents can be used.

ELECTROPHILIC SUBSTITUTION OCCURS AT CARBONS(THE RING) NOT NITROGEN.

(obviously)

Preferable position is C-2, the carbon next to the heteroatom.

If there is an already substitution on C-2 then C-3.

The first substitution is on C-2 because it has more stable intermediates(it stabilizes three resonances structure)

The intermediate resulted from C-3 attack is stabilizes by two resonance structure.

FRIDEL CRAFT ACYLATION

HALOGENATION

MULTI HALOGENATION

DIAZONIUM ION FORMATION

NITRATION

SULPHONATION

PYRIDINE:

Introduction:Pyridine is a basic heterocyclic organic

compound with the chemical formula C5H5N. It is structurally related to benzene, with one methine group (=CH-) replaced by a nitrogen atom. Pyridine have 5 carbon atoms and one nitrogen atom. All are sp² hybridized. The p-orbital of nitrogen and all carbon atoms lie in the same plane. Overlapping of p-orbitals result in delocalization of six π-electrons in the cyclic ring, following Hückel rule, imparts aromatic character to pyridine

HistoryImpure pyridine was undoubtedly prepared by

early alchemists by heating animal bones and other organic matter, but the earliest documented reference is attributed to the Scottish scientist Thomas Anderson. In 1849, Anderson examined the contents of the oil obtained through high-temperature heating of animal bones. Among other substances, he separated from the oil a colorless liquid with unpleasant odor, from which he isolated pure pyridine two years later. He described it as highly soluble in water, readily soluble in concentrated acids and salts upon heating, and only slightly soluble in oils.

OccurrancePyridine is not abundant in nature. In

daily life, trace amounts of pyridine are components of the volatile organic compounds that are produced in roasting and canning processes, roasted coffee, potato chips,  sunflower honey etc

SYNTHESIS

Hantzsch pyridine synthesisIt is a multi-component organic reaction between an aldehyde such as formaldehyde, 2 equivalents of a β-keto ester such as ethyl acetoacetate and a nitrogen donor such as ammonium acetate or ammonia. The initial reaction product is a dihydropyridine which can be oxidized in a subsequent step to a pyridine. The driving force for this second reaction step is aromatization.

Chemical propertiesPyridine is miscible with water

and virtually all organic solvents It is weakly basic pyridine behaves both as a

tertiary amine

Resonance structure:

Electron density is on nitrogen as its lone pair is not taking part in

resonancePositive charge occur on ortho

and para carbons.

Basic property:The nitrogen center of pyridine

features a basic lone pair of electrons. Because this lone pair is not part of the aromatic ring, pyridine is a base, having chemical properties similar to those of tertiary amines.

Pyridine can act as Lewis base, donating its pair of electron to a Lewis acid as in the sulfur trioxide pyridine complex.

Pyridine as nucleophile:Pyridine is a nucleophile as

NITROGEN because its lone pair is not delocalized,

The ring as nucleophile : Electronegative atom like

nitrogen lowers the energy of the ring, this means a less reactive nucleophile

.

But LUMO means a more reactive ELECTROPHILE

The ring as ELECTROPHILE:

Benzene VS pyridine:It is less reactive than benzene in

electrophile substitution.

But nucleophile substitution which is hard for benzene comes easy for pyridine.

Electrophile substitution:Many electrophilic substitutions

on pyridine either do not proceed or proceed only partially they lead only to the addition at the nitrogen atom.

Nucleophile substitution reactions:

FURAN

THIOPHENE

SYNTHESIS :

RESONANCE STRUTURES:

REACTIVITY:

PYRROLE > FURAN >THIOPHENE >> BENZENEACCORDING TO THE MOST ELECTRONE.ATIVE ATOM ATTACHED.

NITRATION:

SULPHONATION:

PYRROLE,FURAN,THIOPHENE undergo

ELECTROPHILIC SUBSTITUTION Easily.

PYRIDINE UNDER NUCLEOPHILIC SUBSTITUTION