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Review : Aromatic

The structure must be ring structure

Alternating bonds or conjugated bonds.

Follows the Hckel's rule: The number of electrons in the

cyclic system is (4N + 2), with N an integer

The structure must be planar

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Ar atic Heter cyclic C p undsy The cyclic part (fr Greek eaning circle) f

heter cyclic indicates that at least ne ring structure is

present in such a c p und.

y prefix heter - (fr Greek eaning ther) refers t the

n ncarb n at s in the ring.

y Heter cyclic c p unds include any f the bi che ical

aterial essential t life. F r exa ple, any naturallyccurring pigments, vitamins, and antibiotics are

heter cyclic c p unds.

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Ar atic Heter cyclic C p undsy Aheterocyclic compound is an rganic c p und in

hich ne r re f the carb n at s in the backb ne

f the lecule has been replaced by an at ther than

carb n. Typical hetero atoms include nitr gen, xygen,

and sulfur.

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Pyridiney A lecule f pyridine c ntains a ring f six at s five

carb n at s and ne nitr gen at .

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Pyrr ley A lecule f pyrr le c ntains a ring f five at s - f ur

at s f carb n ne at f nitr gen.

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Nomenclature of heterocyclic

compoundsy The types of heteroatoms present in a ring are indicated by

prefixes; in particular, oxa-, thia-, and aza- denote oxygen,sulfur, and nitrogen atoms, respectively.

y The numbers of heteroatoms of a particular kind areindicated by number prefixes joined to the heteroatom

prefixes, as dioxa- and triaza-.

y The presence of different kinds of heteroatoms is indicated by

combining the above prefixes, using the following order of

preference: oxa- first, followed by thia- and then aza-.

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N

ome clat reElement oxyge s lf r sele i m itroge phosphoro s silico boro

Valence II II II III III IV III

Prefix Oxa hia Sele a Aza Phospha Sila Bora

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N

ome clat rey I additio , partially sat rated ri gs are i dicated by the

prefixes dihydro-, tetrahydro-, a d so o , accordi g to

the mber of extra hydroge atoms bo ded to theri g atoms.

y

The positio s of heteroatoms, extra hydroge atoms,a d s bstit e ts are i dicated byArabic merals, for

which the mberi g starts at a oxyge atom, if o e is

prese t, or at a s lf r or itroge atom a d co ti es i

s ch away that the heteroatoms are assig ed the lowestpossible mbers.

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Synthesis and

modificati

ono

f hetero

cyclicringsy The importantmethods for synthesizing heterocyclic compounds

can be classified under five headings.y Three areways of forming new heterocyclic rings from forerunner

containing either no rings or one fewer ring than the desiredproduct; one is away ofobtaining a heterocyclic ring from another

heterocyclic ring or from a carbocyclic ring; and one involves themodification of substituents on an existing heterocyclic ring.

y In the formation of rings from no ring, the key step is fre uently

the formation of a carbon-heteroatom linkage (CZ, inwhich Zrepresents an atomofnitrogen, oxygen, sulfur, or amore unusual

element). The actual ring closure, however,may involve theformation of a carbon-carbon bond.

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Synthesis and

modificati

ono

f hetero

cyclicrings1. Nucleophilic ring closure

2. Electrophilic ring closure

3. Ring closure by way of cyclic transition states

4. Conversion of one heterocyclic ring into another

5. Modification of an e isting ring