1. GOVERMENAT ENGINEERING COLLEGE VALSAD
B.E. (Chemical) Year-2nd Semester-III
Subject: Organic Chemistry & Unit Processes
Subject Code:
TOPIC: heterocyclic compounds_

2. Prepared by
NO.
NAME
ENROLLMENT NO. 1.
MANGUKIYA JATIN V.

3. Heterocyclic compounds
are organic compounds that contain rings composed of
carbon and other atoms – heteroatoms – in natural
heterocycles mostly nitrogen, sulfur, and oxygen.
Heterocycles exist as three-, four-, five-, six-, and multi-membered rings.
The stability of heterocycles increases
– with less distorted bond angles (less strain) that are
in five- and six-membered rings, and
– with maximum number of conjugated double bonds,
because the delocalized π-bonding electron pairs
form a molecular π orbital filled with six electrons
and the compound have the aromatic character.

4. Some rings, in spite of being five- or six-membered,
are not stable heterocycles, e.g.
cyclic hemiacetals (cyclic forms of monosaccharides)
are in equilibria with their open (acyclic) forms,
cyclic esters (lactones), amides (lactams), and cyclic
dicarboxylic acid anhydrides
can undergo hydrolytical splitting.
Nonaromatic heterocycles – cyclic ethers, amines, and
sulfides behave like their acyclic analogs with the same
functional group and do not require special discussion.
Aromatic heterocycles (with the maximum number of
conjugated double bonds) are much more important.

5. Survey of the most important heterocycles
pyrrole
pyrrolidine
imidazole
thiazole
thiofene
thiolane
furan N N N N N H N O
pyridine
piperidine
pyrimidine
pyrazine
azepine
1,4-diazepine
4H-pyran N N O
indole
quinoline
isoquinoline
chroman N
purine
pteridine H

6. b) Physical properties of furan, pyrrole, thiophene
At room temperature, thiophene is a colorless liquid with a mildly pleasant odor reminiscent of benzene, with which thiophene shares some similarities. The high reactivity of thiophene towards sulfonation is the basis for the separation of thiophene from benzene, which are difficult to separate by distillation due to their similar boiling points (4 °C difference at ambient pressure). Like benzene, thiophene forms an azeotrope with water.
Furan is typically derived by the thermal decomposition of pentose-containing materials, cellulosic solids especially pine-wood. Furan is a colorless, flammable, highly volatile liquid with a boiling point close to room temperature. It is toxic and may be carcinogenic. Catalytic hydrogenation (see redox) of furan with a palladium catalyst gives tetrahydrofuran.
Pyrrole is a heterocyclic aromatic organic compound. Substituted derivatives are also called pyrroles. For example, C4H4NCH3 is N-methylpyrrole. Porphobilinogen is a trisubstituted pyrrole, which is the biosynthetic precursor to many natural products

7. Electrophilic subistitution :-
The reaction takes place predominantly at the α – position we could account for this orientation of the following basis : the controlling step is the attachment of the electrophilic reagent which takes place in such a way as to yield the most stable intermediate carbonium ion
Attack at position 3 yields carbonium ion that is a hybrid of structure I and II . Attack at position 2 yields a carbonium ion that is a hybrid not only of structure III and IV ( analogous ) to (I and II ) but also of structure V ; the extra stabilization conferred by V makes this ion the more stable one

9. +

10. Nomenclature of heterocycles
Common names for many ring systems are accepted by
the IUPAC rules:
– rings with maximum double bonds
furan, pyrrole, indole, pyridine, etc.
– saturated ring systems
either prefixes dihydro-, tetrahydro-, perhydro-,
or in five-membered rings pyrroline, pyrrolidine,
in six-membered rings piperidine, piperazine. N H
imidazole imidazolidine N H N H N H
pyrrole
pyrroline
pyrrolidine
pyridine piperidine N H

11. Nitrogen-containing heterocycles are basic,
unless the unshared electron pair of nitrogen is conjugated
with π-electron pairs of double bonds, i.e. unless it takes
part on the formation of an aromatic six-electron π cloud..
Pyridine is a weak base. The unshared electron pair of nitrogen
atom can bind a proton by coordination to give pyridinium cation: N – + H
+ H+
Pyrrole
is not a base: N H –
The unshared electron pair of
nitrogen is not able to add proton. + H N – NH
+ H+
Imidazole
is a weak base;
One of the two nitrogen
atoms can add proton:

12. Vitamin B12 (cobalamin) Coenzyme B12
is essential for the maturation and development of red blood cells.
Coenzyme B12
(cobamide cofactors 5´-deoxyadenosylcobalamin, methylcobalamin)
methylcobalamin
(part of the molecule)
5´-deoxyadenosylcobalamin
CH3 N C o + N

13. (pyrrolidine-2-carboxylic acid)
Pyrrolidine is a cyclic secondary amine (pKB = 2.7).
pyrrolidine
pyrrolidinium ion N H +
+ H+
proline
(pyrrolidine-2-carboxylic acid)
COOH N H
Proline is the only standard α-amino
acid that has a secondary amino group:
Hydroxyproline (proline hydroxylated in position 4 or 3) is the
characteristic constituent of collagen – the most abundant protein
of connective tissue.
5-Oxoproline (lactam of
glutamic acid) is formed
in the group translocation
of amino acids (γ-glutamyl
cycle).
4-hydroxyproline HO
COOH N H O
5-oxoproline
(pyroglutamate)

14. Thiolane is completely hydrogenated thiophene.
This ring structure appears (fused with
imidazolidone) in biotin:
1,2-Dithiolane is a cyclic dlsulfide that can be easily reduced. to
acyclic alkan-1.4-dithiol. It occurs as a part of lipoic acid.
thiophene
thiolane
dihydrolipoic acid (transfer of acyls) H C O
lipoic acid (an oxidant)
+ 2H
- 2H
In the cells, biotin may
bind CO2 and resulting
carboxybiotin serves as
a donor of carboxyl in
important carboxylations (e.g., biosynthesis of fatty acids, synthesis of
oxaloacetate from pyruvate).
COOH NH HN C– HO N
carboxybiotin

15. Furan Pyran Oxygen containing five- and six-membered heterocycles
derivatives are rather rare in natural compounds. On the contrary,
tetrahydrofuran ring is quite common in cyclic forms of sugars.
furan O
D-ribofuranose O OH H
HO–CH2 O
tetrahydrofuran
Pyran
is a labile ring (without the aromatic electron sextet), but it is
stabilized through oxidation to pyrone;
tetrahydropyran ring occurs in cyclic forms of
monosaccharides: O
4H-pyran
4-pyranone
(γ-pyrone) O OH H
D-ribopyranose O
tetrahydropyran

16. Pyridine is a weak base (pKB = 8.7).– + H
+ H+ O C OH
NH2
(pyridine-3-carboxylic acid)
nicotinic acid
nicotinamide
pyridine pyridinium
The most important derivatives
of pyridine are two essential
factors:
Nicotinic acid and nicotinamide is called niacin or pellagra-
preventingfactor (PPF); they are included in the group of vitamins B.
alkaloid nicotine
CH2OH HO
H3C
pyridoxine
pyridoxal
CH=O
pyridoxamine
CH2-NH2
Vitamin B6 is the group name for three related derivatives pyridoxine,
pyridoxal, and pyridoxamine.