مم Flashcards
Two isomeric benzopyridines are
quinoline and isoquinoline
Quinoline is the
benzo[b]pyridine
isoquinoline is the
benzo[c]pyridine isomer
Quinoline and isoquinoline are very important heterocyclic units because
their derivatives widely occur in nature as alkaloids.
But unlike benzene the bond lengths of Quinoline and isoquinoline
are irregular
nitration of quinoline with fuming nitric acid in concentrated (conc.) sulfuric acid containing SO3 at room temperature gives
a mixture of 5-nitro-and 8-nitroquinolines,
whereas isoquinoline reacts with same reagent at 0°C to give a mixture of 5- and 8- nitroisoquinolines.
Sulfonation of quinoline with oleum at 92°C gives
quinoline-8-sulfonic acid
Sulfonation of isoquinoline with oleum at 92°C gives
isoquinoline-5- sulfonic acid.
Alkyl and acyl halides react directly with the basic nitrogen atom with quinoline and isoquinoline to give
quaternary salts.
The C=N bond of the pyridine ring in both of these compounds undergoes nucleophilic addition at low temperature with KNH2, and the adduct on oxidation with KMnO4 at low temperature gives
2- aminoquinoline and 1-aminoisoquinoline in a Chichibabin-type reaction.
Indole ring occurs widely in nature as
alkaloids
indole undergoes electrophilic substitution at C-2 in the pyrrole ring and regioselectively at C-3 due to
higher resonance stabilization of the intermediate formed by C-3 attack
Because of the aromatic stability of the benzene ring, the most important contributing structure of indole to its
reso- nance hybrid is its enamine form.
indole easily undergoes protonation to give
indolenium cation
indole is sulfonated at C-3 with
pyridinium–N-sulfonate
brominated at C-3 with
bromine in pyridine at 0°C
acetylated at C-1 and C-3 to give
diacetyl derivative with acetic anhydride in acetic acid,
indole methylated at C-3 with
methyl iodide in DMF at 80°C
indole formylated at C-3 with
POCl3 and DMF at 5°C
indole amino methylated at C-3 with
HCHO and amines (Mannich reaction).
benzene ring is fused with a pyrrole ring and hence behaves as
an aromatic heterocyclic compound
Small heterocyclic rings
epoxides
Epoxides are compounds containing
a three-membered ring with oxygen as a heteroatom
The IUPAC name of such rings is
oxiran. They are cyclic ethers
epoxides synthesized by
the oxidation of a CPC bond of alkenes with peracids like m-chloroperbenzoic acid (MCPBA).
pyridine.
Pyridine is classified as
aromatic
pyridine bond angles of
120°
pyridine heat of combustion indicates a resonance energy of
23 kcal/mol.
pyridine is bonded to other members of the ring by
the use of sp2
the nitrogen atom makes pyridine a much stronger base than
pyrrole
Pyridine is found in
coal tar
Oxidation of the picolines yields the
pyridinecarboxylic acids.
Reactions of pyridine
undergoes the substitution, both electrophilic and nucleophilic
electrophilic substitution pyridine It undergoes
nitration, sulfonation, and halogenation
pyridine does not undergo the
Friedel–Crafts reaction
Nucleophilic substitution takes place readily, particularly at
the 2- and 4-positions
Pyridine is a base with
Kb = 2.3 * 10-9.
Pyridine is a base with Kb = 2.3 * 10-9. It is thus much stronger than
pyrrole (Kb ‘ 2.5 * 10-14) but much weaker than aliphatic amines (Kb ‘ 10-4)
Pyridine has a pair of electrons (in an sp2 orbital)
pyrrole has not
Benzene is a stronger acid than
an alkane
phenyl anion, C H -, is a weaker 65
base than
an alkyl anion, R-.
acetylene is a stronger acid than
benzene
acetylide ion is a weaker base than
the phenyl anion.
p orbital is at some distance from
the nucleus and is held relatively loosely
an s orbital, on the other hand, is close
to the nucleus and is held more tightly
Catalytic hydrogenation of pyridine yields
the aliphatic heterocyclic compound piperidine, C5H11N.
The simplest of the five-membered heterocyclic compounds are
pyrrole, furan,
and thiophene,
pyrrole, furan,
and thiophene have the properties of
conjugated diene and of an amine,
pyrrole, furan,
and thiophene have the properties of
conjugated diene and of an amine,
thiophene does not undergo
the oxidation
pyrrole does not possess the
basic properties typical of amines
pyrrole, furan,
and thiophene derivatives most commonly undergo
elec- trophilic substitution: nitration, sulfonation, halogenation, Friedel–Crafts acylation, even the Reimer–Tiemann reaction and coupling with diazonium salts
Pyrrole and thiophene are found in
small amounts in coal tar.
During the frac- tional distillation of coal tar, thiophene
b.p. 84°C
During the frac- tional distillation of coal tar, thiophene (b.p. 84°C) is collected along with the benzene
(b.p. 80°C)
