CHM 219 Module 7
1. Define Heterocyle as it applies to organic molecules. ans Cyclic molecules
in which one or mroe carbon atoms are replaced by heteratoms - elements other
than carbon or hydrogen.
2. What are two main sub-groups of heterocyclic compounds? ans Aliphatic
and Aromatic
3. Explain why pyridine is soluble in water. ans First - the ability of pyridine to
accept Hydrogen bonding interactions from water molecules due to the lone pair
of electrons in the nitrogen atom. In addition, the electronegative nitrogen creates
a relatively strong dipole moment in the pyridine molecule, thus making pyridine a
polar compound overall.
4. Is pyridine more likely to act as an acid or base? Explain. ans Pyridine,
like all amines, is a weak base. Amines use the lone pair on the nitrogen atom to
accept protons and thus act as bases.
5. Name the two main classes of reactions that the pyridine molecule will
undergo - ans EAS - Electrophilic Aromatic Substitution
NAS - Nucleophilic Aromatic Substitution
6. Heterocyclic six-membered rings with two or more N atoms present as a
part of the ring system are known as what type of heterocycles? ans
Azines
7. Why are pyrimidines the most biologically important members of the di-
azene family of molecules? ans Derivatives of pyrimidines are important bases in
the nucleic acids DNA and RNA , namely cytosine, thymine and uracil.
8. Name the most important five-membered heterocyclic compounds con-
taining a single heteroatom substitution and state which element is the het-
eroatom in each. ans Furan (Oxygen based)
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, CHM 219 Module 7
Pyrrole (Nitrogen based)
Thiophene (sulfur based)
9. What is the most important difference between five and six membered
heterocycles in terms of the bonding a the heteroatom? ans The
heteroatom of a five-membered ring contributes two electrons (one lone pair) to
the aromatic pi system of the ring to make the molecule aromatic. This is in
sharp contrast to six
membered heterocycles where the heteroatom only contributes one electron to the
aromatic pi system.
10. Explain why Pyrrole is a weaker base than pyridine, even though both are
aromatic, heterocyclic amines. ans This is due to the availability of the lone
pair. In pyridine, the lone pair is available (not part of the aromatic pi system) to
accept an acidic proton. In Pyrrole, this is not the case. Protonation of the lone pair
of pyrrole destroys the aromatic pi system and the special stability of aromaticity;
thus, pyrrole is much less likely to accept an acidic proton on its nitrogen atom.
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1. Define Heterocyle as it applies to organic molecules. ans Cyclic molecules
in which one or mroe carbon atoms are replaced by heteratoms - elements other
than carbon or hydrogen.
2. What are two main sub-groups of heterocyclic compounds? ans Aliphatic
and Aromatic
3. Explain why pyridine is soluble in water. ans First - the ability of pyridine to
accept Hydrogen bonding interactions from water molecules due to the lone pair
of electrons in the nitrogen atom. In addition, the electronegative nitrogen creates
a relatively strong dipole moment in the pyridine molecule, thus making pyridine a
polar compound overall.
4. Is pyridine more likely to act as an acid or base? Explain. ans Pyridine,
like all amines, is a weak base. Amines use the lone pair on the nitrogen atom to
accept protons and thus act as bases.
5. Name the two main classes of reactions that the pyridine molecule will
undergo - ans EAS - Electrophilic Aromatic Substitution
NAS - Nucleophilic Aromatic Substitution
6. Heterocyclic six-membered rings with two or more N atoms present as a
part of the ring system are known as what type of heterocycles? ans
Azines
7. Why are pyrimidines the most biologically important members of the di-
azene family of molecules? ans Derivatives of pyrimidines are important bases in
the nucleic acids DNA and RNA , namely cytosine, thymine and uracil.
8. Name the most important five-membered heterocyclic compounds con-
taining a single heteroatom substitution and state which element is the het-
eroatom in each. ans Furan (Oxygen based)
1/8
, CHM 219 Module 7
Pyrrole (Nitrogen based)
Thiophene (sulfur based)
9. What is the most important difference between five and six membered
heterocycles in terms of the bonding a the heteroatom? ans The
heteroatom of a five-membered ring contributes two electrons (one lone pair) to
the aromatic pi system of the ring to make the molecule aromatic. This is in
sharp contrast to six
membered heterocycles where the heteroatom only contributes one electron to the
aromatic pi system.
10. Explain why Pyrrole is a weaker base than pyridine, even though both are
aromatic, heterocyclic amines. ans This is due to the availability of the lone
pair. In pyridine, the lone pair is available (not part of the aromatic pi system) to
accept an acidic proton. In Pyrrole, this is not the case. Protonation of the lone pair
of pyrrole destroys the aromatic pi system and the special stability of aromaticity;
thus, pyrrole is much less likely to accept an acidic proton on its nitrogen atom.
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