CHEM219/ CHEM 219 Module 7 – Principles of
Organic Chemistry with Lab | Portage Learning |
Updated 2026–2027 | Complete Questions &
Verified Answers
... Show More
| Grade A
2026 / 2027 Academic Year
Q: Heterocycles (heterocyclic compounds)
Answer
cyclic organic molecules in which one or more carbon atoms are replaced by heteroatoms
(elements other than carbon or hydrogen)
Q: What are the most common heteroatoms?
Answer
oxygen, nitrogen, and sulfur
Q: How many heteroatoms can be present on a heterocyclic compound?
Answer
More than one heteroatom can be present, and the heteroatoms can be the same or
different
Q: True or False: Heterocyclic compounds may contain multiple bonds and the rings may
have chains or branched chains of carbons attached to them. The rings may be of a variety
of sizes.
Answer
True
Q: True or False: Heterocycles form the largest class of organic compounds known.
Answer
True
Q: True or False: Many natural products and important drug molecules contain
heterocyclic rings.
Answer
True
Q: What are the two main subgroups of heterocyclic compounds?
Answer
Aromatic and aliphatic (non-aromatic)
Example of non-aromatic heterocycles include molecules like cyclic ethers (epoxides) and
sugars like glucose (a cyclic hemiacetal)
Non-aromatic heterocycles behave very similarly to their acyclic counterparts.
Q: Pyridine
Answer
A six-membered heterocyclic analog of benzene. In pyridine, one CH unit of the (benzene)
ring is replaced by a N atom.
Q: What are the bond angles between the atoms of pyridine?
Answer
Approximately 120 degrees
Q: True or False: Like benzene, pyridine is a flat molecule with near-perfect hexagonal
geometry?
Answer
True
Q: What hybridization are the atoms of the pyridine ring?
Answer
sp2
Q: What makes the pyridine ring aromatic just like benzene?
Answer
each atom is sp2 hybridized and each contributes a single electron to the conjugated p
system for a total of six p electrons, making the ring aromatic just like benzene
Q: True or False: The nitrogen atom of pyridine contains a lone pair of electrons in an sp2
hybrid orbital that is NOT orientated in the same plane of the atoms in the ring (like the
hydrogens that project from each carbon atom).
Answer
False - they are orientated in the same plane of the atoms in the ring.
Q: What are the C-C bond lengths in a pyridine molecule?
Answer
139 pm are intermediate between normal C-to-C sigma and pi bonds.
Q: True or False: Pyridine exists as a hybrid of equivalent resonance structures as
evidenced by the C-C bond length?
Answer
True
Q: What does the circle inscribed inside of the pyridine ring represent?
Answer
The delocalization of the pi electrons within the six-membered ring.
Q: What kind of reaction does pyridine tend to undergo and why?
Answer
Substitution reactions because addition reactions would destroy the aromaticity.
Q: True or False: Pyridine is completely miscible in water?
Answer
True; unlike benzene
This is due to the substitution of the N atom in the ring which changes many of the
properties of the molecules.
Pyridine is also miscible in most organic solvents.
Q: What are the several factors that explain the ability of pyridine to dissolve in water?
Answer
1. The ability of pyridine to accept hydrogen bonding interactions from water molecules due
to the lone pair of electrons on the N atom.
2. The electronegative N creates a relatively strong dipole moment in the pyridine molecule,
making it a polar compound overall. There is a shift of electrons away from the Cs and
towards the N, making the ring Cs partially positive and the N partially negative.
Q: What causes the difference in B.P. between pyridine and benzene?
Answer
B.P. of pyridine is 35 degrees higher than that of benzene, even though the molecular
weight is negligible between the two compounds. Dipole-Dipole attractive forces between
the pyridine molecules raises the B.P. as compared to benzene.
Q: Pyridinium salts
Answer
Pyridine is a weak base and will react with strong acids to form pyridinium salts. The ability
of pyridine to act as a base, coupled with its wide-ranging solubility, makes it suited to act
as a proton scavenger in acid-producing reactions, like the conversion of alcohols to alkyl
chlorides using thionyl chloride.
Q: EAS (Electrophilic aromatic substitution) reactions
Answer
Reactions in which a hydrogen is replaced by an electrophile (species that seeks
electrons/negative charge).
Q: Under what conditions will pyridine undergo EAS reactions?
Answer
Slowly and very harsh conditions
i.e. pyridine can be nitrated or brominated, as shown, but the reactions require very high
temperatures and very strong acid catalysis. Electron withdrawal by the N makes the ring
partially positive and, thus, not as receptive to attack by electrophiles like NO2+ or Br+. In
addition, the strongly acidic conditions required for the formation of these electrophiles
causes protonation of the N atom and the positive pyridinium ion formed as a result is even
less attractive to attack by electrophiles.
Q: When EAS does occur, what is the preferred position for the substitution on the ring?
Answer
C3 position
This position is preferred as the positive charge that forms on the C during the reaction is
the one position that does not put the positive charge on the electronegative N via
resonance.
Q: True or False: Pyridine can more easily undergo nucleophilic aromatic substitution
(NAS) than EAS?
Answer
True
In NAS, a nucleophile displaces a hydride (H-) ion or halide ion (on a substituted pyridine)
from the aromatic ring. Due to the presence of the N heteroatom, the ring is partially
positive and therefore is more attractive to nucleophiles, rather than electrophiles.
i.e. Strong nucleophiles like alkoxide ions (RO-) and amide ions (NH2-) help to facilitate
the reaction.
Azines
Answer
heterocyclic six-membered rings with two or more N atoms present as part of the ring
system
Diazines
Answer
contain two N atoms and can exist in three different constitutional isomeric forms;
pyrimidines are the most important because derivatives of pyrimidines are important bases
in the nucleic acids DNA and RNA namely cytosine, thymine, and uracil.
True or False: Tri- and tetrazines are also known, but no others with higher degrees of N
substitution (5 or 6 N atoms) are known.
Answer
True
What are the most important heterocyclic molecules with five-membered rings?
Answer
Furan (oxygen-based)
Pyrrole (nitrogen-based)
Thiophene (sulfur-based)
differences:
the heteroatom of a five-membered ring contributes two electrons (one lone pair) to the
These molecules each contain a carbon ring with a single heteroatom substitution.
What are some similarities and differences between five-membered rings and benzene (six
membered ring)?
Answer
Similarities:
aromatic
undergo EAS rather than other modes, like addition
planar (flat)
aromatic pi system of the ring to make the molecule aromatic while the six-membered
heterocycles have a heteroatom that contributes one electron to the aromatic pi system
What are some important consequences in terms of chemistry of the molecule for the use of
the lone pair of pyrrole in the aromatic pi system?
Pyrrole (five-membered) is a much weaker base than pyridine (six-membered) 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.
True or False: 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 N atom.
True
True or False: Furan and thiophene have two lone pairs on their respective heteroatoms.
While one lone pair is involved in the aromatic pi system, the other is in the same plane as
the atoms of the ring and is available for protonation.
True
"Pyrrole-like"
When the N atom of an aromatic heterocycle has single bonds only, then the lone pair
participates in the conjugated pi system to create aromaticity and is not available to react as
a weak base.
"Pyridine-like"
When the N atom of an aromatic heterocycle is participating in a pi bond, then the lone pair
occupies an sp2 orbital in the same plane as the ring and is available to react as a weak base.
True or False: Furan, pyrrole, and thiophene are all more reactive than benzene in EAS
reactions.
True
Each molecule reacts with electrophiles to substitute (for a hydrogen) at the C-2 position.
Why is substitution preferred at the C-2 position on five-membered heterocycles?
The carbocation intermediate that forms during the reaction is delocalized over more atoms
(and hence, more stable) when substitution occurs at this position.
Azoles
It is possible to introduce more than one heteroatom into a five-membered heterocycle. Of
the many available structural possibilities, the most important heterocycles are formed by
introducing a N atom in place of C-3 in furan, pyrrole, and thiophene. The resulting
molecules are known as azoles.
Thiazole
occurs naturally in thiamin (vitamin B1), a coenzyme required for many biochemical
processes essential to human life
The inclusion of the N at position C-3 results in a "pyridine-like" N, which is involved in a pi
bond.
Imidazole
A very good base, as the positive charge obtained from protonation of the N at position C-3
can be delocalized via resonance over both N atoms.
Imidazole is present in the amino acid histidine and is important in the reactions of many
enzymes.
Why do heterocycles form the largest class of organic compounds known?
Structural diversity in terms of their ring size and the number and identity of heteroatom
substitutions.
Another reason, is the possibility of fused ring systems.
Fused ring system
Two (or more) rings are fused together to form a larger molecule; to be fused means that
each ring shares two (or more) common atoms as a part of the structure of each ring.
Purines
Another biologically important class of fused-ring heterocyclic compounds. Purines contain
a pyrimidine ring fused to an imidazole ring.
Uric acid , caffeine, and theobromine (found in chocolate) are all based on the structure of
purine.
Biochemically, the structures of adenine and guanine, two of the nitrogenous bases present
in DNA and RNA are both purines.
Indole
a bicyclic, fused-ring compound formed by fusing a benzene ring to the C2-C3 bond of
pyrrole
Indole is a component in many biologically active compounds including the amino acid
tryptophan.
Heterocycles
cyclic organic molecule where one or more carbon atoms are replaced by heteroatoms
Heteroatoms
Atoms in an organic compound other than carbon and hydrogen
Ex: Oxygen, Nitrogen, Sulfur
Two main subgroups of heterocycles
1. aromatic
2. aliphatic (non-aromatic)
Pyridine
analogue of benzene where one C is replaced by N
geometry of pyridine
flat/ planar geometry with 120 degree bond angles
atomic structure of pyridine
sp2 hybridized atoms, 6 P electrons just like benzene
resonance structure of pyridine
Why does pyridine undergo substitution rather than addition?
addition would destroy the aromaticity
solubility of pyridine
soluble in most organic solvents and completely soluble in water
why is pyridine soluble in water? -ability of pyridine to accept hydrogen bonding interactions from water molecules d/t the
lone pair of electrons on the nitrogen. - electronegative N creates a relatively strong dipole moment in the pyridine, thus making it
an apolar compound overall.
BP of pyridine vs benzene? why? -pyridine has a higher BP than benzene even though they have a similar weight -why? dipole/dipole forces
Is pyridine an acid or base?
weak base
will form pyridinium salt when reacted with strong acids
Two main reaction types of pyridine?
1. EAS- Electrophilic Aromatic Substitution
2. NAS- Nucleophilic Aromatic Substitution
What happens in the EAS of pyridine?
a hydrogen is replaced by an electrophile
What conditions are needed for pyridine to undergo EAS?
very slow and very harsh conditions- need very hot heat and a strong acid
Why is pyridine less receptive to EAS than other molecules?
ring is partially positive d/t electron withdrawal by the nitrogen
What position does an EAS reaction prefer on a pyridine molecule?
C3 bc the charge wont put a positive charge on the electronegative nitrogen via resonance
Nucleophilic Aromatic Substitution
A substitution reaction in which an aromatic ring is attacked by a nucleophile, which
replaces a leaving group. -displaces a hydride ion or a halide ion from the ring
Why are NAS easier for pyridine than EAS?
the positive partial charge on the Nitrogen atom attracts nucleophiles
Azine
heterocyclic 6-membered rings with 2 or more nitrogen atoms as part of the ring system
Diazines
2 nitrogens replacing carbons in a benzene ring
3 isomeric forms of diazines
1. pyridazine N1, N2
2.pyrimidine N1, N3
3.pyrazine N1, N4
pyridazine
placement: 1, 2
pyrazine
placement 1, 4
Pyrimidine
placement: 1, 3 -most important, found in cyotosine and thymine (DNA/ RNA)
furan
oxygen-based 5-membered aromatic ring
pyrrole
nitrogen-based 5-membered aromatic ring
thiophene
sulfur-based 5-membered aromatic ring
5-membered heterocycles
furan, pyrrole, thiophene -aromatic/ behave like benzene -EAS reactions -Flat/planar geometry
Biggest difference between benzene and membered aromatic rings?
The heteroatom of a 5-membered ring contributes 2 electrons (1 lone pair) to the aromatic
system while the 6-membered heterocycles' heteroatom only contributes 1 electron to the
aromatic system
Pyridine or Pyrrole WEAKER base?
Pyrrole is a weaker base than pyridine bc pyrrole uses the lone pair from the nitrogen in the
aromatic stabilization system.
pronating the lone pair of pyrrole would destroy the aromaticity
Difference in Furan and Thiophene compared to pyrrole
Furan and thiophene have 2 lone pairs instead of one which leaves one lone pair for
protonations after the other is used in the aromatic pi system
Is pyridine or the 3 5-membered heterocycles more reactive in EAS reactions?
Furan, Thiophene, and pyrrole are more reactive in EAS reactions than pyridine
what atom number is preferred for EAS substitution on 5-membered heterocycles?
substitution at C2
carbocation intermediate that is formed during EAS is delocalized over more atoms and
thus more stable when at C2
Azole
more than one heteroatom in a 5-membered heterocycle
oxazole
oxygen at 1, nitrogen at 3
Imidazole
nitrogen at 1, nitrogen at 3
Thiazole
sulfur at 1, nitrogen at 3
What does the additional Nitrogen on azoles allow for?
lone pair on new nitrgoen means there is availability for the molecule to bond to an acidic
proton
Why is Imidazole a good base?
positive charge obtained from the protonation of nitrogen on position 3 can be localized via
resonance over both of the nitrogens
Imidazole is found in:
histidine (amino acid)
thiazole is found in:
Thiamine (B1 vitamin)
fused-ring system
a molecule in which two or more rings share two adjacent carbon atoms
indole
benzene + pyrrole
found in amino acid tryptophan
purine
pyrimidine ring fused to imidazole ring -found in uric acid, adenine and guanine (A & G in DNA)caffeine, theobromine (in
chocolate)
Show Less
Google