CHEM219/ CHEM 219 Module 7 – Principles of Organic Chemistry with Lab | Portage Learning | Updated 2026–2027 | Complete Questions & Verified Answers | Grade A

CHEM219/ CHEM 219 Module 7 – Principles of Organic Chemistry with Lab | Portage Learning | Updated 2026–2027 | Complete Questions & Verified Answers | 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 idine N1, N3 ine 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)