CHEM 219 | CHEM219 Final Exam: Principles of
Organic Chemistry with Lab - Portage Learning
Updated and Latest Questions and Correct
Answers with Rationale
1. A student determines the hybridization of the central carbon in a molecule of
formaldehyde (CH2O). Which of the following describes the hybridization and the orbital
overlap for the C=O bond?
A. sp3 hybridization; one sigma bond and one pi bond.
B. sp2 hybridization; two sigma bonds and no pi bonds.
C. sp hybridization; two pi bonds formed by p-p overlap.
D. sp2 hybridization; one sigma bond formed by sp2-sp2 overlap and one pi bond by p-p
overlap.
Correct Answer: D
Expert Explanation: Formaldehyde contains a carbon atom bonded to three groups, which
corresponds to sp2 hybridization. The carbon-oxygen double bond consists of one sigma
bond and one pi bond. The sigma bond is formed by the head-on overlap of sp2 hybrid
orbitals from carbon and oxygen. The pi bond results from the side-by-side overlap of
unhybridized p orbitals on both atoms. This structural arrangement is fundamental to
understanding the reactivity of carbonyl groups in organic synthesis.
2. According to IUPAC nomenclature, what is the correct name for a six-carbon chain with a
methyl group on carbon 2 and a chlorine atom on carbon 4?
A. 2-methyl-4-chlorohexane
B. 4-chloro-2-methylhexane
C. 3-chloro-5-methylhexane
D. Isomeric chlorohexane
Correct Answer: B
Expert Explanation: The longest carbon chain contains six carbons, making the parent
name hexane. Substituents are numbered to give the lowest possible locants to the
functional groups. Alphabetical order must be followed when listing substituents in the
final name regardless of their position number. Therefore, ‘chloro’ comes before ‘methyl’
alphabetically, resulting in 4-chloro-2-methylhexane. This rule ensures a unique and
standardized name for every organic molecule.
,3. When viewing a Newman projection of butane along the C2-C3 bond, which conformation
represents the global energy minimum?
A. Gauche conformation
B. Eclipsed conformation
C. Anti-periplanar conformation
D. Totally eclipsed conformation
Correct Answer: C
Expert Explanation: The anti-periplanar conformation places the two bulky methyl
groups 180 degrees apart. This arrangement minimizes steric hindrance and torsional
strain between the substituents. Gauche conformations are stable but higher in energy due
to 60-degree steric interactions. Eclipsed conformations represent energy maxima because
of the repulsion between bonding electrons. Mastery of Newman projections is essential for
predicting the physical properties and reactivity of alkanes.
4. In a chair conformation of cis-1,3-dimethylcyclohexane, what is the most stable orientation
of the methyl groups?
A. One axial and one equatorial
B. Both axial
C. One pseudo-axial and one equatorial
D. Both equatorial
Correct Answer: D
Expert Explanation: Cyclohexane derivatives prefer conformations where bulky groups
occupy equatorial positions to avoid 1,3-diaxial interactions. For a cis-1,3 substitution
pattern, both groups can be oriented in the equatorial position simultaneously. If one or
both were axial, the steric strain would significantly increase the potential energy of the
molecule. The ‘both equatorial’ chair is therefore the most stable conformer at equilibrium.
This concept explains why certain isomers react faster or exist in specific shapes in
biological systems.
5. Which of the following compounds is the strongest acid based on its structure and
resonance stabilization of the conjugate base?
A. Ethanol
B. p-Nitrophenol
C. Cyclohexanol
D. Phenol
Correct Answer: B
, Expert Explanation: Acidity is determined by the stability of the conjugate base formed
after deprotonation. p-Nitrophenol is more acidic than phenol because the nitro group is
strongly electron-withdrawing via resonance. This delocalizes the negative charge on the
phenoxide oxygen more effectively across the ring and onto the nitro group. Ethanol and
cyclohexanol lack resonance stabilization, making them much weaker acids. Understanding
inductive and resonance effects is vital for predicting pKa values in organic chemistry.
6. A molecule has two chiral centers with configurations (2R, 3S). What is the relationship
between this molecule and its (2S, 3R) isomer?
A. Diastereomers
B. Enantiomers
C. Constitutional isomers
D. Identical compounds
Correct Answer: B
Expert Explanation: Enantiomers are non-superimposable mirror images where every
chiral center has the opposite configuration. In this case, 2R changes to 2S and 3S changes
to 3R, satisfying the definition of enantiomers. Diastereomers occur when at least one, but
not all, chiral centers are inverted. If the molecule were meso, it would be identical to its
mirror image, but that requires internal symmetry not specified here. Recognizing these
relationships is critical for pharmacology and biochemistry.
7. Which reaction condition favors an SN2 mechanism over an SN1 mechanism for a
secondary alkyl halide?
A. A weak nucleophile in a protic solvent.
B. High temperature and a non-polar solvent.
C. A strong nucleophile in a polar aprotic solvent.
D. Presence of a Lewis acid catalyst.
Correct Answer: C
Expert Explanation: SN2 reactions are bimolecular and require a strong nucleophile to
attack the substrate in a single step. Polar aprotic solvents like DMSO or acetone favor SN2
because they do not solvate the nucleophile strongly, leaving it free to attack. Protic
solvents stabilize ions and favor carbocation formation, which is characteristic of the SN1
pathway. Secondary halides are sensitive to conditions and can undergo either mechanism
depending on the environment. Therefore, choosing a strong nucleophile and aprotic
solvent pushes the reaction toward SN2.
8. What is the major product of the dehydrohalogenation of 2-bromo-2-methylbutane using a
bulky base like potassium tert-butoxide?
A. 2-methyl-1-butene (Hofmann product)
Organic Chemistry with Lab - Portage Learning
Updated and Latest Questions and Correct
Answers with Rationale
1. A student determines the hybridization of the central carbon in a molecule of
formaldehyde (CH2O). Which of the following describes the hybridization and the orbital
overlap for the C=O bond?
A. sp3 hybridization; one sigma bond and one pi bond.
B. sp2 hybridization; two sigma bonds and no pi bonds.
C. sp hybridization; two pi bonds formed by p-p overlap.
D. sp2 hybridization; one sigma bond formed by sp2-sp2 overlap and one pi bond by p-p
overlap.
Correct Answer: D
Expert Explanation: Formaldehyde contains a carbon atom bonded to three groups, which
corresponds to sp2 hybridization. The carbon-oxygen double bond consists of one sigma
bond and one pi bond. The sigma bond is formed by the head-on overlap of sp2 hybrid
orbitals from carbon and oxygen. The pi bond results from the side-by-side overlap of
unhybridized p orbitals on both atoms. This structural arrangement is fundamental to
understanding the reactivity of carbonyl groups in organic synthesis.
2. According to IUPAC nomenclature, what is the correct name for a six-carbon chain with a
methyl group on carbon 2 and a chlorine atom on carbon 4?
A. 2-methyl-4-chlorohexane
B. 4-chloro-2-methylhexane
C. 3-chloro-5-methylhexane
D. Isomeric chlorohexane
Correct Answer: B
Expert Explanation: The longest carbon chain contains six carbons, making the parent
name hexane. Substituents are numbered to give the lowest possible locants to the
functional groups. Alphabetical order must be followed when listing substituents in the
final name regardless of their position number. Therefore, ‘chloro’ comes before ‘methyl’
alphabetically, resulting in 4-chloro-2-methylhexane. This rule ensures a unique and
standardized name for every organic molecule.
,3. When viewing a Newman projection of butane along the C2-C3 bond, which conformation
represents the global energy minimum?
A. Gauche conformation
B. Eclipsed conformation
C. Anti-periplanar conformation
D. Totally eclipsed conformation
Correct Answer: C
Expert Explanation: The anti-periplanar conformation places the two bulky methyl
groups 180 degrees apart. This arrangement minimizes steric hindrance and torsional
strain between the substituents. Gauche conformations are stable but higher in energy due
to 60-degree steric interactions. Eclipsed conformations represent energy maxima because
of the repulsion between bonding electrons. Mastery of Newman projections is essential for
predicting the physical properties and reactivity of alkanes.
4. In a chair conformation of cis-1,3-dimethylcyclohexane, what is the most stable orientation
of the methyl groups?
A. One axial and one equatorial
B. Both axial
C. One pseudo-axial and one equatorial
D. Both equatorial
Correct Answer: D
Expert Explanation: Cyclohexane derivatives prefer conformations where bulky groups
occupy equatorial positions to avoid 1,3-diaxial interactions. For a cis-1,3 substitution
pattern, both groups can be oriented in the equatorial position simultaneously. If one or
both were axial, the steric strain would significantly increase the potential energy of the
molecule. The ‘both equatorial’ chair is therefore the most stable conformer at equilibrium.
This concept explains why certain isomers react faster or exist in specific shapes in
biological systems.
5. Which of the following compounds is the strongest acid based on its structure and
resonance stabilization of the conjugate base?
A. Ethanol
B. p-Nitrophenol
C. Cyclohexanol
D. Phenol
Correct Answer: B
, Expert Explanation: Acidity is determined by the stability of the conjugate base formed
after deprotonation. p-Nitrophenol is more acidic than phenol because the nitro group is
strongly electron-withdrawing via resonance. This delocalizes the negative charge on the
phenoxide oxygen more effectively across the ring and onto the nitro group. Ethanol and
cyclohexanol lack resonance stabilization, making them much weaker acids. Understanding
inductive and resonance effects is vital for predicting pKa values in organic chemistry.
6. A molecule has two chiral centers with configurations (2R, 3S). What is the relationship
between this molecule and its (2S, 3R) isomer?
A. Diastereomers
B. Enantiomers
C. Constitutional isomers
D. Identical compounds
Correct Answer: B
Expert Explanation: Enantiomers are non-superimposable mirror images where every
chiral center has the opposite configuration. In this case, 2R changes to 2S and 3S changes
to 3R, satisfying the definition of enantiomers. Diastereomers occur when at least one, but
not all, chiral centers are inverted. If the molecule were meso, it would be identical to its
mirror image, but that requires internal symmetry not specified here. Recognizing these
relationships is critical for pharmacology and biochemistry.
7. Which reaction condition favors an SN2 mechanism over an SN1 mechanism for a
secondary alkyl halide?
A. A weak nucleophile in a protic solvent.
B. High temperature and a non-polar solvent.
C. A strong nucleophile in a polar aprotic solvent.
D. Presence of a Lewis acid catalyst.
Correct Answer: C
Expert Explanation: SN2 reactions are bimolecular and require a strong nucleophile to
attack the substrate in a single step. Polar aprotic solvents like DMSO or acetone favor SN2
because they do not solvate the nucleophile strongly, leaving it free to attack. Protic
solvents stabilize ions and favor carbocation formation, which is characteristic of the SN1
pathway. Secondary halides are sensitive to conditions and can undergo either mechanism
depending on the environment. Therefore, choosing a strong nucleophile and aprotic
solvent pushes the reaction toward SN2.
8. What is the major product of the dehydrohalogenation of 2-bromo-2-methylbutane using a
bulky base like potassium tert-butoxide?
A. 2-methyl-1-butene (Hofmann product)
