CHEM 219 | CHEM219 Module 8: Principles of
Organic Chemistry with Lab - Portage Learning
Updated and Latest Questions and Correct
Answers with Rationale
1. What is the expected rate law for a reaction proceeding via the SN2 mechanism?
A. Rate = k[Substrate]
B. Rate = k[Nucleophile]
C. Rate = k[Substrate]^2
D. Rate = k[Substrate][Nucleophile]
Correct Answer: D
Expert Explanation: The SN2 mechanism is defined as a bimolecular nucleophilic
substitution reaction. This classification implies that the transition state involves both the
substrate and the nucleophile simultaneously. Consequently, the rate of the reaction is
directly proportional to the concentration of both species. Experimental data consistently
shows that doubling either concentration will double the overall reaction rate. This second-
order kinetic behavior is a primary characteristic used to identify this specific mechanism.
2. Which of the following solvents would be most effective for promoting an SN2 reaction?
A. Dimethyl sulfoxide (DMSO)
B. Ethanol
C. Water
D. Acetic acid
Correct Answer: A
Expert Explanation: SN2 reactions are typically favored in polar aprotic solvents like
DMSO or DMF. These solvents are effective because they solvate the cation of the reagent
but leave the nucleophile relatively ‘naked’ and reactive. In contrast, polar protic solvents
like water or ethanol form hydrogen bonds with the nucleophile. This solvation shell
hinders the nucleophile’s ability to attack the substrate effectively. Therefore, using DMSO
results in a significantly faster reaction rate for bimolecular substitutions.
3. In an SN1 reaction occurring at a chiral center, what is the typical stereochemical outcome?
A. Complete inversion of configuration
B. Complete retention of configuration
C. Formation of a meso compound
,D. Racemization with slight excess of inversion
Correct Answer: D
Expert Explanation: The SN1 mechanism proceeds through a planar carbocation
intermediate which can be attacked from either side. In theory, this leads to a 50:50
mixture of enantiomers known as a racemic mixture. However, in practice, the leaving
group often partially blocks the side from which it departed. This ‘ion-pairing’ effect results
in a slight statistical preference for inversion over retention. Thus, the observed product is
usually a mixture with a small excess of the inverted enantiomer.
4. Which substrate would be the most reactive in an SN1 reaction?
A. Methyl bromide
B. tert-Butyl bromide
C. Isopropyl bromide
D. Ethyl bromide
Correct Answer: B
Expert Explanation: The SN1 reaction rate is determined by the stability of the
carbocation formed during the rate-determining step. Tertiary carbocations are much more
stable than secondary or primary ones due to inductive effects and hyperconjugation. tert-
Butyl bromide forms a tertiary carbocation, making it the most reactive among the given
choices. Methyl and primary halides rarely undergo SN1 because their carbocations are too
high in energy. Therefore, the degree of substitution on the carbon atom is the critical
factor for SN1 reactivity.
5. What is the primary requirement for a substrate to undergo an E2 elimination reaction?
A. Formation of a stable carbocation
B. A highly polar protic solvent
C. Presence of an anti-periplanar beta-hydrogen
D. A very weak base
Correct Answer: C
Expert Explanation: The E2 mechanism is a concerted process where the base removes a
proton while the leaving group departs. For optimal orbital overlap in the transition state,
the beta-hydrogen and the leaving group must be anti-periplanar. This geometric
arrangement allows for the developing pi bond to form efficiently. If this alignment cannot
be achieved, the E2 reaction rate will decrease significantly or fail to occur. Understanding
this stereoelectronic requirement is essential for predicting the major products of
elimination.
, 6. According to Zaitsev’s rule, which alkene is the major product of an elimination reaction?
A. The least substituted alkene
B. The most substituted alkene
C. The alkene with the most terminal double bonds
D. The cis-isomer only
Correct Answer: B
Expert Explanation: Zaitsev’s rule states that in an elimination reaction, the most stable
alkene is generally formed as the major product. Increased substitution around the double
bond provides greater thermodynamic stability through hyperconjugation. This usually
means the alkene with the greatest number of alkyl groups attached to the double bond is
favored. While there are exceptions involving bulky bases, this rule correctly predicts
outcomes for most standard conditions. It is a fundamental concept in organic chemistry
for predicting regioselectivity.
7. Which of the following describes the role of silver nitrate (AgNO3) in an SN1 lab
experiment?
A. It acts as a strong nucleophile
B. It facilitates the removal of the halide as a precipitate
C. It prevents carbocation rearrangement
D. It acts as a catalyst for SN2 reactions
Correct Answer: B
Expert Explanation: Silver nitrate is commonly used in lab scenarios to test for the
reactivity of alkyl halides via SN1. The silver ion coordinates with the halide atom, making
it an even better leaving group. As the halide leaves, it reacts with silver to form an
insoluble silver halide precipitate. This precipitation drives the equilibrium forward and
provides a visible indicator of reaction speed. Consequently, tertiary halides react almost
instantly, while primary halides show no reaction.
8. When using a bulky base like potassium tert-butoxide (t-BuOK), which product is favored in
an E2 reaction?
A. The Zaitsev product
B. The SN2 substitution product
C. The Hofmann product
D. No reaction occurs
Correct Answer: C
Organic Chemistry with Lab - Portage Learning
Updated and Latest Questions and Correct
Answers with Rationale
1. What is the expected rate law for a reaction proceeding via the SN2 mechanism?
A. Rate = k[Substrate]
B. Rate = k[Nucleophile]
C. Rate = k[Substrate]^2
D. Rate = k[Substrate][Nucleophile]
Correct Answer: D
Expert Explanation: The SN2 mechanism is defined as a bimolecular nucleophilic
substitution reaction. This classification implies that the transition state involves both the
substrate and the nucleophile simultaneously. Consequently, the rate of the reaction is
directly proportional to the concentration of both species. Experimental data consistently
shows that doubling either concentration will double the overall reaction rate. This second-
order kinetic behavior is a primary characteristic used to identify this specific mechanism.
2. Which of the following solvents would be most effective for promoting an SN2 reaction?
A. Dimethyl sulfoxide (DMSO)
B. Ethanol
C. Water
D. Acetic acid
Correct Answer: A
Expert Explanation: SN2 reactions are typically favored in polar aprotic solvents like
DMSO or DMF. These solvents are effective because they solvate the cation of the reagent
but leave the nucleophile relatively ‘naked’ and reactive. In contrast, polar protic solvents
like water or ethanol form hydrogen bonds with the nucleophile. This solvation shell
hinders the nucleophile’s ability to attack the substrate effectively. Therefore, using DMSO
results in a significantly faster reaction rate for bimolecular substitutions.
3. In an SN1 reaction occurring at a chiral center, what is the typical stereochemical outcome?
A. Complete inversion of configuration
B. Complete retention of configuration
C. Formation of a meso compound
,D. Racemization with slight excess of inversion
Correct Answer: D
Expert Explanation: The SN1 mechanism proceeds through a planar carbocation
intermediate which can be attacked from either side. In theory, this leads to a 50:50
mixture of enantiomers known as a racemic mixture. However, in practice, the leaving
group often partially blocks the side from which it departed. This ‘ion-pairing’ effect results
in a slight statistical preference for inversion over retention. Thus, the observed product is
usually a mixture with a small excess of the inverted enantiomer.
4. Which substrate would be the most reactive in an SN1 reaction?
A. Methyl bromide
B. tert-Butyl bromide
C. Isopropyl bromide
D. Ethyl bromide
Correct Answer: B
Expert Explanation: The SN1 reaction rate is determined by the stability of the
carbocation formed during the rate-determining step. Tertiary carbocations are much more
stable than secondary or primary ones due to inductive effects and hyperconjugation. tert-
Butyl bromide forms a tertiary carbocation, making it the most reactive among the given
choices. Methyl and primary halides rarely undergo SN1 because their carbocations are too
high in energy. Therefore, the degree of substitution on the carbon atom is the critical
factor for SN1 reactivity.
5. What is the primary requirement for a substrate to undergo an E2 elimination reaction?
A. Formation of a stable carbocation
B. A highly polar protic solvent
C. Presence of an anti-periplanar beta-hydrogen
D. A very weak base
Correct Answer: C
Expert Explanation: The E2 mechanism is a concerted process where the base removes a
proton while the leaving group departs. For optimal orbital overlap in the transition state,
the beta-hydrogen and the leaving group must be anti-periplanar. This geometric
arrangement allows for the developing pi bond to form efficiently. If this alignment cannot
be achieved, the E2 reaction rate will decrease significantly or fail to occur. Understanding
this stereoelectronic requirement is essential for predicting the major products of
elimination.
, 6. According to Zaitsev’s rule, which alkene is the major product of an elimination reaction?
A. The least substituted alkene
B. The most substituted alkene
C. The alkene with the most terminal double bonds
D. The cis-isomer only
Correct Answer: B
Expert Explanation: Zaitsev’s rule states that in an elimination reaction, the most stable
alkene is generally formed as the major product. Increased substitution around the double
bond provides greater thermodynamic stability through hyperconjugation. This usually
means the alkene with the greatest number of alkyl groups attached to the double bond is
favored. While there are exceptions involving bulky bases, this rule correctly predicts
outcomes for most standard conditions. It is a fundamental concept in organic chemistry
for predicting regioselectivity.
7. Which of the following describes the role of silver nitrate (AgNO3) in an SN1 lab
experiment?
A. It acts as a strong nucleophile
B. It facilitates the removal of the halide as a precipitate
C. It prevents carbocation rearrangement
D. It acts as a catalyst for SN2 reactions
Correct Answer: B
Expert Explanation: Silver nitrate is commonly used in lab scenarios to test for the
reactivity of alkyl halides via SN1. The silver ion coordinates with the halide atom, making
it an even better leaving group. As the halide leaves, it reacts with silver to form an
insoluble silver halide precipitate. This precipitation drives the equilibrium forward and
provides a visible indicator of reaction speed. Consequently, tertiary halides react almost
instantly, while primary halides show no reaction.
8. When using a bulky base like potassium tert-butoxide (t-BuOK), which product is favored in
an E2 reaction?
A. The Zaitsev product
B. The SN2 substitution product
C. The Hofmann product
D. No reaction occurs
Correct Answer: C
