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Organic Chemistry - Janice Gorzynski Smith |
Reactions & Mechanisms Practice Exam
200 original mechanism, reagent, and product-prediction questions with
explanations
QUESTIONS FORMAT COVERAGE
200 A-D + Explanations Reactions
Original practice questions for independent study and exam preparation.
,Reactions & Mechanisms Practice Exam
Practice Questions
Choose the best answer for each item. The answer and a detailed explanation follow each question.
Substitution and elimination
1. Which statement best describes 1-bromobutane with NaCN in DMSO?
A. E1 elimination is favored
B. no reaction occurs
C. SN1 substitution is favored
D. SN2 substitution is favored
Answer: D
Explanation: A primary alkyl bromide is accessible to backside attack and cyanide is a strong nucleophile. DMSO is polar aprotic and
enhances nucleophilicity, so SN2 is strongly favored. A primary carbocation would be too unstable for SN1 or E1 to dominate.
Substitution and elimination
2. Which statement best describes ethyl iodide with sodium methoxide in acetone?
A. SN1 substitution is favored
B. E1 elimination is favored
C. only radical substitution occurs
D. SN2 substitution is favored
Answer: D
Explanation: A primary iodide reacts readily with a strong unhindered nucleophile by backside displacement. A polar aprotic solvent
such as acetone supports SN2 reactivity. Primary carbocation pathways are unfavorable.
Substitution and elimination
3. Which statement best describes methyl iodide with sodium ethoxide?
A. SN1 ionization dominates
B. E1 elimination dominates
C. E2 elimination dominates
D. SN2 substitution is the only normal substitution-elimination pathway
Answer: D
Explanation: Methyl halides have no beta carbon, so beta elimination cannot occur. Methyl carbocations are also extremely unstable,
ruling out SN1 and E1. A strong nucleophile therefore reacts through SN2.
Substitution and elimination
4. Which statement best describes 2-bromobutane with sodium ethoxide in ethanol?
A. SN1 must be exclusive
B. SN2 must be exclusive
C. no reaction can occur
D. a mixture is possible, with E2 often important under strong-base conditions
Answer: D
Explanation: A secondary halide with a strong base/nucleophile can undergo both SN2 and E2 pathways. Protic solvent and increased
substitution make SN2 less favorable than for a primary halide, while base strength supports E2. The exact product ratio depends on
temperature and structure.
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,Reactions & Mechanisms Practice Exam
Substitution and elimination
5. Which statement best describes 2-bromobutane with potassium tert-butoxide and heat?
A. SN1 substitution is strongly favored
B. electrophilic addition occurs
C. E2 elimination is strongly favored
D. SN2 is exclusive
Answer: C
Explanation: tert-Butoxide is a strong bulky base and heat favors elimination. Steric bulk makes backside substitution on a secondary
carbon less favorable. E2 therefore becomes the dominant pathway.
Substitution and elimination
6. Which statement best describes tert-butyl bromide in methanol at room temperature?
A. SN2 is dominant
B. SN1/E1 solvolysis is feasible, with substitution often prominent at lower temperature
C. only E2 can occur
D. the C-Br bond cannot ionize
Answer: B
Explanation: A tertiary halide forms a relatively stable tertiary carbocation in a polar protic solvent. Methanol is a weak nucleophile and
weak base, conditions that favor unimolecular pathways. SN2 is blocked by steric hindrance at the tertiary center.
Substitution and elimination
7. Which statement best describes tert-butyl bromide with sodium ethoxide and heat?
A. SN2 substitution dominates
B. no beta hydrogens exist
C. E2 elimination is favored
D. SN1 is the only possible mechanism
Answer: C
Explanation: A tertiary substrate is too hindered for SN2 attack. A strong base such as ethoxide can remove a beta hydrogen in a
concerted E2 step. Heat further favors elimination over substitution.
Substitution and elimination
8. Which statement best describes 1-bromobutane with potassium tert-butoxide?
A. E2 becomes competitive and can be favored because the base is bulky
B. only SN2 is possible regardless of base
C. SN1 dominates through a primary carbocation
D. E1 dominates
Answer: A
Explanation: A primary substrate normally favors SN2, but tert-butoxide is a bulky strong base. Steric hindrance reduces nucleophilic
attack and promotes beta-hydrogen abstraction. This can shift the reaction toward E2 and often toward the less substituted alkene.
Substitution and elimination
9. Which statement best describes benzyl bromide with cyanide ion?
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, Reactions & Mechanisms Practice Exam
A. SN2 substitution is very fast
B. only E1 can occur
C. SN2 is impossible at a benzylic carbon
D. only radical bromination can occur
Answer: A
Explanation: The benzylic carbon is accessible to backside attack and the transition state is stabilized by the adjacent aromatic system.
Cyanide is a strong nucleophile. Benzylic primary halides are therefore exceptionally reactive in SN2 substitution.
Substitution and elimination
10. Which statement best describes allyl bromide with acetate ion?
A. SN2 substitution is favored at the allylic carbon
B. the alkene is hydrogenated
C. only E2 occurs
D. SN2 is impossible because of the nearby double bond
Answer: A
Explanation: Allylic halides are highly reactive in substitution because the transition state benefits from allylic stabilization. Acetate can
attack the carbon bearing bromine by SN2 under suitable conditions. The alkene remains present rather than being hydrogenated.
Substitution and elimination
11. Which statement best describes 2-bromo-2-methylpropane with water?
A. no carbocation can form
B. SN1 solvolysis is favored
C. methyl transfer occurs
D. SN2 is favored
Answer: B
Explanation: Ionization of a tertiary halide gives a stable tertiary carbocation. Water is a weak nucleophile in a polar protic medium,
conditions suited to SN1. Steric hindrance prevents efficient SN2 attack.
Substitution and elimination
12. Which statement best describes 1-chlorobutane with iodide ion in acetone?
A. SN1 is favored
B. only rearrangement occurs
C. SN2 substitution is favored
D. E1 is favored
Answer: C
Explanation: Iodide is a good nucleophile and acetone is polar aprotic. A primary chloride is accessible to backside attack, making SN2
favorable. Formation of a primary carbocation would be highly unfavorable.
Substitution and elimination
13. Which statement best describes 2-chloropropane with water and heat?
A. no reaction is possible
B. SN2 must be exclusive
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Organic Chemistry - Janice Gorzynski Smith |
Reactions & Mechanisms Practice Exam
200 original mechanism, reagent, and product-prediction questions with
explanations
QUESTIONS FORMAT COVERAGE
200 A-D + Explanations Reactions
Original practice questions for independent study and exam preparation.
,Reactions & Mechanisms Practice Exam
Practice Questions
Choose the best answer for each item. The answer and a detailed explanation follow each question.
Substitution and elimination
1. Which statement best describes 1-bromobutane with NaCN in DMSO?
A. E1 elimination is favored
B. no reaction occurs
C. SN1 substitution is favored
D. SN2 substitution is favored
Answer: D
Explanation: A primary alkyl bromide is accessible to backside attack and cyanide is a strong nucleophile. DMSO is polar aprotic and
enhances nucleophilicity, so SN2 is strongly favored. A primary carbocation would be too unstable for SN1 or E1 to dominate.
Substitution and elimination
2. Which statement best describes ethyl iodide with sodium methoxide in acetone?
A. SN1 substitution is favored
B. E1 elimination is favored
C. only radical substitution occurs
D. SN2 substitution is favored
Answer: D
Explanation: A primary iodide reacts readily with a strong unhindered nucleophile by backside displacement. A polar aprotic solvent
such as acetone supports SN2 reactivity. Primary carbocation pathways are unfavorable.
Substitution and elimination
3. Which statement best describes methyl iodide with sodium ethoxide?
A. SN1 ionization dominates
B. E1 elimination dominates
C. E2 elimination dominates
D. SN2 substitution is the only normal substitution-elimination pathway
Answer: D
Explanation: Methyl halides have no beta carbon, so beta elimination cannot occur. Methyl carbocations are also extremely unstable,
ruling out SN1 and E1. A strong nucleophile therefore reacts through SN2.
Substitution and elimination
4. Which statement best describes 2-bromobutane with sodium ethoxide in ethanol?
A. SN1 must be exclusive
B. SN2 must be exclusive
C. no reaction can occur
D. a mixture is possible, with E2 often important under strong-base conditions
Answer: D
Explanation: A secondary halide with a strong base/nucleophile can undergo both SN2 and E2 pathways. Protic solvent and increased
substitution make SN2 less favorable than for a primary halide, while base strength supports E2. The exact product ratio depends on
temperature and structure.
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,Reactions & Mechanisms Practice Exam
Substitution and elimination
5. Which statement best describes 2-bromobutane with potassium tert-butoxide and heat?
A. SN1 substitution is strongly favored
B. electrophilic addition occurs
C. E2 elimination is strongly favored
D. SN2 is exclusive
Answer: C
Explanation: tert-Butoxide is a strong bulky base and heat favors elimination. Steric bulk makes backside substitution on a secondary
carbon less favorable. E2 therefore becomes the dominant pathway.
Substitution and elimination
6. Which statement best describes tert-butyl bromide in methanol at room temperature?
A. SN2 is dominant
B. SN1/E1 solvolysis is feasible, with substitution often prominent at lower temperature
C. only E2 can occur
D. the C-Br bond cannot ionize
Answer: B
Explanation: A tertiary halide forms a relatively stable tertiary carbocation in a polar protic solvent. Methanol is a weak nucleophile and
weak base, conditions that favor unimolecular pathways. SN2 is blocked by steric hindrance at the tertiary center.
Substitution and elimination
7. Which statement best describes tert-butyl bromide with sodium ethoxide and heat?
A. SN2 substitution dominates
B. no beta hydrogens exist
C. E2 elimination is favored
D. SN1 is the only possible mechanism
Answer: C
Explanation: A tertiary substrate is too hindered for SN2 attack. A strong base such as ethoxide can remove a beta hydrogen in a
concerted E2 step. Heat further favors elimination over substitution.
Substitution and elimination
8. Which statement best describes 1-bromobutane with potassium tert-butoxide?
A. E2 becomes competitive and can be favored because the base is bulky
B. only SN2 is possible regardless of base
C. SN1 dominates through a primary carbocation
D. E1 dominates
Answer: A
Explanation: A primary substrate normally favors SN2, but tert-butoxide is a bulky strong base. Steric hindrance reduces nucleophilic
attack and promotes beta-hydrogen abstraction. This can shift the reaction toward E2 and often toward the less substituted alkene.
Substitution and elimination
9. Which statement best describes benzyl bromide with cyanide ion?
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, Reactions & Mechanisms Practice Exam
A. SN2 substitution is very fast
B. only E1 can occur
C. SN2 is impossible at a benzylic carbon
D. only radical bromination can occur
Answer: A
Explanation: The benzylic carbon is accessible to backside attack and the transition state is stabilized by the adjacent aromatic system.
Cyanide is a strong nucleophile. Benzylic primary halides are therefore exceptionally reactive in SN2 substitution.
Substitution and elimination
10. Which statement best describes allyl bromide with acetate ion?
A. SN2 substitution is favored at the allylic carbon
B. the alkene is hydrogenated
C. only E2 occurs
D. SN2 is impossible because of the nearby double bond
Answer: A
Explanation: Allylic halides are highly reactive in substitution because the transition state benefits from allylic stabilization. Acetate can
attack the carbon bearing bromine by SN2 under suitable conditions. The alkene remains present rather than being hydrogenated.
Substitution and elimination
11. Which statement best describes 2-bromo-2-methylpropane with water?
A. no carbocation can form
B. SN1 solvolysis is favored
C. methyl transfer occurs
D. SN2 is favored
Answer: B
Explanation: Ionization of a tertiary halide gives a stable tertiary carbocation. Water is a weak nucleophile in a polar protic medium,
conditions suited to SN1. Steric hindrance prevents efficient SN2 attack.
Substitution and elimination
12. Which statement best describes 1-chlorobutane with iodide ion in acetone?
A. SN1 is favored
B. only rearrangement occurs
C. SN2 substitution is favored
D. E1 is favored
Answer: C
Explanation: Iodide is a good nucleophile and acetone is polar aprotic. A primary chloride is accessible to backside attack, making SN2
favorable. Formation of a primary carbocation would be highly unfavorable.
Substitution and elimination
13. Which statement best describes 2-chloropropane with water and heat?
A. no reaction is possible
B. SN2 must be exclusive
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