OAT Organic Chemistry Reactions Questions and Answers — Questions and Answers
Question 1: Which of the following reagents is most suitable for oxidizing a primary alcohol, such as 1-butanol, directly to a carboxylic acid, butanoic acid, in a single step?
- Pyridinium chlorochromate (PCC)
- Potassium permanganate (KMnO₄) (Correct answer)
- Lithium aluminum hydride (LiAlH₄)
- Dess-Martin periodinane (DMP)
Correct answer: Potassium permanganate (KMnO₄)
Strong oxidizing agents are required to convert a primary alcohol directly to a carboxylic acid. Potassium permanganate (KMnO₄) is a powerful oxidizing agent capable of this transformation. PCC and DMP are considered 'weak' oxidants that will oxidize a primary alcohol only to the aldehyde stage. LiAlH₄ is a reducing agent, not an oxidizing agent.
Question 2: In an electrophilic aromatic substitution reaction, which of the following substituents on a benzene ring would direct an incoming electrophile primarily to the meta position?
- –OH (Hydroxyl)
- –CH₃ (Methyl)
- –NH₂ (Amino)
- –NO₂ (Nitro) (Correct answer)
Correct answer: –NO₂ (Nitro)
The nitro group (–NO₂) is a strong electron-withdrawing group, which deactivates the benzene ring towards electrophilic attack. Deactivating groups direct incoming electrophiles to the meta position. The hydroxyl (–OH), methyl (–CH₃), and amino (–NH₂) groups are all electron-donating, activating groups that direct incoming electrophiles to the ortho and para positions.
Question 3: A student treats acetone (propanone) with methylmagnesium bromide (CH₃MgBr), followed by an acidic workup with H₃O⁺. What is the major organic product of this reaction sequence?
- 2-Propanol
- 2-Butanone
- tert-Butanol (2-methyl-2-propanol) (Correct answer)
- 1-Propanol
Correct answer: tert-Butanol (2-methyl-2-propanol)
This is a Grignard reaction. The nucleophilic methyl group from the Grignard reagent attacks the electrophilic carbonyl carbon of acetone. This forms a magnesium alkoxide intermediate. The subsequent acidic workup protonates the alkoxide to yield the final product. Since acetone is a ketone, the product will be a tertiary alcohol. The addition of a methyl group to acetone's three carbons results in tert-butanol.
Question 4: The reaction of (2E,4E)-hexa-2,4-diene with maleic anhydride is a classic example of which type of reaction?
- SN2 Nucleophilic Substitution
- E2 Elimination
- Aldol Condensation
- Diels-Alder Cycloaddition (Correct answer)
Correct answer: Diels-Alder Cycloaddition
The Diels-Alder reaction is a [4+2] cycloaddition that occurs between a conjugated diene (the 4π electron component) and a dienophile (the 2π electron component) to form a six-membered ring. (2E,4E)-hexa-2,4-diene is a conjugated diene, and maleic anhydride is a classic dienophile, making this a Diels-Alder reaction.
Question 5: When 2-bromopropane is treated with sodium ethoxide (NaOEt) in ethanol, two products are formed via competing reaction pathways. Which statement best describes the expected outcome?
- The SN1 product, 2-ethoxypropane, will be the exclusive product.
- The E1 product, propene, will be the major product due to the weak base.
- The SN2 product, 2-ethoxypropane, and the E2 product, propene, will both be formed. (Correct answer)
- The reaction will not proceed as 2-bromopropane is an unreactive substrate.
Correct answer: The SN2 product, 2-ethoxypropane, and the E2 product, propene, will both be formed.
2-bromopropane is a secondary alkyl halide. Sodium ethoxide is a strong nucleophile and also a strong, non-bulky base. With a secondary substrate and a strong nucleophile/base, both SN2 and E2 reactions will occur competitively. The SN2 pathway yields 2-ethoxypropane, and the E2 pathway yields propene. Neither pathway is typically exclusive under these conditions.
Question 6: Which of the following molecules could undergo a self-aldol condensation reaction in the presence of a dilute base?
- Formaldehyde (methanal)
- Benzaldehyde
- Acetaldehyde (ethanal) (Correct answer)
- 2,2-Dimethylpropanal
Correct answer: Acetaldehyde (ethanal)
The aldol condensation requires an aldehyde or ketone to have at least one alpha-hydrogen (a hydrogen on the carbon adjacent to the carbonyl group). This hydrogen is acidic and can be removed by a base to form an enolate nucleophile. Acetaldehyde has three alpha-hydrogens. Formaldehyde, benzaldehyde, and 2,2-dimethylpropanal all lack alpha-hydrogens and therefore cannot form an enolate to initiate the self-condensation reaction.
OAT Organic Chemistry Reactions Questions and Answers
Which of the following reagents is most suitable for oxidizing a primary alcohol, such as 1-butanol, directly to a carboxylic acid, butanoic acid, in a single step?