OAT Organic Chemistry Reactions 3 — Questions and Answers
Question 1: Which reaction converts an aldehyde to a cyanohydrin?
- Addition of HCN (Correct answer)
- Addition of H2O2
- Wolff-Kishner reduction
- Wittig reaction
Correct answer: Addition of HCN
HCN adds to the carbonyl of an aldehyde via nucleophilic addition to produce a cyanohydrin (α-hydroxy nitrile).
Question 2: Which of the following is the best description of Markovnikov's rule for HBr addition to propene?
- Br adds to the less substituted carbon
- Br adds to the more substituted carbon (Correct answer)
- H adds to the more substituted carbon
- Addition is anti-Markovnikov when peroxides are absent
Correct answer: Br adds to the more substituted carbon
Markovnikov's rule states the halide (nucleophile) adds to the more substituted carbon, forming the more stable carbocation intermediate.
Question 3: What reagent is used in ozonolysis to work up the ozonide and obtain aldehydes rather than carboxylic acids?
- H2O2
- Zn/H2O (reductive workup) (Correct answer)
- KMnO4
- NaOH
Correct answer: Zn/H2O (reductive workup)
Reductive workup with Zn and H2O (or Me2S) cleaves the ozonide to give aldehydes, preventing further oxidation.
Question 4: In the aldol condensation, what type of intermediate is formed in the first step under basic conditions?
- Carbocation
- Enolate ion (Correct answer)
- Carbanion via radical
- Acylium ion
Correct answer: Enolate ion
Base removes an α-hydrogen to form an enolate, which then acts as the nucleophile attacking the carbonyl of another molecule.
Question 5: Which reaction mechanism is involved in the saponification of an ester?
- Nucleophilic acyl substitution with OH⁻ (Correct answer)
- Electrophilic addition
- Free radical substitution
- SN2 at the alkyl carbon
Correct answer: Nucleophilic acyl substitution with OH⁻
Saponification proceeds via nucleophilic acyl substitution where hydroxide attacks the carbonyl carbon, forming a tetrahedral intermediate that expels the alkoxide.
Question 6: What is the product of treating an alkyl halide with alcoholic KOH at high temperature?
- Alcohol
- Alkene via elimination (Correct answer)
- Grignard reagent
- Ether via SN2
Correct answer: Alkene via elimination
Alcoholic KOH at high temperature is a classic condition for E2 elimination, producing an alkene.
Question 7: Which of the following best describes the Claisen rearrangement?
- Nucleophilic addition to a ketone
- Concerted [3,3]-sigmatropic rearrangement of an allyl vinyl ether (Correct answer)
- Electrophilic aromatic substitution
- Radical chain reaction
Correct answer: Concerted [3,3]-sigmatropic rearrangement of an allyl vinyl ether
The Claisen rearrangement is a thermal [3,3]-sigmatropic reaction of allyl vinyl ethers that forms γ,δ-unsaturated carbonyl compounds.
OAT Organic Chemistry Reactions 3
Which reaction converts an aldehyde to a cyanohydrin?