OAT Organic Chemistry Reactions 4 — Questions and Answers
Question 1: Which reagent selectively reduces an ester to a primary alcohol without reducing isolated alkene groups?
- LiAlH4 (Correct answer)
- NaBH4
- DIBAL-H at −78°C
- H2/Pd-C
Correct answer: LiAlH4
LiAlH4 is a strong enough hydride donor to reduce esters fully to primary alcohols, whereas NaBH4 cannot reduce esters.
Question 2: What product results from treating a ketone with hydroxylamine (NH2OH)?
- Oxime (Correct answer)
- Hydrazone
- Imine (Schiff base)
- Semicarbazone
Correct answer: Oxime
Hydroxylamine condenses with the carbonyl group of a ketone to form an oxime (C=N-OH) and water.
Question 3: In electrophilic aromatic substitution, nitrobenzene undergoes nitration to give primarily which isomer?
- Ortho-dinitrobenzene
- Para-dinitrobenzene
- Meta-dinitrobenzene (Correct answer)
- Equal mixture of ortho and para
Correct answer: Meta-dinitrobenzene
The nitro group is a strong meta-director because it destabilizes carbocation intermediates at ortho and para positions.
Question 4: Which conditions convert a carboxylic acid to an acyl chloride?
- SOCl2 or PCl5 (Correct answer)
- HCl gas
- Cl2/hv
- NaCl/H2SO4
Correct answer: SOCl2 or PCl5
Thionyl chloride (SOCl2) or phosphorus pentachloride (PCl5) replaces the OH of a carboxylic acid with Cl to give the acyl chloride.
Question 5: What type of reaction is the Beckmann rearrangement?
- Conversion of a ketoxime to an amide via acid-catalyzed rearrangement (Correct answer)
- Reduction of a nitrile to an amine
- Hofmann rearrangement of a primary amide
- Oxidation of an amine to a nitroso compound
Correct answer: Conversion of a ketoxime to an amide via acid-catalyzed rearrangement
The Beckmann rearrangement converts a ketoxime to an amide under acidic conditions via a 1,2-shift of the group anti to the hydroxyl.
Question 6: Which mechanism operates during the Friedel-Crafts alkylation of benzene?
- Electrophilic aromatic substitution via a carbocation electrophile (Correct answer)
- Nucleophilic aromatic substitution
- Free radical chain mechanism
- Concerted cycloaddition
Correct answer: Electrophilic aromatic substitution via a carbocation electrophile
A Lewis acid (AlCl3) generates a carbocation electrophile that attacks the π system of benzene in an electrophilic aromatic substitution.
Question 7: What is the major product when 1-butyne reacts with 2 equivalents of HBr?
- 1,1-dibromobutane (Correct answer)
- 1,2-dibromobutane
- 2,2-dibromobutane
- 3,4-dibromobutane
Correct answer: 1,1-dibromobutane
Two Markovnikov additions of HBr to the terminal alkyne place both bromines on C-1, giving 1,1-dibromobutane (gem-dibromide).
OAT Organic Chemistry Reactions 4
Which reagent selectively reduces an ester to a primary alcohol without reducing isolated alkene groups?