OAT Organic Chemistry Reactions 5 — Questions and Answers
Question 1: Which reaction is used to convert a primary amide to a primary amine with one fewer carbon (chain shortening)?
- Hofmann rearrangement (Correct answer)
- Curtius rearrangement
- Schmidt reaction
- Lossen rearrangement
Correct answer: Hofmann rearrangement
The Hofmann rearrangement treats a primary amide with Br2/NaOH, causing migration of the R group to nitrogen and loss of CO2 to give a primary amine.
Question 2: What is the stereochemical result of bromine addition to (E)-2-butene?
- Meso-2,3-dibromobutane (Correct answer)
- Racemic mixture of (2R,3R) and (2S,3S)-2,3-dibromobutane
- Only (2R,3S)-2,3-dibromobutane
- Only (2S,3R)-2,3-dibromobutane
Correct answer: Meso-2,3-dibromobutane
Anti addition of Br2 to (E)-2-butene through a bromonium ion gives the meso-2,3-dibromobutane because the two stereocenters are opposite but the molecule has an internal mirror plane.
Question 3: Which nucleophile is used in the Wittig reaction to convert a ketone into an alkene?
- Phosphorus ylide (R3P=CH2) (Correct answer)
- Grignard reagent
- Enolate ion
- Cyanide ion
Correct answer: Phosphorus ylide (R3P=CH2)
A phosphorus ylide reacts with the carbonyl to form a four-membered oxaphosphetane that collapses to give an alkene and triphenylphosphine oxide.
Question 4: In the Fischer esterification, what is the role of the acid catalyst and excess alcohol?
- The acid protonates the carbonyl oxygen to activate it toward nucleophilic attack, and excess alcohol drives equilibrium toward ester (Correct answer)
- The acid deprotonates the alcohol to form an alkoxide nucleophile
- The acid oxidizes the carboxylic acid to an acyl radical
- Excess alcohol is oxidized to an aldehyde
Correct answer: The acid protonates the carbonyl oxygen to activate it toward nucleophilic attack, and excess alcohol drives equilibrium toward ester
The acid catalyst activates the carbonyl toward nucleophilic addition, and excess alcohol shifts the equilibrium toward product by Le Chatelier's principle.
Question 5: Which reaction converts a terminal alkyne to an aldehyde via anti-Markovnikov hydration?
- Hydroboration-oxidation (9-BBN, then H2O2/NaOH) (Correct answer)
- H2SO4/HgSO4/H2O
- Ozonolysis
- LiAlH4 reduction
Correct answer: Hydroboration-oxidation (9-BBN, then H2O2/NaOH)
Hydroboration of a terminal alkyne with a bulky borane followed by oxidation gives an aldehyde via anti-Markovnikov addition.
Question 6: Which carbonyl compound is most reactive toward nucleophilic addition?
- Acyl chloride (Correct answer)
- Ester
- Amide
- Carboxylic acid
Correct answer: Acyl chloride
Acyl chlorides are the most reactive toward nucleophilic acyl substitution because Cl is a poor electron donor and an excellent leaving group.
Question 7: What is the product of a Favorskii rearrangement of an α-haloketone treated with base?
- A ring-contracted carboxylic acid or ester with the same carbon skeleton (Correct answer)
- An α,β-unsaturated ketone
- A secondary alcohol
- An epoxide
Correct answer: A ring-contracted carboxylic acid or ester with the same carbon skeleton
The Favorskii rearrangement of an α-haloketone with base forms a cyclopropanone intermediate that opens to give a ring-contracted ester or carboxylic acid.
OAT Organic Chemistry Reactions 5
Which reaction is used to convert a primary amide to a primary amine with one fewer carbon (chain shortening)?