OAT Organic Chemistry 3 — Questions and Answers
Question 1: What type of isomers are (2R,3S)-2,3-dibromobutane and (2S,3R)-2,3-dibromobutane?
- Enantiomers (Correct answer)
- Diastereomers
- Constitutional isomers
- Conformational isomers
Correct answer: Enantiomers
They are non-superimposable mirror images with opposite configurations at every chiral center, making them enantiomers.
Question 2: Which reaction mechanism describes the addition of HBr to propene in the absence of peroxides?
- Free-radical addition (anti-Markovnikov)
- Electrophilic addition (Markovnikov) (Correct answer)
- Nucleophilic addition
- Electrophilic substitution
Correct answer: Electrophilic addition (Markovnikov)
Without peroxides, HBr adds via electrophilic addition following Markovnikov's rule, placing Br on the more substituted carbon.
Question 3: An amide bond is best described as having which character due to resonance?
- Pure single bond character
- Pure double bond character
- Partial double bond character (restricted rotation) (Correct answer)
- Triple bond character
Correct answer: Partial double bond character (restricted rotation)
Lone pair donation from nitrogen into the carbonyl gives the C–N bond partial double bond character, restricting rotation.
Question 4: What is the major product of the dehydration of 2-methyl-2-butanol with H2SO4?
- 2-methyl-1-butene
- 2-methyl-2-butene (Correct answer)
- 3-methyl-1-butene
- 1-pentene
Correct answer: 2-methyl-2-butene
Zaitsev's rule predicts preferential formation of 2-methyl-2-butene, the more substituted (more stable) alkene.
Question 5: Which spectroscopic technique is most useful for identifying the connectivity of carbon atoms in an organic molecule?
- IR spectroscopy
- Mass spectrometry
- 13C NMR spectroscopy (Correct answer)
- UV-Vis spectroscopy
Correct answer: 13C NMR spectroscopy
13C NMR reveals the chemical environment of each distinct carbon, mapping carbon skeleton connectivity.
Question 6: Which of the following undergoes SN1 reaction most readily?
- Methyl bromide
- Ethyl bromide
- Isopropyl bromide
- tert-Butyl bromide (Correct answer)
Correct answer: tert-Butyl bromide
tert-Butyl bromide forms the most stable tertiary carbocation intermediate, making SN1 highly favorable.
Question 7: The Diels-Alder reaction requires the diene to adopt which conformation?
- s-trans
- s-cis (Correct answer)
- Boat
- Chair
Correct answer: s-cis
The diene must be in the s-cis conformation so its terminal carbons can overlap simultaneously with the dienophile.
OAT Organic Chemistry 3
What type of isomers are (2R,3S)-2,3-dibromobutane and (2S,3R)-2,3-dibromobutane?