DAT Organic Chemistry: Stereochemistry 3 — Questions and Answers
Question 1: When the lowest priority group points toward the viewer, the apparent rotation direction must be:
- Kept as observed
- Reversed (Correct answer)
- Doubled
- Ignored entirely
Correct answer: Reversed
If the lowest priority group faces you, you reverse the assigned rotation to get the correct R/S.
Question 2: Which of the following is achiral?
- 2-Bromobutane
- CHFClBr
- Glycine (aminoacetic acid) (Correct answer)
- 2-Chlorobutane
Correct answer: Glycine (aminoacetic acid)
Glycine's alpha carbon bears two hydrogens, so it has no stereocenter and is achiral.
Question 3: Diastereomers differ from enantiomers because diastereomers:
- Are always achiral
- Have different physical properties (Correct answer)
- Are mirror images
- Have identical melting points
Correct answer: Have different physical properties
Diastereomers are not mirror images and therefore have different physical properties.
Question 4: A compound with specific rotation [α] = 0 but containing stereocenters is most likely:
- A single enantiomer
- A meso compound or racemate (Correct answer)
- A constitutional isomer
- An sp2 alkene
Correct answer: A meso compound or racemate
Zero rotation with stereocenters points to either a meso compound or a racemic mixture.
Question 5: Converting a Fischer projection: horizontal lines represent bonds pointing:
- Away from viewer
- Toward the viewer (Correct answer)
- Straight up
- Into the page diagonally
Correct answer: Toward the viewer
In a Fischer projection, horizontal bonds point toward the viewer and vertical bonds point away.
Question 6: Epimers are diastereomers that differ at:
- Every stereocenter
- Exactly one stereocenter (Correct answer)
- No stereocenters
- The carbonyl carbon only
Correct answer: Exactly one stereocenter
Epimers are diastereomers that differ in configuration at only a single stereocenter.
Question 7: Which describes an optically inactive compound that can be separated into active forms?
- Meso compound
- Racemic mixture (Correct answer)
- Single enantiomer
- Achiral molecule
Correct answer: Racemic mixture
A racemic mixture is optically inactive overall but can be resolved into its active enantiomers.
DAT Organic Chemistry: Stereochemistry 3
When the lowest priority group points toward the viewer, the apparent rotation direction must be: