DAT Organic Chemistry: Stereochemistry 2 — Questions and Answers
Question 1: How many stereoisomers are possible for a molecule with 3 stereocenters and no internal symmetry?
- 6
- 8 (Correct answer)
- 9
- 4
Correct answer: 8
The maximum number of stereoisomers is 2^n, so 2^3 = 8.
Question 2: Two compounds are enantiomers. Which property differs between them?
- Boiling point
- Direction of plane-polarized light rotation (Correct answer)
- Density
- Melting point
Correct answer: Direction of plane-polarized light rotation
Enantiomers rotate plane-polarized light in equal but opposite directions while sharing all other physical properties.
Question 3: A meso compound is best described as:
- Optically active with no stereocenters
- Achiral despite having stereocenters (Correct answer)
- A pair of enantiomers
- A constitutional isomer
Correct answer: Achiral despite having stereocenters
A meso compound contains stereocenters but has an internal plane of symmetry, making it achiral.
Question 4: What is the relationship between (2R,3R) and (2S,3R) of the same molecule?
- Enantiomers
- Diastereomers (Correct answer)
- Identical
- Constitutional isomers
Correct answer: Diastereomers
They differ at one stereocenter but not all, so they are diastereomers.
Question 5: In assigning R/S, which atom gets highest priority?
- Highest atomic number (Correct answer)
- Smallest atomic radius
- Highest electronegativity
- Lowest mass
Correct answer: Highest atomic number
CIP rules rank substituents by atomic number, highest first.
Question 6: A racemic mixture has what observed optical rotation?
- Strongly positive
- Strongly negative
- Zero (Correct answer)
- Equal to one enantiomer
Correct answer: Zero
Equal amounts of two enantiomers cancel each other's rotation, giving zero.
Question 7: Geometric (cis/trans) isomerism requires:
- A tetrahedral carbon
- Restricted rotation such as a double bond or ring (Correct answer)
- An sp carbon
- A lone pair
Correct answer: Restricted rotation such as a double bond or ring
Cis/trans isomerism arises when rotation is restricted, as in alkenes or rings.
DAT Organic Chemistry: Stereochemistry 2
How many stereoisomers are possible for a molecule with 3 stereocenters and no internal symmetry?