DAT - Dental Admission Organic Chemistry: Stereochemistry Questions and Answers 1 — Questions and Answers
Question 1: A compound has a specific rotation of +32°. What would be the specific rotation of its enantiomer?
- 0°
- -32° (Correct answer)
- +32°
- It cannot be determined from the information given.
Correct answer: -32°
Enantiomers are non-superimposable mirror images that rotate plane-polarized light to an equal magnitude but in opposite directions. If one enantiomer is dextrorotatory (+), its mirror image will be levorotatory (-) by the same amount.
Question 2: Which of the following terms best describes the relationship between (2R,3S)-2,3-dibromobutane and (2S,4R)-2,4-dichloropentane?
- Enantiomers
- Diastereomers
- Identical compounds
- Constitutional isomers (Correct answer)
Correct answer: Constitutional isomers
Constitutional isomers have the same molecular formula but different connectivity. The two compounds have different carbon skeletons (butane vs. pentane) and different substituents (bromo vs. chloro), meaning their connectivity is different, though they are not isomers at all as their molecular formulas are different. However, among the choices, constitutional isomers is the only one describing different connectivity, which is the fundamental difference here. Enantiomers and diastereomers are types of stereoisomers, which must have the same connectivity.
Question 3: What is the maximum number of stereoisomers possible for a molecule with the structure of 3,4-dimethylhexane?
- 2
- 3
- 4 (Correct answer)
- 8
Correct answer: 4
The maximum number of stereoisomers is calculated by the formula 2^n, where 'n' is the number of chiral centers. 3,4-dimethylhexane has two chiral centers (at C3 and C4). Therefore, the maximum number of stereoisomers is 2^2 = 4. These stereoisomers exist as two pairs of enantiomers.
Question 4: Which of the following molecules is a meso compound?
- (2R,3R)-Tartaric acid
- (2R,3S)-Tartaric acid (Correct answer)
- trans-1,2-Dimethylcyclopropane
- (S)-2-Butanol
Correct answer: (2R,3S)-Tartaric acid
A meso compound is an achiral molecule that contains chiral centers and an internal plane of symmetry. (2R,3S)-Tartaric acid has two chiral centers but possesses an internal plane of symmetry, making it achiral and thus a meso compound. The optical rotations from its two chiral centers cancel each other out.
Question 5: A 50:50 mixture of two enantiomers is known as what?
- A diastereomeric mixture
- A constitutional isomer
- An optically active solution
- A racemic mixture (Correct answer)
Correct answer: A racemic mixture
A racemic mixture, or racemate, is a 50:50 mixture of two enantiomers. Because the two enantiomers rotate plane-polarized light equally but in opposite directions, their effects cancel out, and the racemic mixture is optically inactive.
Question 6: What is the absolute configuration (R/S) of the chiral center in (S)-2-chlorobutane?
- R
- S (Correct answer)
- E
- Z
Correct answer: S
The question explicitly states the configuration in the IUPAC name. (S)-2-chlorobutane indicates that the chiral center at carbon 2 has the S configuration according to the Cahn-Ingold-Prelog priority rules. This is a direct knowledge question based on interpreting nomenclature.
DAT - Dental Admission Organic Chemistry: Stereochemistry Questions and Answers 1
A compound has a specific rotation of +32°.
What would be the specific rotation of its enantiomer?