MCAT® ExamChemical and Physical Foundations of Biological SystemsHard
A student is asked to determine the number of possible stereoisomers for a compound with the formula C₄H₈Br₂ that contains two chiral centers. What is the maximum number of stereoisomers possible for such a compound?
- A8
- B6
- C4
- D2
Show answer & explanationAnswer & explanation
Correct answer: C. 4
For a molecule with 'n' chiral centers, the maximum number of possible stereoisomers is 2ⁿ. In this case, the compound has two chiral centers, so the maximum number of stereoisomers is 2² = 4. These would typically be a pair of enantiomers and a pair of diastereomers, or potentially a meso compound reducing the total if there is an internal plane of symmetry for one of the forms.
Why the other options are wrong
- A. This would be for a molecule with three chiral centers (2³ = 8).
- B. Incorrect. This number is not typically derived directly from the number of chiral centers using the 2ⁿ rule.
- D. This would be for a molecule with one chiral center, or if a meso compound existed reducing the total from 4 to 3 (e.g., in a specific case).
Maximum Stereoisomers (2ⁿ Rule)
For a molecule with 'n' chiral centers (stereocenters), the maximum possible number of stereoisomers is 2ⁿ.
- A chiral center is typically a carbon atom bonded to four different groups.
- This rule gives the *maximum* number; meso compounds can reduce the actual count.
- Stereoisomers include enantiomers and diastereomers.
Memory trick: Chiral Centers Count, Two to the Power.