MCAT® ExamChemical and Physical Foundations of Biological SystemsEasy
A chemist is attempting to synthesize a chiral molecule. Which of the following conditions is absolutely necessary for a molecule to be considered chiral?
- AIt must contain at least one double bond.
- BIt must be non-superimposable on its mirror image.
- CIt must be superimposable on its mirror image.
- DIt must have a plane of symmetry.
Show answer & explanationAnswer & explanation
Correct answer: B. It must be non-superimposable on its mirror image.
Chirality is defined by a molecule's non-superimposability on its mirror image. This means that despite having a mirror image, the two forms cannot be perfectly overlaid, similar to a left and right hand.
Why the other options are wrong
- A. Double bonds are not a requirement for chirality; many chiral molecules (e.g., amino acids) do not have double bonds.
- C. Molecules that are superimposable on their mirror image are achiral, not chiral.
- D. Chiral molecules *lack* a plane of symmetry. Molecules with a plane of symmetry are achiral.
Chirality
The property of a molecule that is non-superimposable on its mirror image, often due to the presence of a chiral center (e.g., a carbon atom bonded to four different groups).
- Molecules are 'handed' like human hands.
- Often (but not always) involves a chiral carbon (sp3 hybridized carbon with four different substituents).
- Lack of a plane of symmetry is a key indicator.
Memory trick: Chiral Can't Copy, it's not Superimposable.