Organic Chemistry MCAT Practice Exam

What characterizes a chiral center in a molecule?

1. sp3 carbon with four identical substituents

2. sp3 carbon with three identical substituents

3. sp3 carbon with four different substituents

4. sp2 carbon with two different substituents

The correct answer is: sp3 carbon with four different substituents

A chiral center in a molecule is characterized by a carbon atom that is bonded to four different substituents. This unique arrangement prevents the molecule from being superimposable on its mirror image, which is the primary requirement for chirality. The presence of four different substituents creates two non-superimposable mirror images known as enantiomers. When considering the other options, having a carbon with four identical substituents (the first option) or three identical substituents (the second option) both lead to a lack of chirality as these configurations result in a symmetrical molecule, making it superimposable on its mirror image. A carbon with two different substituents and sp2 hybridization (the fourth option) does not meet the criteria for a chiral center either because chirality requires the presence of four distinct groups. Thus, the defining feature of a chiral center is indeed an sp3 carbon with four different substituents, making the correct choice clear.