Organic Chemistry MCAT Practice Exam

Which type of molecules are stereoisomers?

1. Same molecular formula and connectivity with different properties

2. Same molecular formula but different atomic weights

3. Different molecular formula and connectivity

4. Identical molecules in all respects

The correct answer is: Same molecular formula and connectivity with different properties

Stereoisomers are characterized by having the same molecular formula and connectivity of their atoms, but they differ in the spatial arrangement of those atoms. This distinction in spatial arrangement leads to different properties for these isomers. For example, consider the case of cis and trans isomers of alkenes; they possess the same molecular formula (like C4H8), the same connectivity (each carbon is connected in the same way), but the orientation of the substituents around the double bond varies. This can significantly impact their physical and chemical properties, such as boiling points and reactivity. Other choices do not correctly describe stereoisomers. Having different atomic weights suggests differences in isotopes or molecular formulas, which do not pertain to stereoisomers. Similarly, different molecular formulas and connectivity (as mentioned in another option) would suggest entirely different compounds rather than stereoisomerism. Lastly, identical molecules in all respects do not qualify as stereoisomers, as they would be considered the same compound without any differences in arrangement or properties. Thus, the definition provided in the first choice accurately captures the nature of stereoisomers in organic chemistry.