Two compounds have the molecular formula C₄H₈Cl₂. Compound A has both chlorines on adjacent carbons with a specific spatial arrangement; Compound B has the same connectivity but the two substituents arranged differently in space. What is the relationship between A and B?
AConstitutional isomers — they have the same formula but different connectivity
BIdentical compounds — same formula and same connectivity means the same compound
CStereoisomers — same connectivity, different spatial arrangement
DResonance structures — they interconvert without breaking bonds
Stereoisomers share the same molecular formula AND the same connectivity (the same sequence of atom-to-atom bonds), but differ in how those atoms are arranged in three-dimensional space. Constitutional (structural) isomers, by contrast, differ in which atoms are bonded to which — different connectivity. Since A and B have the same connectivity but different spatial arrangement, they are stereoisomers. Resonance structures are not real molecules but representations of electron delocalization. This distinction is foundational: stereochemistry begins precisely where connectivity stops telling the whole story.
Question 2 Multiple Choice
The drug thalidomide caused birth defects in one of its enantiomers while the other treated morning sickness. Why can two enantiomers produce such different biological effects despite having identical molecular formulas and connectivity?
AOne enantiomer is more soluble in blood, so it reaches target tissues at higher concentrations
BBiological receptors and enzymes are themselves chiral molecules, so they interact differently with the two mirror-image forms — like a right glove fitting a right hand but not a left
CThe enantiomers have different melting points, so one is absorbed faster through the digestive tract
DOne enantiomer spontaneously converts to the other inside the body, doubling the effective dose
Biological macromolecules (enzymes, receptors, transport proteins) are themselves chiral — they are built from L-amino acids and exist in one specific three-dimensional form. A chiral molecule interacts with these structures like a key in a lock: the two enantiomers are mirror-image keys that fit differently into the same lock. One may bind to the target receptor and trigger the desired response; the other may bind to a completely different receptor with harmful effects, or not bind at all. This is why enantiomers can have wildly different pharmacological profiles even though they have the same atoms connected in the same order. Option C is false — enantiomers have identical scalar physical properties including melting point.
Question 3 True / False
Any two molecules that are non-superimposable mirror images of each other are enantiomers.
TTrue
FFalse
Answer: False
False — this is one of the most common misconceptions in stereochemistry. For two molecules to be enantiomers, they must be (1) mirror images AND (2) non-superimposable. But they must also be the same compound — you cannot call two entirely different molecules with different connectivity 'enantiomers' just because they happen to be mirror images of each other. More subtly, some molecules are their own mirror images — they are superimposable on their mirror image — and are called 'achiral.' The mirror image of an achiral molecule is identical to the original, not a separate enantiomer. So: non-superimposability is necessary but you must also be dealing with a pair of the same compound.
Question 4 True / False
Enantiomers have identical melting points, boiling points, and solubilities in achiral solvents, but can interact differently with polarized light and with chiral biological molecules.
TTrue
FFalse
Answer: True
True. Enantiomers are related by a mirror reflection and therefore have exactly the same scalar physical properties: melting point, boiling point, density, refractive index, solubility in achiral solvents. The only physical property that distinguishes them is their interaction with plane-polarized light — one rotates it clockwise (dextrorotatory, '+') and the other counterclockwise (levorotatory, '−') by equal magnitudes. In biology, where molecules are inherently chiral, the two enantiomers interact differently with enzymes, receptors, and other chiral environments. Diastereomers, by contrast, differ in all their physical properties because their internal spatial geometry is not simply a mirror reflection.
Question 5 Short Answer
Explain why a biological receptor can distinguish between two enantiomers even though both molecules contain the same atoms connected in the same order.
Think about your answer, then reveal below.
Model answer: A biological receptor is itself a chiral three-dimensional structure built from chiral components (L-amino acids). Binding between a molecule and its receptor depends on the three-dimensional geometric complementarity — shape, charge distribution, and orientation must match at the binding site. Two enantiomers are mirror images: their atoms are connected identically, but their spatial arrangements are non-superimposable. Just as a left glove cannot fit a right hand (despite being made of the same material with the same pattern), one enantiomer may fit the receptor's binding pocket while the other cannot — or fits a different receptor entirely. The receptor does not 'read' connectivity; it senses three-dimensional shape.
This question tests whether students understand that chirality has consequences because chirality is recognized by other chiral structures. The connectivity (graph structure) of both enantiomers is identical — a 2D structural formula cannot distinguish them. What differs is their three-dimensional arrangement, and that is what matters for fitting into three-dimensionally specific biological binding sites. The hand-glove analogy makes this intuitive: the 'glove' (receptor) is specific to one 'handedness.'