2,3-Dibromobutane has two stereocenters. A student systematically draws all four configurations: (R,R), (S,S), (R,S), and (S,R). How many distinct stereoisomers of 2,3-dibromobutane actually exist?
A4 — each configuration is a unique compound
B2 — the (R,S) and (S,R) forms are identical, and so are (R,R) and (S,S)
C3 — the (R,R) and (S,S) forms are enantiomers, and the (R,S)/(S,R) forms are the same meso compound
D1 — all configurations are the same compound due to rotational symmetry
The 2ⁿ formula predicts 4 stereoisomers for 2 stereocenters, but meso compounds reduce this count. The (R,R) and (S,S) forms of 2,3-dibromobutane are non-superimposable mirror images — they are enantiomers. The (R,S) form has an internal plane of symmetry that makes it superimposable on the (S,R) form — they are the same meso compound. So only 3 distinct stereoisomers exist: the enantiomeric pair and the meso form. This is a recurring pattern whenever a molecule with two identical stereocenters can be drawn with equal and opposite configurations.
Question 2 Multiple Choice
A student claims that (2R,3R)-tartaric acid and (2S,3S)-tartaric acid have different melting points because they are stereoisomers with different three-dimensional structures. Is this claim correct?
AYes — all stereoisomers have different physical properties due to their different spatial arrangements
BNo — enantiomers have identical physical properties in achiral environments; only their optical rotation differs
CYes — tartaric acid stereoisomers are diastereomers with different properties
DNo — tartaric acid only exists as a meso compound and cannot form enantiomers
The (2R,3R) and (2S,3S) forms of tartaric acid are enantiomers — non-superimposable mirror images. Enantiomers have identical physical properties (melting point, boiling point, solubility, density) in an achiral environment, because their intermolecular interactions are mirror images of each other with the same energy. The only property that differs is the direction in which they rotate plane-polarized light. Diastereomers — stereoisomers that are NOT mirror images — do have different physical properties and can be separated by conventional techniques. This is a critical distinction for synthetic chemistry.
Question 3 True / False
A meso compound is achiral overall despite containing two or more stereocenters, because an internal plane of symmetry makes it superimposable on its mirror image.
TTrue
FFalse
Answer: True
This is the defining feature of a meso compound. The stereocenters are present and real — one may be R and the other S — but the molecule as a whole is achiral because an internal mirror plane relates one half to the other. When you reflect the molecule through this plane, you get an identical molecule. The optical rotations contributed by each stereocenter cancel exactly, producing zero net rotation of plane-polarized light. The meso form of 2,3-dibromobutane and meso-tartaric acid are classic examples: both contain stereocenters, yet both are achiral.
Question 4 True / False
Diastereomers share the same melting points and solubilities as one another, just as enantiomers do.
TTrue
FFalse
Answer: False
This is the most important practical distinction between diastereomers and enantiomers. Enantiomers are mirror images and have identical physical properties in achiral environments — they cannot be separated by ordinary means. Diastereomers are NOT mirror images; they have different three-dimensional shapes, different intermolecular interactions, and therefore different physical constants (melting points, boiling points, solubilities, densities). This means diastereomers can be separated by column chromatography, recrystallization, or distillation — standard laboratory techniques. This separability makes diastereomers crucial in asymmetric synthesis, where chemists deliberately create diastereomers to separate desired stereoisomers.
Question 5 Short Answer
Explain why a meso compound is achiral even though it contains stereocenters, and describe how you would identify one from its structure.
Think about your answer, then reveal below.
Model answer: A meso compound is achiral because it possesses an internal plane of symmetry — a mirror plane that divides the molecule into two halves that are mirror images of each other. The stereocenter in one half has the opposite configuration to the corresponding stereocenter in the other half (e.g., one R and one S), so their optical rotations cancel. When you superimpose the molecule on its mirror image, they are identical — the defining criterion for an achiral compound. To identify a meso compound: look for two or more stereocenters with identical substituents, then check whether any plane through the molecule makes the two halves mirror images of each other.
The practical test is to build or draw the molecule, reflect it, and check for superimposability. In 2,3-dibromobutane, the internal mirror plane is perpendicular to the C2–C3 bond in the eclipsed conformation, cutting through the middle of the carbon chain. In cis-1,2-dimethylcyclohexane, the plane of the ring serves as the internal mirror. The key conceptual move is distinguishing 'this molecule has stereocenters' from 'this molecule is chiral' — meso compounds are the proof that stereocenters do not guarantee chirality.