Organic Chemistry 1 · Stereochemistry of alkene reactions
Spot the error: OsO₄ + cyclopentene “gives the trans diol”
Conditions: OsO4, then aqueous NaHSO3.
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The question
A student writes: "Cyclopentene with OsO4, then NaHSO3, gives trans-cyclopentane-1,2-diol, because the two OH groups repel each other." What is wrong?
Reveal the answer and explanation
Answer and explanation
Correct answer The osmate ester delivers both oxygens to one face, so the diol is cis; the trans diol needs mCPBA then H3O⁺
Syn addition through the cyclic ester fixes the cis geometry, and only the epoxide route gives anti addition.
Not this one Nothing is wrong; the trans diol is more stable, so it forms
Product stability does not control a stereospecific concerted addition, and the ring bonds cannot rotate to swap faces.
Not this one OsO4 does not react with cyclopentene; it cleaves the ring
OsO4 dihydroxylates the alkene; ring cleavage needs ozone or hot KMnO4.
Not this one The product is trans, but only because NaHSO3 inverts one carbon
NaHSO3 cleaves the O–Os bonds and never breaks the C–O bonds, so no carbon is inverted.
Worked solution
OsO4 adds to the alkene in one concerted step to form a five-membered cyclic osmate ester, so both C–O bonds are made on the same face. Hydrolysis by NaHSO3 breaks the O–Os bonds, not the C–O bonds, so the cis relationship survives: the product is cis-cyclopentane-1,2-diol. Repulsion between OH groups cannot move them after the bonds form. The trans diol comes from epoxidation with mCPBA followed by acid-catalysed anti opening with water.
What you'll practice
Diagnose a wrong stereochemical claim about osmylation of a ring alkene and name the route that really gives the trans diol.
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