Difficulty: Intermediate | Prerequisites: Lecture 35 addition mechanisms (Mechanisms I, II, III), alkene structure, basic stereochemistry.
Oxidation reactions of alkenes add one or two oxygen atoms across the double bond. These reactions are among the most synthetically useful transformations in organic chemistry: dihydroxylation installs two OH groups, ozonolysis cleaves the double bond entirely, and epoxidation creates a strained three-membered ring with oxygen.
All three reactions follow Mechanism III (concerted, syn addition) or involve concerted steps. Understanding the mechanism tells you the stereochemistry of the product. This topic also introduces the important synthetic distinction between forming a syn diol (OsO₄/KMnO₄) and an anti diol (epoxidation followed by ring opening).
OsO₄ and KMnO₄ convert alkenes to syn diols (both OH groups on the same face). Ozone (O₃) cleaves double bonds entirely, giving aldehydes/ketones (reductive workup) or carboxylic acids/ketones (oxidative workup). mCPBA converts alkenes to epoxides via syn addition, and those epoxides can be opened with water under acid conditions to give anti diols.
Dihydroxylation
The addition of two hydroxyl (OH) groups across a double bond, converting an alkene into a 1,2-diol (vicinal diol). Think of it as painting both carbons of the double bond with OH from the same side.
OsO₄ (osmium tetroxide)
A strong oxidising agent that performs syn dihydroxylation on alkenes. Forms a cyclic osmate ester intermediate. Workup: Na₂SO₃ and H₂O.
KMnO₄ (potassium permanganate)
Another oxidising agent that performs syn dihydroxylation. Forms a cyclic manganate ester intermediate. Workup: NaOH and H₂O. Under harsh conditions can over-oxidise and cleave the bond.
Ozonolysis
Cleavage of a C=C double bond using ozone (O₃), breaking both the sigma and pi bonds to give two carbonyl fragments. In simple terms, ozone cuts the double bond in half.
Molozonide
The initial, unstable cyclic intermediate formed when ozone reacts with the alkene. It rearranges to the more stable ozonide.
Ozonide
The key intermediate in ozonolysis, a five-membered ring containing three oxygen atoms. It is cleaved during workup to give the carbonyl products.
Reductive workup
Treatment of the ozonide with a mild reducing agent, either Zn/H₂O or dimethyl sulfide ((CH₃)₂S). Gives aldehydes and/or ketones.
Oxidative workup
Treatment of the ozonide with H₂O₂. Oxidises any aldehyde fragments to carboxylic acids; ketones remain unchanged.
Epoxidation
The addition of a single oxygen atom across the double bond to form an epoxide (a three-membered ring containing oxygen). Follows Mechanism III: concerted, syn addition.
mCPBA (meta-chloroperoxybenzoic acid)
The standard peroxyacid reagent used for epoxidation. The reactive O-O bond delivers one oxygen to the alkene in a single concerted step.
Peroxyacid (peracid)
A carboxylic acid with an extra oxygen: RCO₃H. The weak O-O bond is the source of the oxygen transferred to the alkene.
Epoxide
A three-membered cyclic ether. Highly strained and reactive; opens easily under acidic or basic conditions. Also called an oxirane.
Syn diol
A 1,2-diol where both OH groups are on the same face of the original double bond. Formed by OsO₄ or KMnO₄.
Anti diol
A 1,2-diol where the OH groups are on opposite faces. Formed by epoxidation (mCPBA) followed by acid-catalysed ring opening with water.
Both reagents add two OH groups across the double bond via a cyclic intermediate (Mechanism III, concerted).
OsO₄: forms a cyclic osmate ester. Workup with Na₂SO₃/H₂O cleaves the Os-O bonds and releases the syn diol.
KMnO₄: forms a cyclic manganate ester. Workup with NaOH/H₂O releases the syn diol.
Stereoselectivity: syn addition (both OHs delivered from the same face).
Regioselectivity: not applicable (both groups are the same).
The mechanism involves a [3+2] cycloaddition of the metal oxide across the pi bond.
Ozone reacts with the alkene in a concerted [3+2] cycloaddition to form a molozonide (unstable).
The molozonide rearranges to the ozonide (the key intermediate).
The workup determines the oxidation level of the products:
Reductive workup (Zn/H₂O or (CH₃)₂S): ozonide breaks into aldehydes and/or ketones. A hydrogen on the original alkene carbon becomes an aldehyde C-H.
Oxidative workup (H₂O₂): aldehydes are further oxidised to carboxylic acids. Ketones are unaffected.
Key rule for predicting products: erase the double bond, put =O on each carbon. If that carbon had an H, it becomes an aldehyde (reductive) or carboxylic acid (oxidative). If it had no H, it becomes a ketone (either workup).
When the double bond is in a ring, ozonolysis opens the ring and produces a single dicarbonyl product rather than two fragments.
mCPBA (meta-chloroperoxybenzoic acid) delivers one oxygen atom to the double bond in a single concerted step.
The mechanism is a "butterfly" transition state: the alkene's pi electrons attack the electrophilic O of the peroxyacid's O-O bond while the O-H proton transfers to the carbonyl oxygen.
Stereoselectivity: syn addition. The oxygen is delivered from one face; cis substituents remain cis in the epoxide.
Product: an epoxide (oxirane), a three-membered ring containing one oxygen.
No intermediate, no carbocation, no regioselectivity issue (one oxygen atom bridges both carbons symmetrically).
Under acid conditions (H₃O⁺), the epoxide is protonated and a nucleophile (H₂O) opens the ring from the back side (anti to the departing oxygen), analogous to Mechanism II (like a bromonium ion opening).
This gives an anti diol: the two OH groups end up on opposite faces.
Synthetic comparison: syn diol via OsO₄ or KMnO₄, anti diol via mCPBA followed by acid-catalysed ring opening with water.
Reaction | Reagent | Workup | Product | Stereoselectivity | Mechanism |
|---|---|---|---|---|---|
Dihydroxylation | OsO₄ | Na₂SO₃, H₂O | syn diol | syn | III |
Dihydroxylation | KMnO₄ | NaOH, H₂O | syn diol | syn | III |
Ozonolysis (reductive) | O₃ | Zn/H₂O or (CH₃)₂S | aldehydes + ketones | n/a | III |
Ozonolysis (oxidative) | O₃ | H₂O₂ | carboxylic acids + ketones | n/a | III |
Epoxidation | mCPBA | none | epoxide | syn | III |
Anti diol formation | mCPBA, then H₃O⁺/H₂O | acid ring opening | anti diol | anti (overall) | III then II-like |
Students often confuse the two ozonolysis workups. Reductive workup (Zn/H₂O or (CH₃)₂S) preserves aldehydes. Oxidative workup (H₂O₂) converts aldehydes to carboxylic acids. The workup choice does not affect ketone products.
Assuming OsO₄ and mCPBA give the same diol. OsO₄ gives a syn diol directly. mCPBA gives an epoxide, which must be opened in a separate step, and that opening gives an anti diol. They are complementary, not interchangeable.
Forgetting that ozonolysis of a cyclic alkene does not produce two separate fragments. The ring opens to give a single chain with two carbonyl groups at the cleavage points.
Treating epoxidation as if it has regioselectivity. Because a single oxygen atom bridges both carbons, there is no regiochemical choice in the epoxidation step itself.
⚠️ A classic exam question gives you an alkene and asks you to form either a syn diol or an anti diol. You must know: syn = OsO₄ or KMnO₄, anti = mCPBA then H₃O⁺/H₂O.
⚠️ Ozonolysis product prediction is heavily tested. Draw the two fragments by erasing the double bond and adding =O to each carbon. Then decide: aldehyde or carboxylic acid based on workup.
⚠️ The reverse of ozonolysis (given the carbonyl products, work backwards to find the original alkene) is a common exam question. Join the two carbonyls at their C=O carbons to reconstruct the double bond.
⚠️ Stereochemistry of epoxidation: if the alkene is cis, the epoxide retains that cis relationship. If trans, the epoxide retains trans.
True or false: KMnO₄ gives an anti diol. (False, it gives a syn diol.)
Fill in the blank: Ozonolysis with reductive workup converts a terminal alkene (R-CH=CH₂) into an aldehyde and ______. (Formaldehyde, CH₂O.)
True or false: mCPBA directly produces a diol. (False, it produces an epoxide. A separate ring-opening step is needed for the diol.)
Fill in the blank: Oxidative workup of ozonolysis uses ______ and converts aldehydes to ______. (H₂O₂; carboxylic acids.)
True or false: Ozonolysis of a cyclic alkene gives two separate carbonyl fragments. (False, it gives a single open-chain dicarbonyl compound.)
Q: What reagents convert cyclohexene into a syn diol? Draw the stereochemistry.
A: OsO₄ followed by Na₂SO₃/H₂O (or KMnO₄ followed by NaOH/H₂O). Both OH groups add to the same face of the ring, giving cis-cyclohexane-1,2-diol.
Q: 2-Butene is treated with (1) O₃, then (2) Zn/H₂O. What are the products?
A: Two equivalents of acetaldehyde (CH₃CHO). The double bond is cleaved and each carbon gets a C=O. Because both carbons had one H, reductive workup gives aldehydes.
Q: How would you convert cyclohexene into an anti-1,2-diol?
A: First treat with mCPBA to form cyclohexene oxide (the epoxide), then open the ring with H₃O⁺/H₂O. Water attacks from the opposite face (anti), giving trans-cyclohexane-1,2-diol.
Q: An unknown alkene gives acetone (CH₃COCH₃) and formaldehyde (CH₂O) on ozonolysis with reductive workup. What is the structure of the alkene?
A: 2-Methylpropene (isobutylene), (CH₃)₂C=CH₂. Reconstruct by joining the carbonyl carbons: the ketone carbon was doubly substituted, the aldehyde carbon was terminal.
Q: What product forms when mCPBA reacts with cis-2-butene?
A: cis-2,3-Dimethyloxirane (cis-2-butene oxide). The cis relationship of the methyl groups is retained because epoxidation is syn addition.
Dihydroxylation and epoxidation connect to the broader theme of syn vs. anti selectivity introduced in the electrophilic addition mechanisms (Mechanisms I, II, III). The epoxide ring opening under acid is mechanistically analogous to the opening of a bromonium ion (Mechanism II).
Ozonolysis connects to carbonyl chemistry later in the course: the aldehyde and ketone products are substrates for nucleophilic addition, aldol reactions, and Grignard reactions.
The syn diol vs. anti diol distinction appears in total synthesis problems and in carbohydrate chemistry, where the stereochemistry of hydroxyl groups determines the identity of the sugar.
Related Terms / Search Tags
dihydroxylation, osmium tetroxide, OsO4, potassium permanganate, KMnO4, ozonolysis, ozone, molozonide, ozonide, reductive workup, oxidative workup, dimethyl sulfide, epoxidation, mCPBA, meta-chloroperoxybenzoic acid, peroxyacid, peracid, epoxide, oxirane, syn diol, anti diol, anti dihydroxylation, epoxide ring opening, CHEM 210, organic chemistry II, University of Michigan