Difficulty: Intermediate | Prerequisites: Electrophilic addition basics (first half of Ch. 5), carbocation stability
Once you understand basic electrophilic addition, the next question is: how do chemists control which product forms? Oxymercuration-reduction and hydroboration-oxidation are two methods for hydrating alkenes to alcohols, but they give opposite regiochemistry. Ozonolysis, meanwhile, is a completely different beast: it cleaves double bonds entirely. These three reactions round out the toolkit for alkene transformations and are tested heavily because they force you to think about selectivity and mechanism type.
Oxymercuration-reduction gives Markovnikov alcohols without rearrangement. Hydroboration-oxidation gives anti-Markovnikov alcohols via a concerted (no-carbocation) mechanism. Ozonolysis cleaves the double bond entirely, producing ketones, aldehydes, or carboxylic acids depending on substitution and workup conditions.
Oxymercuration-reduction
A two-step method for adding water across an alkene with Markovnikov regiochemistry. Step 1: mercuric acetate (Hg(OAc)₂) and water add across the double bond. Step 2: NaBH₄ reduces the C-Hg bond to C-H, giving the alcohol.
Think of it as a "clean" Markovnikov hydration that avoids carbocation rearrangements.
Hydroboration-oxidation
A two-step method for adding water across an alkene with anti-Markovnikov regiochemistry. Step 1: BH₃ adds across the double bond in a concerted, syn addition (H and B add to the same face). Step 2: H₂O₂/NaOH oxidises the C-B bond to C-OH.
Think of it as the "opposite" hydration: OH ends up on the less substituted carbon.
Concerted mechanism
A reaction in which bond breaking and bond forming happen simultaneously in a single step, with no intermediate. Hydroboration is concerted: the B-H bond adds across the pi bond in one step.
In simple terms, everything happens at once, so there is no carbocation and no chance of rearrangement.
Syn addition
Both new groups add to the same face of the double bond. Hydroboration gives syn addition. Contrast with anti addition (groups add to opposite faces), as in bromination.
Anti-Markovnikov addition
The regiochemistry in which H adds to the more substituted carbon and the functional group (OH, Br) adds to the less substituted carbon. Opposite of Markovnikov's rule.
Ozonolysis
Cleavage of a C=C double bond by ozone (O₃), followed by a workup step. The double bond is completely broken and replaced by C=O bonds on each fragment.
Ozonide
The cyclic intermediate formed during ozonolysis before workup. It contains an O-O bond and is unstable.
Reductive workup ((CH₃)₂S or Zn)
Treatment of the ozonide under mild reducing conditions. Produces aldehydes and/or ketones (does not further oxidise aldehydes).
Oxidative workup (H₂O₂)
Treatment of the ozonide under oxidising conditions. Aldehydes are further oxidised to carboxylic acids; ketones are unaffected.
Triisobutylborane
The product when BH₃ has added across three equivalents of an alkene (e.g. 2-methylpropene). Demonstrates that one BH₃ molecule can hydroborate up to three alkene molecules.
Reagents: Hg(OAc)₂, H₂O (step 1), then NaBH₄ (step 2).
Mercuric acetate acts as the electrophile. The pi bond attacks mercury, and water acts as the nucleophile.
An intermediate mercurinium ion forms (analogous to the bromonium ion). This is not a free carbocation.
Because there is no carbocation intermediate, no rearrangement occurs.
NaBH₄ replaces the mercury with hydrogen, giving a Markovnikov alcohol.
Regioselectivity: OH goes to the more substituted carbon (Markovnikov).
Reagents: BH₃·THF (step 1), then H₂O₂/NaOH (step 2).
BH₃ adds to the alkene in a single concerted step. The boron attaches to the less substituted carbon and hydrogen to the more substituted carbon.
This is syn addition: both B and H add to the same face of the double bond.
One BH₃ molecule can add across up to three alkene molecules, ultimately forming a trialkylborane (e.g. triisobutylborane).
Oxidation with H₂O₂/NaOH replaces the C-B bond with C-OH.
Regioselectivity: OH ends up on the less substituted carbon (anti-Markovnikov).
No carbocation, no rearrangement.
Which is faster? When comparing two alkenes undergoing hydroboration, the one that gives a lower-energy, more stable transition state reacts faster. A transition state where boron's partial positive charge sits on a more substituted (tertiary) carbon is more stable than one where it sits on a secondary carbon.
These two reactions are presented as a pair precisely because they give opposite products from the same alkene. If you need a Markovnikov alcohol without rearrangement, use oxymercuration-reduction. If you need an anti-Markovnikov alcohol, use hydroboration-oxidation.
Reagent: O₃ (ozone), followed by either a reductive or oxidative workup.
Ozone reacts with the alkene in a concerted cycloaddition, forming a molozonide, which rearranges to the more stable ozonide.
The workup determines the final products.
Reductive workup ((CH₃)₂S or Zn):
Cleaves the ozonide to give aldehydes and/or ketones.
A disubstituted carbon on the alkene becomes a ketone (R₂C=O).
A monosubstituted carbon becomes an aldehyde (RCH=O).
A terminal =CH₂ becomes formaldehyde (H₂C=O).
Oxidative workup (H₂O₂):
Same cleavage, but any aldehyde is further oxidised to a carboxylic acid.
Ketones are unaffected.
A monosubstituted carbon becomes a carboxylic acid (RCOOH) instead of an aldehyde.
Ozonolysis is used in forensic and analytical chemistry to determine the position of double bonds in unknown alkenes: you break the molecule at the double bond and identify the fragments. Hydroboration-oxidation is widely used in synthetic chemistry whenever anti-Markovnikov alcohols are needed.
Students often confuse the regiochemistry of oxymercuration and hydroboration. Remember: oxymercuration = Markovnikov (OH on more substituted carbon). Hydroboration = anti-Markovnikov (OH on less substituted carbon).
A frequent error is thinking hydroboration goes through a carbocation. It does not. The mechanism is concerted, which is why there is no rearrangement and why syn addition occurs.
Students sometimes assume the ozonolysis workup does not matter. It does. Reductive workup preserves aldehydes; oxidative workup converts them to carboxylic acids. Getting this wrong changes your predicted products.
Some students think ozonolysis is reversible. It is not. The double bond is permanently cleaved.
⚠️ You will almost certainly be asked to choose between oxymercuration-reduction and hydroboration-oxidation to produce a specific alcohol. Know which gives Markovnikov and which gives anti-Markovnikov.
⚠️ Expect a problem where you must predict ozonolysis products and choose the correct workup. Pay close attention to whether the question specifies (CH₃)₂S or H₂O₂.
⚠️ "No carbocation, no rearrangement" is a principle that applies to both the bromonium ion pathway and the concerted hydroboration pathway. Be ready to explain why.
⚠️ Syn vs. anti addition is a classic exam question. Hydroboration = syn. Bromination = anti.
Fill in the blank: Oxymercuration-reduction gives ______ regiochemistry, while hydroboration-oxidation gives ______ regiochemistry.
True or False: Hydroboration involves a carbocation intermediate.
Fill in the blank: Ozonolysis with (CH₃)₂S workup produces aldehydes, while ozonolysis with H₂O₂ workup converts those aldehydes to ______.
True or False: Hydroboration gives anti addition across the double bond.
True or False: Oxymercuration-reduction avoids carbocation rearrangements.
Answers: 1. Markovnikov; anti-Markovnikov. 2. False (concerted mechanism, no intermediate). 3. Carboxylic acids. 4. False (syn addition). 5. True.
Q: You need to convert 1-methylcyclohexene to trans-2-methylcyclohexanol. Which reagent set do you use?
A: Hydroboration-oxidation (BH₃·THF, then H₂O₂/NaOH). This gives anti-Markovnikov, syn addition, placing OH on the less substituted carbon with syn stereochemistry.
Q: What products result from ozonolysis of 2-butene (CH₃CH=CHCH₃) with reductive workup ((CH₃)₂S)?
A: Two equivalents of acetaldehyde (CH₃CHO). The symmetrical alkene gives two identical aldehyde fragments.
Q: Same alkene, but with oxidative workup (H₂O₂). What changes?
A: The aldehydes are further oxidised to acetic acid (CH₃COOH). Two equivalents of acetic acid.
Q: A student performs oxymercuration-reduction on 3,3-dimethyl-1-butene and predicts a rearranged product. Is this prediction correct?
A: No. Oxymercuration-reduction does not involve a free carbocation, so rearrangement does not occur. The product is the Markovnikov alcohol without any skeletal rearrangement.
Q: Explain why hydroboration is described as a concerted mechanism.
A: In hydroboration, the B-H bond and the pi bond break and form new bonds simultaneously in a single transition state. There is no discrete intermediate, no carbocation, and no separate steps.
Oxymercuration and hydroboration connect back to the stereochemistry of Chapter 4: syn vs. anti addition determines the three-dimensional arrangement of atoms in the product. Ozonolysis connects forward to carbonyl chemistry (aldehydes, ketones, carboxylic acids) covered in later chapters.
The concept of concerted vs. stepwise mechanisms reappears throughout organic chemistry, particularly in pericyclic reactions and in SN2 substitutions.
Oxymercuration-reduction, hydroboration-oxidation, Markovnikov hydration, anti-Markovnikov hydration, syn addition, anti addition, concerted mechanism, ozonolysis, ozonide, reductive workup, oxidative workup, dimethyl sulfide workup, BH₃·THF, mercuric acetate, NaBH₄, alkene cleavage, carbonyl formation, trialkylborane, regioselectivity, stereoselectivity, organic chemistry chapter 5