Difficulty: Intermediate | Prerequisites: Alkene structure, Markovnikov's rule, basic stereochemistry (R/S, cis/trans)
This material sits in the middle of CHM 255's coverage of alkene reactivity. You need a solid grasp of how alkenes react with electrophiles and how to assign stereochemistry before tackling these reactions. The worksheet tests your ability to predict products, classify reactions as oxidations or reductions, and draw stereochemical outcomes for additions to alkenes. If you are comfortable with HBr addition and Markovnikov's rule, you are ready for this.
Alkenes undergo several important transformations covered in this worksheet: hydroboration-oxidation gives anti-Markovnikov alcohols, ozonolysis cleaves the double bond into carbonyl fragments, and OsO4 dihydroxylation adds two OH groups on the same face. Each reaction is classified as an oxidation or reduction by tracking how the carbon's bonds to oxygen and hydrogen change. Stereochemistry (syn vs anti addition, Markovnikov vs anti-Markovnikov regiochemistry) is the thread running through all of it.
Hydroboration-oxidation
A two-step reaction sequence: (1) BH3/THF adds B and H across the double bond in a syn fashion, placing boron on the less substituted carbon; (2) H2O2/OH− replaces boron with OH. The net result is an anti-Markovnikov alcohol with syn stereochemistry.
In simple terms, this gives you the alcohol on the "wrong" (less substituted) carbon, with both new groups added from the same face.
Ozonolysis
Cleavage of a C=C double bond using ozone (O3), followed by a reductive workup such as (CH3)2S or Zn. Each carbon of the former double bond becomes a carbonyl (C=O), producing aldehydes and/or ketones depending on the substitution.
Think of it as cutting the double bond in half and capping each side with an oxygen.
Dihydroxylation (OsO4)
Addition of two hydroxyl groups across a double bond using OsO4 as the oxidant, followed by a reductive workup (NaHSO3/H2O). Both OH groups add to the same face of the alkene (syn addition), producing a 1,2-diol (vicinal diol).
In simple terms, you glue two OH groups onto the double bond from the same side.
Syn addition
Both new groups (or atoms) add to the same face of the double bond. Hydroboration-oxidation and OsO4 dihydroxylation are classic syn additions. Catalytic hydrogenation (H2/Pd) is another.
Anti-Markovnikov addition
The incoming group ends up on the less substituted carbon of the double bond, the opposite of what Markovnikov's rule predicts. Hydroboration-oxidation is the textbook example.
Oxidation (organic context)
A reaction that increases the number of C–O bonds (or C–X bonds to electronegative atoms) or decreases the number of C–H bonds on a carbon. Ozonolysis and dihydroxylation are oxidations.
Reduction (organic context)
A reaction that increases the number of C–H bonds or decreases the number of C–O (or C–X) bonds on a carbon. Hydroboration-oxidation is a reduction because a C=C becomes a C–C with new C–H bonds, even though an OH is introduced.
The quick test: count the number of C–H and C–O bonds before and after.
More C–H bonds after = reduction
More C–O bonds after = oxidation
Same overall oxidation state = neither
Reagents: BH3/THF, then H2O2/OH−
Starting material: a terminal alkene (2-methylbut-1-ene type structure)
Product: an anti-Markovnikov alcohol (OH on the less substituted carbon)
Classification: Reduction (the pi bond is replaced by C–H and C–OH; net gain of C–H bonds)
Stereochemistry: syn addition (both H and OH delivered to the same face)
Reagents: O3, then (CH3)2S (dimethyl sulfide, a mild reductant)
Starting material: an internal alkene (trisubstituted cyclohexene derivative)
Product: two carbonyl fragments (an aldehyde and a ketone)
Classification: Oxidation (C=C becomes two C=O bonds)
Key point: the carbon skeleton is cleaved at the double bond. Work backwards from the products to identify the original alkene on exams.
Reagents: OsO4, then NaHSO3/H2O
Starting material: a bicyclic alkene (norbornene derivative)
Product: a syn-1,2-diol (both OH groups on the same face)
Classification: Oxidation (two new C–O bonds are formed)
Stereochemistry: syn addition, producing a cis-diol
The worksheet gives a trisubstituted cyclohexene (1-methyl-4-methylcyclohexene type) and asks for products of three different reactions.
HBr addition:
Markovnikov regiochemistry (Br goes to the more substituted carbon)
No stereochemical control: the carbocation intermediate is planar, so nucleophilic attack occurs from both faces
Result: a pair of enantiomers (both products are chiral)
Hydroboration-oxidation (BH3, then H2O2/NaOH):
Anti-Markovnikov regiochemistry (OH on the less substituted carbon)
Syn addition: H and OH both delivered from the same face
Product is chiral; a single enantiomer is drawn from each face, but both faces are accessible, so a racemic mixture results from an achiral starting material
Catalytic hydrogenation (H2/Pd):
H2 adds across the double bond from the same face (syn addition)
Both hydrogens are delivered simultaneously via the metal surface
Produces the cis product
Students often think hydroboration-oxidation is an oxidation because the product contains an OH group. It is classified as a reduction because the overall change is a net gain of C–H bonds when the pi bond is converted to sigma bonds.
Students frequently confuse syn and anti addition. Syn means both groups add from the same face. OsO4, BH3, and H2/Pd are all syn. HBr goes through a carbocation, which is planar, so both faces are attacked (no syn/anti preference).
Ozonolysis products are sometimes drawn incorrectly because students forget that the double bond is fully cleaved. Each carbon of the original C=C ends up with a C=O. If a carbon was disubstituted, it becomes a ketone; if monosubstituted, an aldehyde; if unsubstituted, formaldehyde.
Students mix up the reductive workup agents. (CH3)2S and Zn/AcOH are reductive workups for ozonolysis (giving aldehydes/ketones). H2O2 is the oxidative step of hydroboration, not ozonolysis.
⚠️ Expect a question asking you to classify a reaction as oxidation, reduction, or neither. Count C–H and C–O bond changes.
⚠️ Hydroboration-oxidation's regiochemistry (anti-Markovnikov) and stereochemistry (syn) are tested constantly. Know both.
⚠️ Ozonolysis questions often run in reverse: you are given the carbonyl fragments and asked to reconstruct the original alkene.
⚠️ OsO4 dihydroxylation gives syn-diols. If the question asks for an anti-diol, you need a different reaction (epoxidation followed by acid-catalysed ring opening).
⚠️ Stereochemistry of HBr addition: because it goes through a planar carbocation, there is no facial selectivity. Both enantiomers form. Contrast this with syn additions.
True or False: Hydroboration-oxidation produces a Markovnikov alcohol.
Answer: False. It produces the anti-Markovnikov alcohol.
Fill in the blank: Ozonolysis followed by (CH3)2S workup converts a C=C into two ______ groups.
Answer: carbonyl (C=O)
True or False: OsO4 dihydroxylation is classified as a reduction.
Answer: False. It is an oxidation (two new C–O bonds).
Fill in the blank: In syn addition, both new groups are delivered to the ______ face of the double bond.
Answer: same
True or False: HBr addition to an alkene produces a single enantiomer.
Answer: False. The planar carbocation intermediate allows attack from both faces, giving a racemic mixture.
Q: An alkene is treated with BH3/THF followed by H2O2/NaOH. Is this reaction an oxidation, a reduction, or neither? Explain.
A: It is a reduction. The pi bond is converted to sigma bonds with a net increase in C–H bonds. Although an OH is introduced, the overall oxidation state of the relevant carbons decreases.
Q: Draw the products of ozonolysis (O3, then (CH3)2S) of 2-methylbut-2-ene.
A: The double bond is between C2 and C3. Cleavage gives acetone (from the more substituted side) and acetaldehyde (from the less substituted side).
Q: What is the stereochemical outcome of OsO4 dihydroxylation of cyclohexene?
A: Syn addition of two OH groups, giving cis-cyclohexane-1,2-diol.
Q: A trisubstituted alkene reacts with HBr. How many stereoisomeric products are possible, and why?
A: Two enantiomers. HBr follows Markovnikov addition through a carbocation. The carbocation is sp2 (planar), so Br− attacks from both faces equally, producing a racemic mixture.
Q: You need an anti-Markovnikov alcohol from an alkene with syn stereochemistry. Which reagent set do you use?
A: BH3/THF followed by H2O2/OH− (hydroboration-oxidation). It is the only common reagent combination that gives both anti-Markovnikov regiochemistry and syn addition.
Hydroboration-oxidation, BH3 THF, H2O2 NaOH, anti-Markovnikov alcohol, syn addition, ozonolysis, O3 DMS workup, reductive ozonolysis, oxidative cleavage of alkenes, OsO4 dihydroxylation, osmium tetroxide, syn-diol, vicinal diol, cis-diol, catalytic hydrogenation, H2 Pd, oxidation reduction organic chemistry, alkene addition reactions, stereochemistry of addition, Markovnikov rule, regioselectivity, facial selectivity, CHM 255 Purdue, PSO Worksheet 7
Connections to other topics: These alkene reactions connect directly to alkyne chemistry (covered in the companion notes), where similar reagents produce different outcomes due to the triple bond. Ozonolysis and dihydroxylation also connect to retrosynthetic analysis, since working backwards from carbonyl or diol products to identify the parent alkene is a common exam strategy. Stereochemistry concepts here carry forward into substitution and elimination reactions later in the course.