Difficulty: Intermediate | Prerequisites: Alkene structure, pi bonds, stereochemistry basics (cis/trans, syn/anti), electrophilic addition reactions.
Oxidation and reduction reactions of alkenes and alkynes round out the toolkit for transforming unsaturated hydrocarbons. Where electrophilic addition adds atoms across a double bond, oxidation can break the double bond entirely (ozonolysis) or install two hydroxyl groups simultaneously (dihydroxylation). Reduction removes the pi bond by adding hydrogen. The selective reduction of alkynes is particularly important: depending on your choice of reagent, you can stop at a cis alkene, a trans alkene, or go all the way to an alkane.
OsO4 adds two OH groups to the same face of an alkene (syn dihydroxylation, giving a cis-diol). Ozonolysis cleaves the double bond completely, producing ketones or aldehydes. Catalytic hydrogenation (H2/Pd) reduces alkenes to alkanes and alkynes to alkanes. Lindlar's catalyst stops alkyne reduction at the cis alkene; Na/NH3 (dissolving metal) gives the trans alkene.
Dihydroxylation (syn-dihydroxylation)
The addition of two hydroxyl groups to both carbons of a double bond. With OsO4, both OH groups add to the same face (syn), giving a cis-diol.
Diol (glycol)
A compound with two hydroxyl groups on adjacent carbons. Think of it as a double alcohol.
Ozonolysis
Cleavage of a C=C double bond using ozone (O3), followed by a reductive workup (typically (CH3)2S or Zn). The double bond is completely broken and replaced by two C=O bonds.
Catalytic hydrogenation
The addition of H2 across a double or triple bond using a metal catalyst (Pd, Pt, or Ni). Syn addition of hydrogen to the same face of the pi bond.
Lindlar's catalyst
A poisoned palladium catalyst (Pd on CaCO3, treated with lead acetate and quinoline) that is selective enough to reduce an alkyne to a cis (Z) alkene without going further to the alkane.
Dissolving metal reduction
Reduction of an alkyne using Na (or Li) in liquid NH3. Gives the trans (E) alkene selectively. The mechanism involves radical anion intermediates, and the geometry arises because the bulkier groups prefer to be on opposite sides.
Reagents: 1) OsO4, 2) NaHSO3 and H2O (reductive workup)
Product: cis-1,2-diol (syn-diol)
Stereochemistry: syn addition (both OH groups on the same face)
OsO4 forms a cyclic osmate ester across the double bond, delivering both oxygens from one face
The reductive workup releases the diol and regenerates osmium
Reagents: 1) O3, 2) (CH3)2S (dimethyl sulfide) or Zn
Product: carbonyl compounds (ketones and/or aldehydes)
The double bond is cleaved completely: each carbon of the former C=C becomes a C=O
If a carbon of the double bond bore two R groups, it becomes a ketone; if it bore one R and one H, it becomes an aldehyde
Useful for determining the position of a double bond in an unknown structure: work backwards from the ozonolysis products to reconstruct the alkene
Reagents: H2, Pd (or Pt, or Ni)
Product: alkane
Stereochemistry: syn addition (both H atoms delivered to the same face of the double bond from the metal surface)
The reaction occurs on the surface of the metal catalyst
Full reduction to alkane: H2 (excess) with Pd gives the alkane. Both pi bonds are reduced.
Selective reduction to cis (Z) alkene: H2 with Lindlar's catalyst. The poisoned catalyst stops the reduction after one equivalent of H2 has been added. Syn addition gives the cis (Z) geometry.
Selective reduction to trans (E) alkene: Na in liquid NH3 (dissolving metal reduction). The mechanism involves radical anion intermediates, and the trans geometry is favoured because the two larger groups end up on opposite sides of the double bond to minimise steric strain.
Reaction | Reagents | Product | Stereochemistry | Key Detail |
|---|---|---|---|---|
Dihydroxylation | 1) OsO4, 2) NaHSO3/H2O | cis-1,2-Diol | Syn | Both OHs same face |
Ozonolysis | 1) O3, 2) (CH3)2S | Ketones / aldehydes | N/A (bond cleaved) | Breaks C=C completely |
Alkene reduction | H2, Pd | Alkane | Syn | H's from metal surface |
Alkyne to alkane | H2 (excess), Pd | Alkane | Syn | Full reduction |
Alkyne to cis alkene | H2, Lindlar's catalyst | cis (Z) Alkene | Syn | Poisoned Pd stops at alkene |
Alkyne to trans alkene | Na, NH3 (liq.) | trans (E) Alkene | Anti | Dissolving metal reduction |
Ozonolysis is used in forensic and structural chemistry to determine where a double bond sits in an unknown molecule. Catalytic hydrogenation is the basis for the industrial hardening of vegetable oils into margarine (partial hydrogenation of the C=C bonds in unsaturated fatty acids). Dihydroxylation with OsO4 was part of the chemistry recognised in K. Barry Sharpless's Nobel Prize work on asymmetric synthesis.
Students often confuse syn and anti dihydroxylation. OsO4 gives syn (cis-diol). If the exam asks about anti dihydroxylation, that is a different reagent set (mCPBA followed by ring-opening of the epoxide with aqueous acid or base), not covered in this review but worth knowing the distinction.
Mixing up Lindlar's catalyst and Na/NH3. Lindlar's gives cis (Z), Na/NH3 gives trans (E). A useful mnemonic: Lindlar = cis, just remember "L" for "less distance" between substituents on the same side.
Forgetting that ozonolysis breaks the double bond entirely. Some students draw addition products instead of cleavage products.
Assuming catalytic hydrogenation of an alkyne always stops at the alkene. With a standard Pd catalyst and excess H2, it goes all the way to the alkane. You need Lindlar's catalyst specifically to stop at the alkene stage.
⚠️ Predicting ozonolysis products (and working backwards from products to the starting alkene) is a classic exam question.
⚠️ The three selective reduction methods for alkynes (to alkane, to cis alkene, to trans alkene) and which reagent gives which product are tested frequently.
⚠️ Stereochemistry of dihydroxylation (syn) and catalytic hydrogenation (syn) could appear as draw-the-product questions on cyclic substrates.
True or False: OsO4 dihydroxylation gives a trans-diol. (False, it gives a cis-diol via syn addition.)
Fill in the blank: Ozonolysis of a tetrasubstituted alkene produces two ________. (Ketones.)
True or False: Lindlar's catalyst reduces an alkyne to a trans alkene. (False, Lindlar's gives cis (Z). Na/NH3 gives trans (E).)
Fill in the blank: Catalytic hydrogenation delivers hydrogen via ________ addition. (Syn.)
True or False: Treating an alkyne with excess H2 and Pd gives an alkene. (False, excess H2 and Pd gives the alkane.)
Q: What products result from ozonolysis of 2-butene (followed by (CH3)2S workup)?
A: Two equivalents of acetaldehyde (ethanal, CH3CHO). The symmetrical double bond is cleaved, and each carbon becomes an aldehyde because each bore one H.
Q: You need to convert 2-butyne into cis-2-butene. What reagents do you use?
A: H2 with Lindlar's catalyst. This gives syn addition of one equivalent of H2, producing the cis (Z) alkene.
Q: How would you convert 2-butyne into trans-2-butene instead?
A: Na in liquid NH3 (dissolving metal reduction). This selectively gives the trans (E) alkene.
Q: Cyclohexene is treated with OsO4 followed by NaHSO3/H2O. Draw the product and explain the stereochemistry.
A: cis-1,2-Cyclohexanediol. Both OH groups are on the same face of the ring (syn addition), so the product is the cis diol.
Q: An unknown alkene is subjected to ozonolysis and gives formaldehyde (CH2O) and acetone ((CH3)2CO). What was the starting alkene?
A: 2-Methylpropene (isobutylene). Reconnect the two carbonyl carbons with a double bond: CH2=C(CH3)2.
Syn addition appears in both dihydroxylation (OsO4) and catalytic hydrogenation (H2/Pd), as well as hydroboration-oxidation from the previous set of notes. Recognising syn vs anti is a thread that runs through every reaction in this unit.
Ozonolysis connects to carbonyl chemistry later in the course. The aldehydes and ketones it produces are the same functional groups you will study in nucleophilic addition, aldol reactions and oxidation/reduction of carbonyls.
The selective reduction of alkynes (Lindlar vs Na/NH3) ties back to alkene stereochemistry: you need to assign E/Z configurations to the products, so comfort with Cahn-Ingold-Prelog priority rules is essential.
Oxidation reactions, reduction reactions, dihydroxylation, OsO4, osmium tetroxide, syn-diol, cis-diol, ozonolysis, O3, ozone cleavage, reductive workup, dimethyl sulfide, catalytic hydrogenation, H2 Pd, H2 Pt, Lindlar's catalyst, poisoned palladium, cis alkene from alkyne, trans alkene from alkyne, dissolving metal reduction, Na NH3, sodium ammonia, alkyne reduction, alkene reduction, CHM 255, organic chemistry, Purdue, midterm 2