Chapter 16 — Addition Reactions of Alkenes: The Full Toolbox

"An alkene is the most versatile starting material in synthesis. With the right reagent, it becomes an alcohol, a halohydrin, a diol, an epoxide, a halide, a carbonyl, a saturated alkane. Master the toolbox, and you can install almost any functional group at a C=C." — paraphrase from a synthesis text

"There is a reagent for every regioselectivity and a reagent for every stereoselectivity. Knowing which reagent to choose is the heart of synthetic chemistry." — Clayden, Greeves, Warren — Organic Chemistry

Chapter 15 covered the archetype (HX addition to alkenes). This chapter surveys the rest of the alkene addition toolbox: - Hydration (acid-catalyzed and via mercurinium). - Hydroboration-oxidation (anti-Markovnikov hydration). - Halogenation and halohydrin formation. - Epoxidation (with peroxyacids). - Dihydroxylation (OsO₄). - Ozonolysis (alkene cleavage). - Catalytic hydrogenation (alkene → alkane). - Asymmetric versions (Sharpless, Jacobsen, Knowles, Noyori).

Each reagent has characteristic regioselectivity (Markovnikov vs anti-Markovnikov), stereoselectivity (syn vs anti), and chemoselectivity (which functional groups it touches and which it leaves alone). Mastering the toolbox is essential for synthesis.

By the end of this chapter you should be able to: - Choose the right reagent to install a specific functional group across a C=C. - Predict regio- and stereochemistry for each reaction type. - Recognize which reactions go through carbocations (and risk rearrangement) vs. which are concerted (and don't). - Combine multiple alkene additions in multi-step syntheses. - Apply asymmetric variants for enantioselective synthesis.

Mechanism Map 16.1 — Every reaction in this chapter shares one feature: a π bond is broken, and two new σ bonds are formed. What differs is how those σ bonds form: through an open carbocation (rearrangements possible), through a cyclic 3-membered intermediate (anti opening), or through a concerted TS (syn delivery). Identify which class a reagent falls into, and you've predicted its stereochemistry.

16.1 Acid-catalyzed hydration

$$\text{alkene} + H_2O \xrightarrow{H_2SO_4} \text{alcohol (Markovnikov)}$$

Mechanism: same as HX addition (Ch 15), but water is the nucleophile. 1. H⁺ adds to alkene; carbocation forms at the more-substituted C (Markovnikov). 2. Water attacks the carbocation; positively-charged oxonium intermediate. 3. Deprotonation gives the alcohol.

Result: Markovnikov alcohol. Two-step via carbocation; rearrangements possible.

Industrial use

Acid-catalyzed hydration of ethylene → ethanol (the basis of industrial ethanol production from petroleum, supplementing fermentation ethanol). Annual scale: ~5 million tons of synthetic ethanol globally; complemented by ~100 million tons of fermentation ethanol.

The industrial process: - Catalyst: solid phosphoric acid on silica (heterogeneous; easier to remove than H₂SO₄). - Conditions: 300 °C, 60-70 atm H₂O. - Yield: 95% ethanol. - Used: solvents, fuel additives, beverages (after distillation).

Reversibility

Acid-catalyzed hydration is reversible. The reverse reaction is acid-catalyzed dehydration (Ch 12 alcohol → alkene by E1).

The equilibrium position depends on temperature and water concentration: - High water + low temperature → favors hydration (alcohol product). - Low water + high temperature → favors dehydration (alkene product).

This is Le Chatelier's principle in action; same reaction in either direction.

Limitations

Goes through a carbocation, so: - Anti-Markovnikov alternatives needed for terminal alcohols (use hydroboration instead). - Rearrangements can give unexpected products on substrates with adjacent stable cations. - Highly substituted substrates can give E1 elimination instead of hydration.

Worked example

3-methyl-1-butene + H₂SO₄/H₂O →

The terminal alkene gives a 2° cation initially: $\text{CH}_3-\text{CH}(\text{CH}_3)-\text{CH}^+-\text{CH}_3$

But this 2° cation rearranges via 1,2-H shift to give a more stable 3° cation: $\text{CH}_3-\text{C}^+(\text{CH}_3)-\text{CH}_2-\text{CH}_3$

Water attacks the 3° cation → 2-methyl-2-butanol (NOT the expected 2-methyl-1-butanol).

This is a Markovnikov product PLUS rearrangement — characteristic of the carbocation-mediated mechanism.

16.2 Oxymercuration-demercuration

$$\text{alkene} + Hg(OAc)_2, H_2O \to \text{Markovnikov alcohol (no rearrangement)}$$

Step 1 (oxymercuration): Hg(OAc)₂ + alkene → mercurinium ion intermediate (3-membered ring with Hg + 2 C). Water attacks at the more-substituted C (the one with more cation character). Result: Markovnikov-substituted alcohol with HgOAc at the other C.

Step 2 (demercuration): NaBH₄ replaces -HgOAc with -H. The C-Hg bond is reduced.

Net result: Markovnikov alcohol, no rearrangement (the mercurinium ring prevents the open-chain cation from forming).

Mechanism details

The mercurinium intermediate is structurally similar to the bromonium ion: a 3-membered ring with mercury bridging the two carbons of the alkene. Two key features: - The mercury carries a positive charge but bears it through partial bonds to both carbons (no full open-chain cation). - The C atoms of the ring are partially positive; the more-substituted C is more positive (more cation-like). - Water attacks the more-positive C from the opposite face of the Hg (anti-attack on the ring).

This anti-attack means the resulting -OH and -HgOAc are on opposite faces of the original alkene — geometrically anti, even though no carbocation forms.

After demercuration with NaBH₄, the -HgOAc is replaced by -H. The stereochemistry of this step is poorly defined (NaBH₄ can attack from either face), so the final product's stereochemistry at that carbon is largely scrambled.

Why oxymercuration is preferred over acid hydration

• No rearrangements: the mercurinium ring blocks 1,2-shifts.
• Mild conditions: room temperature, water solvent.
• Selective for Markovnikov: even on substrates that would give rearrangement issues.

The reagent: Hg(OAc)₂ — mercury(II) acetate. Some toxicity concerns (Hg is a heavy metal); modern alternatives use other metal-mediated hydration (Au, Pt, etc.).

Modern green alternatives

Mercury catalysis has fallen out of favor due to toxicity. Modern alternatives: - Gold-catalyzed hydration: Au(I) and Au(III) complexes catalyze Markovnikov hydration without mercury. Less toxic. - Pd-catalyzed Wacker oxidation: alkene + O₂ + Pd/Cu → ketone (anti-Markovnikov for terminal). Industrially important for acetaldehyde synthesis. - Brønsted acid catalysis: in some cases simple H₂SO₄ or H₃PO₄ works without rearrangement issues.

The classical oxymercuration is now mostly in textbooks and old industrial processes; modern industrial chemistry has moved away from heavy-metal catalysis where possible.

16.3 Hydroboration-oxidation

$$\text{alkene} + BH_3 \cdot THF \xrightarrow{H_2O_2/NaOH} \text{alcohol (anti-Markovnikov, syn)}$$

The standard method for anti-Markovnikov hydration. Discovered by H. C. Brown (Purdue, Nobel Prize 1979 — shared with Wittig for boron and phosphorus chemistry).

Mechanism

Step 1 (hydroboration): BH₃ adds across the alkene in a single concerted step (4-centered TS). - B and H add to opposite carbons of the C=C. - Both add to the same face: syn addition. - B goes to the less-substituted C (steric reasons + electronic). - H goes to the more-substituted C.

The 4-centered transition state has partial bonds: H...C and B...C forming, and the alkene π bond breaking. The TS is like a parallelogram: alkene C=C on one side, H-B on the other, with two new bonds forming simultaneously.

Step 2 (oxidation): H₂O₂ + NaOH converts the C-B bond to C-OH with retention of stereochemistry.

The mechanism involves: 1. Hydroxide deprotonates H₂O₂ to give peroxide anion (HOO⁻). 2. HOO⁻ attacks boron, displacing the C-B bond and migrating the alkyl group from B to O. 3. The result: C-O-B intermediate. 4. Hydrolysis cleaves C-O-B to give C-OH + boronate.

The migration step (alkyl from B to O) goes with retention of configuration at carbon. This is why the overall hydroboration-oxidation gives syn addition: the H and B added syn in step 1, and the B → OH in step 2 retains the configuration.

Net result: - Anti-Markovnikov alcohol (OH at less-substituted C, since B was there and B → OH with retention). - Syn addition (the new -OH and the new -H are on the same face of the original alkene).

Why anti-Markovnikov?

The boron's vacant p orbital (B is electron-deficient: 3 bonds = 6 electrons; empty p orbital) makes B+ act partially as the electrophile. So the alkene's more-substituted C is the more nucleophilic; it bonds to H. The less-substituted C bonds to B. In the oxidation step, B becomes OH at the less-substituted C — anti-Markovnikov.

There are two contributing factors: 1. Electronic: the carbocation-like character on the more-substituted C in the TS draws the more nucleophilic C (less-substituted) toward B. 2. Steric: B is bigger than H; B prefers to land on the less hindered (less substituted) C.

Both factors point to the same regiochemistry: B at less-substituted, H at more-substituted, → -OH at less-substituted = anti-Markovnikov.

BH₃ and other boron reagents

• BH₃·THF: standard reagent. BH₃ in pure form is unstable (B₂H₆ dimer in gas phase); coordinated to THF for stability. Reacts 3 times with alkene, forming R₃B (trialkylborane).
• BH₃·SMe₂: similar; even more stable in solution. Stinky but useful.
• 9-BBN (9-borabicyclo[3.3.1]nonane): bulky borane; gives clean monohydroboration; stops after one alkene reaction. Discovered by Brown and Knights, 1968.
• Disiamylborane ([(CH₃)₂CHCH(CH₃)]₂BH): bulky; very anti-Markovnikov selective.
• Catecholborane (a B-H of catecholatoborate): less reactive; useful for sensitive substrates.
• Pinacolborane (HB(pin)): used in modern boron chemistry; product can couple via Suzuki (Ch 37).

No rearrangements

Hydroboration is concerted (single-step transition state). No carbocation intermediate. So no 1,2-shifts, no rearrangements. This is a major advantage over acid hydration.

Stereo: syn

Both H and B add to the same face of the alkene. After oxidation, the new -OH and the new -H are on the same face. This is syn addition — different from the anti pattern of Br₂.

For (Z)-2-butene + BH₃ → (after oxidation) → (2R,3S)-2-butanol = meso-2,3-butanediol's monoalcohol cousin.

Synthetic applications

• Anti-Markovnikov primary alcohols: the only way to get a -CH₂OH from a terminal alkene at scale.
• Suzuki-coupling intermediates: hydroboration of an alkene gives an alkylborane that can be coupled with aryl halides via Pd catalysis (Ch 37).
• Asymmetric hydroboration: chiral boranes (e.g., diisopinocampheylborane, Ipc₂BH) give chiral alcohols with high ee.

Industrial scale

Modest industrial use; mostly in pharmaceutical synthesis where anti-Markovnikov regiochemistry is needed.

16.4 Halogenation: full picture

Br₂, Cl₂ in CCl₄ (no nucleophile)

$$\text{alkene} + Br_2 \to \text{vicinal dibromide (anti)}$$

Bromonium ion → backside attack → anti addition. Stereospecific. Covered in Ch 15.

The reaction works for both Br₂ and Cl₂. F₂ is too aggressive (oxidizes the alkene); I₂ is too weak (the diiodide is unstable, equilibrium favors the alkene).

For Cl₂, the reaction is similar but the chloronium ion is less symmetric; for highly polarized alkenes (with EDG or EWG substituents), Cl₂ can give some Markovnikov-like preference.

Br₂, Cl₂ in H₂O (halohydrin)

$$\text{alkene} + Br_2 + H_2O \to \text{bromohydrin (OH on more-substituted C; Br on less-substituted C)}$$

Mechanism: 1. Bromonium ion forms. 2. Water (more abundant nucleophile in aqueous solution) attacks the more-substituted C (the one with more cation character). 3. Result: -OH at the more-substituted C; -Br at the less-substituted C. 4. Net: anti addition (water attacks from opposite face of Br).

This is Markovnikov-like: OH at the more-substituted C.

Useful for synthesizing halohydrins, which can be cyclized to epoxides (Williamson ether synthesis-style; with base).

N-halosuccinimides (NBS, NCS)

N-bromosuccinimide (NBS) and N-chlorosuccinimide (NCS) are convenient solid sources of bromine and chlorine respectively. They release Br₂ (or Cl₂) slowly into the reaction mixture.

NBS in DMSO/H₂O is the standard reagent for bromohydrin formation: the slow release of Br₂ keeps the bromonium intermediate concentration low, suppressing side reactions.

Practical example

Cyclohexene + Br₂/H₂O → trans-2-bromo-cyclohexan-1-ol. - The bromonium opens trans (anti) by water attack. - The -OH ends up at one carbon, -Br at the other, on opposite faces.

This trans-bromohydrin can be cyclized to cyclohexene oxide (an epoxide) by treatment with base — see section 16.10.

Halogenation in synthesis

Halohydrins are key intermediates in: - Epoxide synthesis (intramolecular SN2 to close the 3-membered ring). - Pharmaceutical intermediates (chiral halohydrins as building blocks). - Polymer chemistry (vinyl chloride is made from ethylene + HCl/Cu catalysis or from ethylene + Cl₂ + heat).

16.5 Epoxidation

$$\text{alkene} + RCO_3H \to \text{epoxide (3-membered O-ring) + RCOOH}$$

Mechanism: peroxyacid (RCO₃H, an O-O containing acid) delivers its O atom to the alkene face in a concerted, syn-addition TS (a "butterfly" TS). The peroxyacid oxygen bridges the two C atoms; the resulting epoxide retains the alkene's stereochemistry.

The butterfly transition state

The TS is called "butterfly" because the geometry resembles butterfly wings. Specifically: - The alkene π bond breaks. - The O-O bond of the peracid breaks (homolytically? heterolytically? — it's concerted). - Two new C-O bonds form on the alkene face. - The acid -OH migrates to give the carboxylic acid byproduct.

The reaction is concerted: no carbocation, no radical, just a single TS. This is why it's stereospecific (syn addition; no scrambling).

Common peracid: mCPBA

meta-chloroperoxybenzoic acid (mCPBA) is the standard reagent. Stable, handleable, available commercially. Crystalline solid; storable in the freezer.

Other peracids: - Trifluoroperacetic acid (CF₃CO₃H): much more reactive; used for less reactive alkenes. - Peracetic acid (CH₃CO₃H): industrial scale; cheaper than mCPBA. - Performic acid (HCO₃H): even more reactive but unstable. - Dimethyldioxirane (DMDO): a similar oxidant; cyclic dioxirane that delivers oxygen.

Stereo: syn

The peroxyacid TS adds both ends of the new bonds (the two new C-O bonds) to the same face of the alkene. Result: cis-epoxide from cis-alkene; trans-epoxide from trans-alkene. Stereospecific.

Substrate scope

• Electron-rich alkenes (with alkyl substituents) react faster than electron-poor alkenes.
• Trisubstituted > disubstituted > monosubstituted: more substituted alkenes are more electron-rich.
• Electron-poor alkenes (with EWG) need very reactive peracid (CF₃CO₃H).
• Conjugated alkenes can give enol ether or other products (the C=O oxygen is more nucleophilic than the C=C).

Asymmetric epoxidation

Sharpless asymmetric epoxidation (SAE) uses chiral diethyl tartrate (DET) + Ti(OiPr)₄ + tert-butyl hydroperoxide (TBHP) on allylic alcohols to give chiral epoxides with high ee. Sharpless Nobel 2001 (Ch 8 case study).

For non-allylic alkenes, Jacobsen epoxidation (Mn-salen catalyst + NaOCl) gives chiral epoxides. The Jacobsen catalyst is a chiral salen-Mn(III) complex that delivers oxygen to one face of the alkene preferentially.

Industrial epoxidation

Ethylene + O₂ → ethylene oxide (used to make ethylene glycol, antifreeze, polyester): - Catalyst: Ag/α-Al₂O₃. - Process: ethylene + O₂ at 250 °C and 15-25 atm. - Annual scale: ~30 million tons of ethylene oxide globally.

Propylene + organic peroxide → propylene oxide (HPPO process): - Catalyst: Ti-doped silica (TS-1). - Process: propylene + H₂O₂ catalyzed by TS-1. - Annual scale: ~10 million tons globally.

These industrial processes use heterogeneous (Ag) or homogeneous (Ti) catalysts. Different mechanism details from the laboratory mCPBA reaction but same overall transformation.

16.6 Dihydroxylation: OsO₄ for syn-diol

$$\text{alkene} + OsO_4 + NMO \to \text{cis-1,2-diol (syn)}$$

Mechanism: 1. OsO₄ (a strong oxidant) adds across the alkene in a concerted [3+2] cycloaddition. 2. A cyclic osmate ester forms (5-membered ring: 2 C atoms + 2 O atoms + Os). 3. Hydrolysis releases the diol with both -OH on the same face: syn-diol (cis). 4. NMO (N-methylmorpholine N-oxide) recycles Os(VI) back to Os(VIII), allowing catalytic Os usage.

The osmate ester intermediate

The 5-membered ring of the osmate ester locks the geometry: both oxygens are bonded to the alkene carbons on the same face. When the osmate is hydrolyzed (water cleaves the Os-O bonds), the two -OH groups are released on the same face — cis-diol.

This is the key to the syn stereochemistry: it comes from the cyclic intermediate's geometry, not from any face-selective attack.

Sharpless asymmetric dihydroxylation (AD)

Adds chiral cinchona alkaloid ligands to OsO₄ + NMO. Gives chiral cis-diols with high ee. Two ligand families: - (DHQ)₂PHAL: derived from quinine; gives one enantiomer. - (DHQD)₂PHAL: derived from quinidine; gives the other enantiomer.

Both are commercial; the AD-mix-α and AD-mix-β reagent kits are pre-mixed for easy use: - AD-mix-α (with DHQ): gives (R,R)-diol from many simple alkenes. - AD-mix-β (with DHQD): gives (S,S)-diol.

This is one of the most widely used asymmetric reactions in modern synthesis.

KMnO₄ alternative

Cold dilute KMnO₄ also gives syn-diols, but is more aggressive (oxidizes further at higher T or higher concentration). KMnO₄ is cheaper than OsO₄ but less selective.

Modern preference: OsO₄ + catalyst (NMO) + chiral ligand for asymmetric work; KMnO₄ for simple, achiral cases.

Stereochemistry detailed

For (Z)-2-butene + OsO₄ + NMO: - Add cis-1,2-diol (syn addition). - Both -OH on same face. - The two carbons are now (R,S) or (S,R) — the meso form.

For (E)-2-butene + OsO₄ + NMO: - Add cis-1,2-diol (syn). - Both -OH on same face. - The two carbons are now (R,R) or (S,S) — the racemic chiral diol.

Note: the alkene's E/Z geometry directly controls which diastereomer is obtained — this is stereospecificity.

Anti-dihydroxylation: an alternative path

Sometimes a trans-diol is wanted. The route: 1. Alkene + mCPBA → epoxide (syn). 2. Epoxide + H₂O/H⁺ → trans-diol (anti opening of the epoxide).

So overall: alkene + mCPBA + H₂O/H⁺ → trans-diol. The two-step sequence gives the opposite stereochemistry from OsO₄/NMO directly.

16.7 Ozonolysis: cleaving the C=C

$$\text{alkene} + O_3 \to \text{ozonide} \xrightarrow{\text{workup}} \text{two carbonyls}$$

Step 1 (ozonide formation): O₃ adds across the alkene to form a primary ozonide (a 3-O-2-C 5-membered ring), which rearranges to a secondary ozonide (a different 5-membered O-O-C-O-O-C ring).

Step 2 (workup): the secondary ozonide is decomposed to give two carbonyl fragments. - Reductive workup (Zn/HOAc, or Me₂S, or PPh₃): gives aldehydes or ketones (or one of each). - Oxidative workup (H₂O₂): gives carboxylic acids or ketones (aldehydes are oxidized to COOHs).

Mechanism details

The primary ozonide rearrangement (Criegee mechanism): 1. Primary ozonide cleaves homolytically to give a carbonyl and a carbonyl oxide. 2. The carbonyl oxide and carbonyl recombine to give the secondary ozonide.

The secondary ozonide is the isolated intermediate; it's stable enough to handle but is typically not isolated because of explosion risk (ozonides can be shock-sensitive).

Practical considerations

• Solvent: typically methanol, ethanol, or methylene chloride at -78 °C.
• Indicator: a dye (Sudan III) that decolorizes when ozone consumes alkene; tells you when the alkene has reacted.
• Workup: reductive (Me₂S, Zn/HOAc) for carbonyl product; oxidative (H₂O₂) for carboxylic acid product.
• Safety: ozonolysis produces ozone and ozonides, both potentially hazardous. Conducted in well-ventilated fume hood, never on large scale without engineering controls.

Net result

The C=C is cleaved. Each carbon of the original alkene becomes a separate carbonyl (or COOH).

Example: - 2-methyl-2-butene + O₃ + Zn/HOAc → acetone + acetaldehyde. - The same alkene + O₃ + H₂O₂ → acetone + acetic acid.

Use in structure determination

Ozonolysis fragments a complex alkene into known carbonyl pieces, helping identify where the C=C was. Used historically (and still) for structure elucidation.

For example, an unknown C₆H₁₂ that ozonolyzes to acetone (×2) is identified as 2,3-dimethyl-2-butene.

Industrial alternatives

For industrial-scale C=C cleavage, catalytic methods are preferred over ozonolysis: - Wacker oxidation (Pd/Cu + O₂): converts terminal alkene to methyl ketone (industrial acetaldehyde from ethylene). - OsO₄ + NaIO₄: oxidative diol cleavage; cleaner and safer than ozone. - KMnO₄: hot KMnO₄ cleaves alkenes oxidatively.

For laboratory use, ozonolysis remains widely used due to its mild conditions and predictable fragmentation.

16.8 Catalytic hydrogenation

$$\text{alkene} + H_2 \xrightarrow{Pd, Pt, Ni \text{ catalyst}} \text{alkane (syn)}$$

Mechanism (heterogeneous)

The alkene adsorbs on the metal catalyst surface (Pd, Pt, Ni). H₂ also adsorbs and dissociates to atomic H. Both H atoms are delivered from the surface to the same face of the alkene → syn addition.

The detailed steps: 1. H₂ approaches the metal surface; the metal weakens the H-H bond. 2. H₂ dissociates to give two surface-bound H atoms. 3. The alkene coordinates to the metal surface (π-bonded). 4. One H atom is transferred to the alkene C. 5. The other H atom is transferred to the other alkene C. 6. The product alkane desorbs.

Both H atoms come from the same surface — so they add to the same face of the alkene (syn).

Catalysts

• Pd/C (5-10% Pd on activated carbon): mild, selective; doesn't reduce aromatics, esters, amides at typical conditions.
• Pt/C, PtO₂ (Adams catalyst): more reactive; reduces some aromatics under harsher conditions.
• Raney Ni: very reactive (porous Ni created from Ni-Al alloy + NaOH); reduces almost everything including aromatics, sulfides.
• Lindlar Pd (Pd on CaCO₃ poisoned with Pb): reduces alkynes selectively to cis-alkenes (Ch 17). Stops at the alkene; doesn't reduce further to alkane.
• Wilkinson's catalyst (RhCl(PPh₃)₃): homogeneous; soluble; doesn't reduce aromatics.
• H₂ gas at 1 atm to 100 atm: pressure varies with substrate.

Stereo: syn

Both H atoms add to the same face. For a cis-alkene, the product is meso or racemic (depending on substitution pattern).

Industrial use

• Hydrogenation of vegetable oils: makes margarines (saturated or partially-saturated fats from unsaturated oils). Industrial scale; ~30 million tons/year. Partial hydrogenation produces trans fats (now banned/restricted in many countries due to health concerns).
• Hydrogenation of cyclohexene → cyclohexane: precursor for nylon synthesis (caprolactam, adipic acid).
• Hydrogenation of benzene → cyclohexane: industrial; makes the nylon precursor.
• Petroleum hydrogenation (hydrocracking, hydrodesulfurization): refines crude oil into clean fuels.

Selectivity issues

For substrates with multiple unsaturated groups, chemoselectivity matters: - Pd/C: alkenes ✓; aldehydes/ketones ✗ (don't reduce); esters ✗; amides ✗; aromatic ✗ (under mild conditions); nitro ✓ (reduces -NO₂ to -NH₂!). - Pt/C: alkenes ✓; aldehydes/ketones partially. - Raney Ni: most things reduce.

For asymmetric hydrogenation: chiral Rh or Ru catalysts (Knowles, Noyori, Nobel 2001).

Asymmetric hydrogenation: Knowles, Noyori

Chiral phosphine ligands (DIPAMP, BINAP) on Rh or Ru convert prochiral alkenes (with a coordinating group like enamide or β-keto ester) to chiral alkanes with >95% ee.

Examples (industrial): - L-DOPA (Knowles, 1974): asymmetric hydrogenation of an enamide → L-DOPA for Parkinson's treatment. - (S)-Naproxen: asymmetric hydrogenation of an α,β-unsaturated acid → (S)-naproxen. - Sitagliptin (Januvia): asymmetric hydrogenation of an enamide intermediate. - Many β-blockers, statins, and other chiral pharmaceuticals.

The 2001 Nobel Prize in Chemistry went to Knowles (asymmetric hydrogenation) and Noyori (asymmetric ketone hydrogenation) for this work, plus Sharpless (asymmetric epoxidation).

16.9 The reagent toolbox: summary table

This is the menu. Choose the reagent based on the desired regio-, stereo-, and chemo-selectivity, plus any required asymmetry.

Common Mistake 16.1 — Students sometimes assume catalytic hydrogenation reduces all double bonds in a molecule. It often doesn't! Pd/C at 1 atm reduces alkenes but leaves aromatics, ketones, esters, and amides untouched. Selectivity comes from choice of catalyst and conditions. Always specify the catalyst and conditions; "H₂ + catalyst" is not enough information.

Common Mistake 16.2 — "Anti-Markovnikov" doesn't mean the OH goes to the less-substituted side of the molecule; it means the H goes to the more-substituted carbon and the OH goes to the less-substituted carbon (relative to the original alkene). Read the regiochemistry from the alkene's substitution pattern, not from the molecule's overall orientation.

16.10 Multistep alkene chemistry

Many syntheses chain multiple alkene reactions:

Example: alkene → diol → diacid

1. Alkene + OsO₄ + NMO → cis-diol.
2. Diol + KMnO₄ + heat → 1,2-diacid (oxidative cleavage; alternative to ozonolysis).

Example: alkene → epoxide → opened nucleophilic ring

1. Alkene + mCPBA → epoxide.
2. Epoxide + R-OH or R-NH₂ → ring-opened ether or amine.

The epoxide opening is highly regioselective: - Acid catalysis: nucleophile attacks the more-substituted C (Markovnikov-like, via partial cation). - Base catalysis: nucleophile attacks the less-substituted C (SN2-like, anti).

Example: alkene → halohydrin → epoxide

1. Alkene + Br₂/H₂O → bromohydrin (-OH on more-substituted C; -Br on less-substituted C).
2. Bromohydrin + base → epoxide (intramolecular SN2; Williamson ether synthesis-like).

The base deprotonates the -OH; the alkoxide intramolecularly attacks the C-Br, displacing Br⁻ and closing the 3-membered epoxide ring. Net result: a chiral epoxide with stereochemistry derived from the starting alkene.

Example: alkene → diol → ketone (via pinacol rearrangement)

1. Alkene + OsO₄/NMO → diol.
2. Diol + acid + heat → ketone (pinacol rearrangement; carbocation rearrangement chemistry).

The pinacol rearrangement: protonation of one -OH, loss as water gives a carbocation; 1,2-H shift; the new cation has -OH on it that can lose a proton to give a ketone.

Example: alkene → epoxide → alcohol (regioselective opening)

1. Alkene + mCPBA → epoxide.
2. Epoxide + LiAlH₄ → alcohol (anti-Markovnikov, since hydride attacks less-substituted C).

This gives the same regiochemistry as hydroboration but via a different route. Useful when the alkene has functional groups that don't tolerate BH₃.

Example: alkene → vicinal diol → carbonyl (single C cleavage)

For a 1,1-disubstituted alkene → 1,2-diol → cleavage: 1. Alkene + OsO₄ → diol. 2. Diol + NaIO₄ (Malaprade reaction) → carbonyl + HCHO (formaldehyde).

This is used for selective C=C cleavage where ozonolysis is too aggressive.

These chains are how synthesis chemistry is built up.

16.11 Biological applications

Alkene chemistry is everywhere in biology:

Terpene biosynthesis (Ch 34)

Terpenes are built from isoprene units (5-carbon C=C compounds). The biosynthesis involves: - Mevalonate pathway → isopentenyl pyrophosphate (IPP) → dimethylallyl pyrophosphate (DMAPP). - Prenyltransferases combine IPP + DMAPP units, with new C=C formation at each step. - Cyclization: protonation triggers cation formation; cation cyclizes; new C=C formed.

This is alkene chemistry done by enzymes — same Markovnikov rules, same stereoselectivity, but with enzyme control.

Prostaglandin synthesis

Prostaglandins are derived from arachidonic acid (20:4 fatty acid with 4 alkenes). Biosynthesis involves: - Cyclooxygenase (COX): oxidizes one C=C to make a 5-membered ring + new C=C. - Peroxide intermediate: an alkene-O-O-X structure analogous to mCPBA. - Reductive workup: NADPH reduces the peroxide to a hydroxyl + new alkene.

This biosynthesis is the target of NSAIDs (aspirin, ibuprofen) — they inhibit COX and prevent prostaglandin formation.

Biological epoxide chemistry

• Squalene → squalene 2,3-epoxide → cholesterol: the first committed step of cholesterol biosynthesis.
• Squalene epoxidase is targeted by terbinafine (antifungal, blocks squalene epoxide → ergosterol synthesis in fungi).
• Epoxide hydrolases open epoxides to diols in liver detoxification (Phase II metabolism).

Asymmetric biosynthesis

Most enzymatic reactions are highly stereoselective. The active site is a chiral environment (built from L-amino acids); it presents one face of the substrate to the catalytic groups. Chemists mimic this with asymmetric catalysts (Sharpless, etc.).

16.12 Connections to spectroscopy

IR for alkene products

• Alcohol O-H at 3200-3600 cm⁻¹ (broad): identifies hydration/hydroboration products.
• C=O at 1715 for ketones from oxidative cleavage; 1735-1750 for esters; 2720 + 1725 for aldehydes (sharp aldehyde C-H at 2720).
• C-O at 1100-1300 for ether/epoxide products.
• Disappearance of C=C at 1600-1680: confirms alkene reaction occurred.

NMR for alkene products

• Vinyl H at 5-6 ppm: disappears after alkene reaction.
• New CH-OH at 3.5-4.5 ppm.
• New CH-O (epoxide) at 2.5-3.5 ppm (epoxide carbons appear as multiplets at characteristic chemical shifts).
• ¹³C of new sp³ carbons: 50-90 ppm range.

MS for alkene products

• Hydroxyl loss (-18) common in alcohols.
• Acid loss (-44 = CO₂) in carboxylic acids from oxidative cleavage.
• α-cleavage in ketones from ozonolysis: characteristic Mass loss = R'• + acyl cation.

Spectroscopy Clue 16.1 — When you reduce an alkene by hydroboration-oxidation, the reaction moves the IR alkene C=C peak (1640) to disappear, replaces it with the broad O-H (3300) of an alcohol, and shifts the ¹H NMR vinyl peaks (5-6) to a CH-OH peak at ~3.5-4. Three independent spectroscopic checks confirm the transformation.

16.13 Why this chapter matters

Mastering Chapter 16 means having the alkene functional group conversion toolbox. From any alkene, you can install: - Alcohol (Markovnikov or anti-Markovnikov, with or without stereocontrol). - Halide (anti-1,2 or as bromohydrin). - Epoxide (precursor to many other functional groups). - Diol (cis or trans). - Carbonyl (via ozonolysis). - Alkane (via hydrogenation).

Plus, with chiral catalysts: - Chiral epoxide, chiral diol, chiral alcohol, chiral alkane.

This toolbox is central to organic synthesis. Many natural product syntheses involve sequential alkene transformations — an alkene → epoxide → diol → carbonyl chain might span several synthesis steps.

The mechanism-first thesis is on display: instead of memorizing each reaction's regiochemistry and stereochemistry independently, you derive them from the mechanism. Cyclic intermediate + back-side attack = anti. Cyclic concerted TS = syn. Carbocation intermediate = Markovnikov + possible rearrangement. These three patterns cover essentially every alkene reaction.

16.14 Industrial alkene chemistry (preview)

Polyethylene production

Ethylene + Ziegler-Natta catalyst (TiCl₄/AlR₃) → polyethylene. Industrial scale: ~110 million tons/year. The alkene addition is to itself (polymerization).

Polypropylene production

Propylene + Ziegler-Natta or metallocene catalysts → isotactic polypropylene. Industrial scale: ~75 million tons/year.

Ethylene oxide for ethylene glycol

Ethylene + O₂ over Ag catalyst → ethylene oxide → ethylene glycol (antifreeze). 30 million tons/year.

Vinyl chloride to PVC

Ethylene + Cl₂ → ethylene dichloride → vinyl chloride → PVC. Industrial scale: ~50 million tons/year.

Acrylonitrile from propylene

Propylene + NH₃ + O₂ → acrylonitrile (used in nitrile rubber, acrylic fibers). 7 million tons/year.

These industrial processes are alkene chemistry at the megaton scale. Modern process chemistry refinements add catalysts (metallocenes for polymerization), and adapt the chemistry of Chapter 16 to industrial constraints (continuous flow, heterogeneous catalysis, atom economy).

16.15 Deep dive: key historical experiments

Brown's discovery of hydroboration (1956)

Herbert C. Brown (Purdue, 1912-2004) was studying borohydride chemistry when he made a remarkable observation: NaBH₄ + diborane (B₂H₆) in ether reacted with alkenes to give organoboranes. The boron-carbon bond turned out to be highly versatile.

By 1956, Brown had developed: - Hydroboration: standard alkene + BH₃ → trialkylborane. - Oxidative workup: H₂O₂/NaOH → alcohols (anti-Markovnikov). - Carbonylation: CO + reducing agent → ketones, alcohols, etc. - Disiamylborane: bulky borane for monohydroboration. - 9-BBN: another bulky borane (1968, with Knights). - Asymmetric hydroboration: chiral borane (Ipc₂BH) for chiral alcohols.

The cumulative impact was the development of an entirely new branch of organic chemistry — borane reagents — that earned Brown the Nobel Prize in 1979 (shared with Wittig for organic phosphorus chemistry).

The Nobel committee specifically noted Brown's hydroboration as "one of the most important discoveries in synthetic organic chemistry of the 20th century."

Sharpless's epoxidation discovery (1980)

K. Barry Sharpless (MIT, then Scripps) was studying titanium catalysis when he discovered that Ti(OiPr)₄ + a chiral diethyl tartrate (DET) catalyzed asymmetric epoxidation of allylic alcohols using TBHP (tert-butyl hydroperoxide).

The remarkable features: - High ee (typically 90-95%). - Predictable absolute configuration from the catalyst's chirality (the "Sharpless mnemonic"). - Both enantiomers of DET commercially available. - Catalytic in chirality.

By 1990, Sharpless had also developed: - Asymmetric dihydroxylation (AD): with cinchona alkaloid + OsO₄ → chiral diols. - Asymmetric aminohydroxylation: → β-amino alcohols.

The 2001 Nobel Prize in Chemistry was awarded to Sharpless (asymmetric oxidation) jointly with Knowles and Noyori (asymmetric hydrogenation).

Sharpless went on to win a second Nobel in 2022 for click chemistry — making him only the second person to win two Chemistry Nobels (after Frederick Sanger).

The thalidomide and asymmetric synthesis story

Recall from Ch 7-8: thalidomide's tragedy showed why chiral drugs need single-enantiomer purity. Asymmetric synthesis (Sharpless, Knowles, Noyori, etc.) provides the methodology to make single-enantiomer drugs reliably.

Modern pharmaceutical chemistry depends on the asymmetric reactions in this chapter. ~80% of new drug candidates are chiral; most are launched as pure single enantiomers. The chemistry of Chapter 16 — particularly the asymmetric variants — is the workhorse of modern pharmaceutical process chemistry.

16.16 Reagent selection in practice

When to use H₂SO₄/H₂O hydration

• Industrial scale: cheap, simple.
• Substrates that don't rearrange: e.g., simple terminal alkenes that give 2° cations (no further stable cation accessible).
• Markovnikov regiochemistry desired: matches the carbocation pathway.

When to use oxymercuration

• Markovnikov hydration without rearrangement: substrates that would rearrange under H₂SO₄.
• Mild conditions: when other functional groups are sensitive to acid.
• Modern alternatives: Au- or Pt-catalyzed hydration (less toxic).

When to use hydroboration

• Anti-Markovnikov alcohols: when you need -CH₂OH from a terminal alkene.
• No rearrangement tolerated: when carbocation rearrangements would scramble the product.
• Stereoselective syn addition needed: for diastereoselectivity in chiral substrates.
• Suzuki coupling: when you need a borane for subsequent C-C coupling (Ch 37).

When to use halogenation (Br₂)

• Vicinal dibromides: as protective groups; later removed by Zn/HOAc.
• Halohydrins: as epoxide precursors.
• NBS in DMSO/H₂O: clean halohydrin formation.

When to use epoxidation

• 3-membered O-ring: as a versatile electrophile for nucleophilic attack.
• Asymmetric synthesis target: Sharpless or Jacobsen for chiral epoxide.
• trans-Diol synthesis: epoxide + H⁺/H₂O → trans-diol (anti opening).

When to use OsO₄/NMO (cis-diol)

• cis-1,2-Diol target: directly from alkene.
• Asymmetric variant (Sharpless AD): for chiral cis-diols.
• NaIO₄ cleavage afterward: to make a vicinal carbonyl pair (Malaprade reaction).

When to use ozonolysis

• C=C cleavage: when you need to break the bond entirely.
• Structure determination: to identify where C=C was located in an unknown.
• Partial alkene chains: to fragment a complex polyene into pieces.

When to use hydrogenation

• Saturate a C=C without further reaction: simple Pd/C.
• Aromatic + C=C selectively: choose Pd/C (only C=C reduced) or Pt (both reduce).
• Asymmetric (Knowles, Noyori): for chiral product.
• Selective alkyne → alkene: Lindlar Pd (Ch 17).

Decision tree

For a synthesis problem requiring alkene → ?:

1. What functional group do I need? (alcohol, halide, epoxide, diol, carbonyl, alkane?)
2. What regiochemistry? (Markovnikov or anti-Markovnikov?)
3. What stereochemistry? (syn, anti, or no preference?)
4. Asymmetric? (Need pure enantiomer?)
5. Functional group tolerance? (What other groups in the molecule?)

The toolbox table in 16.9 lets you pick the right reagent.

16.17 Mechanism comparisons

Three "categories" of alkene addition mechanism:

Category 1: Cyclic 3-membered intermediate (anti opening)

• Bromonium: from Br₂ → anti dibromide.
• Mercurinium: from Hg(OAc)₂ → Markovnikov alcohol (anti-like).
• Halohydrin formation: bromonium + H₂O → bromohydrin (anti).
• Epoxide formation in some cases: if the epoxide forms from a halohydrin (intramolecular).

Common features: 3-membered cyclic intermediate; nucleophile attacks from anti face; stereospecific.

Category 2: Concerted cyclic TS (syn)

• Hydroboration: 4-centered TS; syn delivery of H and B.
• Epoxidation (mCPBA): butterfly TS; syn delivery of O.
• Dihydroxylation (OsO₄): [3+2] cycloaddition; syn delivery of two -O- groups.
• Hydrogenation (catalytic): surface delivery; syn.

Common features: single TS; both new groups arrive from the same face; stereospecific syn.

Category 3: Carbocation intermediate (Markovnikov, possibly rearranged)

• Acid-catalyzed hydration: H⁺ adds; cation forms; water attacks.
• HX addition (Ch 15): similar.
• Cation rearrangement: 1,2-H or 1,2-CH₃ shift to give a more stable cation.

Common features: open-chain cation; nucleophile attacks from either face (some racemization or modest preference); rearrangement possible.

How to identify which category for any given reaction

Look at the reagent: - Three-membered cyclic intermediate: Br₂, Cl₂, Hg(OAc)₂, peracids forming halonium-like → anti or syn depending on which oxygen is delivered. - Concerted single-step: BH₃, RCO₃H (mCPBA), OsO₄, H₂/catalyst → syn. - Open carbocation: H⁺ + nucleophile (H₂O, X⁻, etc.) → Markovnikov, possibly rearranged.

16.18 Computational chemistry of alkene additions

DFT calculations

Modern computational tools (e.g., Gaussian, ORCA, NWChem) can compute: - Reaction TS energies for alkene + reagent. - Activation barriers for SN1 vs SN2 (for hydration mechanism). - Bromonium ion structure (3-membered ring) and energies. - Mercurinium ion symmetry (whether it's a true 3-membered ring or unsymmetric).

These calculations confirm experimental mechanisms and reveal subtle details (e.g., the unsymmetric mercurinium ion has more cation character on the more-substituted C).

Visualization

Avogadro and other free tools let students: - Build the alkene + reagent TS. - Optimize geometries. - Visualize MOs (HOMO of alkene, LUMO of reagent). - See partial bonds in TS.

Computational Exercise 16.1 — Build (Z)-2-butene in Avogadro. Add Br₂ above the alkene plane. Optimize. The bromonium ion intermediate should appear as a 3-membered ring (Br above the C=C). Now visualize the bromonium HOMO; it has antibonding character in the 3-membered ring, supporting back-side attack opening.

Computational Exercise 16.2 — Compute the reaction barrier for HBr + propene → 2-bromopropane (Markovnikov) vs 1-bromopropane (anti-Markovnikov). The barrier difference reflects the cation stability difference.

16.19 Spectroscopy storyline

Confirming each transformation

Acid hydration of an alkene → alcohol: - Loss: vinyl H at 5-6 ppm in ¹H NMR; C=C at 1640 cm⁻¹ in IR. - Gain: broad OH at 3300 cm⁻¹; sp³ CH at 3.5-4 ppm in ¹H NMR.

Hydroboration-oxidation (anti-Markov alcohol): - Same pattern as above; but the alcohol is on the less-substituted C. - Distinguish from acid hydration by checking the regiochemistry.

Br₂ addition (vicinal dibromide): - Loss: vinyl H, C=C peak. - Gain: CH-Br at 3.5-4.5 ppm; ¹³C C-Br at 30-50 ppm; characteristic isotope pattern in MS (M, M+2 = 1:1 for one Br; 1:2:1 for two Br's).

Epoxidation (epoxide formation): - Loss: vinyl H, C=C peak. - Gain: epoxide CH at 2.5-3.5 ppm; ¹³C of epoxide C at 50-60 ppm.

Dihydroxylation (cis-diol): - Loss: vinyl H, C=C peak. - Gain: 2 broad OH (or one if intramolecular H-bonded) at 3300 cm⁻¹; 2 CH-OH at 3.5-4.5 ppm.

Ozonolysis (cleavage): - Loss: alkene region entirely (C=C cleaved). - Gain: new C=O peaks at 1700-1750 cm⁻¹; new aldehyde/ketone signals in ¹H and ¹³C NMR.

Hydrogenation (alkane): - Loss: vinyl H, C=C peak. - Gain: 2 new sp³ CH at 1-2 ppm; ¹³C of new sp³ at 10-50 ppm.

These spectroscopic signatures verify each transformation. Modern alkene synthesis is verified by NMR, IR, and MS; the alkene starting material's signature is gone, the product's signature is there.

16.20 Worked synthesis problems

Problem A: 2-methyl-2-butene → 2-methyl-2-butanol

The alkene is trisubstituted; tertiary cation stable. Strategy: - Acid-catalyzed hydration (H₂SO₄/H₂O): direct Markovnikov; tertiary alcohol; no rearrangement (3° cation already most stable). - Or oxymercuration (cleaner but unnecessary here).

Problem B: 1-pentene → pentan-1-ol (anti-Markovnikov)

• BH₃·THF, then H₂O₂/NaOH → 1-pentanol.
• Anti-Markovnikov regiochemistry; B (then OH) at less-substituted C.

Problem C: cyclopentene → cis-1,2-cyclopentanediol

• OsO₄/NMO → cis-1,2-cyclopentanediol (syn dihydroxylation).
• Both -OH on the same face of the ring.

Problem D: cyclohexene → trans-1,2-cyclohexanediol

• mCPBA → cyclohexene oxide (epoxide).
• H₂O/H⁺ → trans-1,2-cyclohexanediol (anti opening of epoxide).
• Two-step pathway gives the opposite stereochemistry to the OsO₄ direct method.

Problem E: 1-pentene → pentanal (anti-Markov aldehyde)

Strategy: 1. BH₃·THF → trialkylborane (anti-Markov). 2. H₂O₂/NaOH → 1-pentanol (anti-Markov primary alcohol). 3. PCC or Swern → pentanal.

Or alternative: 1. Hydroboration with 9-BBN. 2. CrO₃·HCl or PCC → aldehyde directly.

Problem F: cyclohexene → adipaldehyde (cleave + 2 CHO)

• O₃, then Zn/HOAc (reductive workup) → 1,6-hexanedial = adipaldehyde.

Problem G: cyclohexene → adipic acid (cleave + 2 COOH)

• O₃, then H₂O₂ (oxidative workup) → adipic acid (1,6-hexanedioic acid).
• Major industrial synthesis (~3 million tons/year for nylon).

Problem H: terminal alkene → chiral epoxide

• Sharpless asymmetric epoxidation (with allylic alcohol functional group).
• Or Jacobsen Mn-salen catalyst (for non-allylic alcohols).

Problem I: ester-protected alkene → diol without affecting ester

• OsO₄/NMO is selective for the alkene; the ester is unreactive.
• Pd/C + H₂ would also leave the ester unreacted.
• Don't use KMnO₄ (might oxidize the ester).

Problem J: alkene + ester → both reacted

• Need different reagents for different groups.
• For the alkene + ester both reduced: H₂/Pt + LiAlH₄ in sequence.

These problems illustrate the kind of decision-making that synthesis chemists do every day.

16.21 Stereoelectronic considerations

Why cyclic 3-membered intermediates open anti

The bromonium ion (or mercurinium, or epoxide) has its 3-membered ring's σ orbital antibonding between the two C atoms. A nucleophile approaching from anti to one C-X bond attacks into this σ, breaking the C-X bond. From the syn direction, the σ* is not accessible.

This is why all 3-membered cyclic intermediates open anti. It's a stereoelectronic requirement, not just a geometric one.

Why concerted cyclic TSs are syn

The cyclic concerted TS (e.g., the butterfly TS of mCPBA, the [3+2] of OsO₄, the 4-centered of BH₃) has bonds forming on one face of the alkene only. The transition state geometry forces this — the new bonds can only form on the side where the reagent is approaching.

Hyperconjugation and Markovnikov direction

In a carbocation pathway, the more-substituted cation is preferred because of hyperconjugation: the σ-bonded alkyl groups donate electron density into the empty p orbital. This stabilizes the cation. Markovnikov's rule reflects this: the more-substituted side gets the cation.

Steric effects in hydroboration

For BH₃, the larger boron prefers the less hindered (less-substituted) C. This is steric control of regiochemistry. Bulkier boranes (9-BBN, disiamylborane) accentuate this preference.

16.22 The chemistry of common alkene products

Vinyl chloride → PVC

Industrial: ethylene + HCl/Cu catalyst → vinyl chloride (CH₂=CHCl). Vinyl chloride polymerizes via radical addition to PVC (polyvinyl chloride). PVC is the third most produced plastic globally (~50 Mt/year), used in pipes, flooring, and many other applications.

Styrene → polystyrene

Ethylbenzene → styrene (by dehydrogenation; Pd or other catalyst). Styrene polymerizes to polystyrene (~25 Mt/year). Used in foam packaging, disposable cups, plastic models. Polystyrene is the alkene addition polymerization at largest scale.

Acrylonitrile

Propylene + NH₃ + O₂ → acrylonitrile. ~7 Mt/year. Used in nitrile rubber (gloves, hoses), acrylic fibers (Orlon, Acrilan), and plastic resins.

Ethylene glycol

Ethylene + O₂ → ethylene oxide → ethylene glycol. ~30 Mt/year. Used in antifreeze and as a building block for polyethylene terephthalate (PET, ~80 Mt/year).

These industrial polymers depend on alkene addition chemistry — the chemistry of Chapter 16 — at megaton scale.

16.23 Modern developments and future directions

Photoredox alkene chemistry

Modern photoredox catalysis (Ch 40) uses visible light + iridium or ruthenium photocatalysts to generate radical intermediates from alkenes. New transformations: - Anti-Markovnikov hydration via radical addition. - Hydroamination (alkene + N-H → amine). - Carboamination (alkene + amine + carbon source → β-amino carbonyl).

These complement classical methods and offer milder conditions.

Earth-abundant metal catalysis

Replacing precious metals (Pd, Pt, Rh, Ru, Os) with earth-abundant first-row metals (Fe, Co, Ni, Cu, Mn) is a major modern trend: - Fe-catalyzed hydroboration: same regiochemistry as Pd. - Ni-catalyzed alkene hydrofunctionalization: many new products. - Mn-catalyzed asymmetric epoxidation: Jacobsen-Katsuki methods.

These reduce cost and environmental impact.

Electrochemical alkene chemistry

Electrochemical oxidation/reduction of alkenes is gaining traction: - Anodic oxidation to give radical cations → C-C coupling, dihydroxylation. - Cathodic reduction to give radical anions → carbanion-like chemistry.

Avoids stoichiometric oxidants/reductants; uses electrons as the "reagent."

Flow chemistry

Continuous-flow reactors handle alkene additions at scale: - Hydrogenation in flow with packed-bed Pd catalyst. - Ozonolysis in flow (safer than batch, no ozonide accumulation). - Asymmetric epoxidation in flow.

Improves safety, scalability, and reproducibility.

Biocatalysis

Engineered enzymes can perform alkene additions: - Hydratases: alkene + H₂O → alcohol (reversible). - Halohydrin halogenases: alkene + HOCl → halohydrin with high ee. - Engineered P450s: epoxidation with high ee. - Squalene-hopene cyclases: alkene cyclization to terpenoids.

Often greener (water solvent, mild conditions) than chemical catalysis.

16.24 The mechanism-first thesis revisited

Looking back at this chapter, notice the recurring pattern:

For each new reaction, you don't memorize an arbitrary list of regiochemistry/stereochemistry. You ask: 1. What kind of intermediate? (cation, cyclic 3-mem, concerted) 2. What direction does the nucleophile come from? (anti for 3-mem cyclic, syn for concerted) 3. What's the most stable cation? (for Markovnikov direction) 4. Could it rearrange? (for cation pathways)

These four questions reproduce all the rules of Chapter 16. Once you've internalized this pattern, you don't need to memorize each reaction individually — you derive its outcomes from the mechanism.

This is the mechanism-first thesis of this textbook in action: a small number of principles applied consistently is more powerful than a long memorized list.

16.25 Common mistakes and pitfalls

Common Mistake 16.3 — Confusing regiochemistry across reactions. Acid hydration gives Markovnikov (-OH on more-substituted C); hydroboration gives anti-Markovnikov (-OH on less-substituted C). Read the reagent first; the regiochemistry follows.

Common Mistake 16.4 — Forgetting that catalytic hydrogenation is selective. Pd/C reduces alkenes but not aromatics, ketones, or amides at typical conditions. To reduce the ketone, you'd need Pt or Raney Ni. To reduce just the alkene of an alkene-containing aromatic ester, Pd/C is your tool.

Common Mistake 16.5 — Drawing the bromonium opening with both Br's on the same face. The bromonium opens anti — water (or other nucleophile) attacks from the opposite face of Br. Result: trans dibromide (or trans bromohydrin), not cis.

Common Mistake 16.6 — Ignoring rearrangement potential in acid hydration. A 2° → 3° hydride shift happens fast in superacids. Always check whether the initial cation could rearrange to a more stable one.

Common Mistake 16.7 — Confusing OsO₄ syn-diol with mCPBA + H₂O anti-diol. Same molecular formula (1,2-diol) but different stereochemistry. OsO₄ direct = cis (syn); mCPBA + acid hydrolysis = trans (anti). Choose carefully based on your synthetic target.

Common Mistake 16.8 — Using ozonolysis for partial cleavage. Ozonolysis cleaves all alkenes in the molecule; it's not selective for one. For partial cleavage, use OsO₄/NaIO₄ on a specific alkene with selective masking of others.

16.26 Summary

1. Acid-catalyzed hydration: alkene + H₂O + H⁺ → Markovnikov alcohol (cation; rearrangement possible).
2. Oxymercuration: alkene + Hg(OAc)₂/H₂O + NaBH₄ → Markovnikov alcohol, no rearrangement.
3. Hydroboration-oxidation: alkene + BH₃ + H₂O₂/NaOH → anti-Markovnikov alcohol, syn addition. Brown's discovery; Nobel 1979.
4. Halogenation (Br₂): alkene + Br₂ → 1,2-dibromide, anti.
5. Halohydrin (Br₂/H₂O): bromohydrin with -OH at more-substituted C, anti.
6. Epoxidation (mCPBA): alkene + RCO₃H → epoxide, syn (retains alkene stereochemistry). Butterfly TS.
7. Asymmetric epoxidation (Sharpless or Jacobsen): chiral catalyst gives chiral epoxide. Sharpless Nobel 2001.
8. Dihydroxylation (OsO₄/NMO): alkene → cis-diol, syn. NMO is catalyst regenerator.
9. Sharpless AD: asymmetric version with chiral cinchona ligands. AD-mix-α and AD-mix-β kits.
10. Ozonolysis: alkene + O₃ → carbonyls (reductive workup) or COOHs (oxidative workup). Cleaves the C=C.
11. Hydrogenation (H₂/Pd): alkene → alkane, syn. Pd/C is mildest; Pt and Ni more reactive.
12. Asymmetric hydrogenation (Knowles, Noyori): chiral Rh or Ru catalyst gives chiral alkane. Nobel 2001.
13. Lindlar Pd: alkyne → cis-alkene (Ch 17).
14. Multi-step chains: alkene → epoxide → diol; alkene → halohydrin → epoxide; alkene → diol → carbonyl; etc.
15. Industrial scale: polyethylene (110 Mtons/year), polypropylene (75), ethylene oxide (30), PVC (50), acrylonitrile (7).
16. Biological connections: terpene biosynthesis, prostaglandin synthesis, cholesterol biosynthesis, COX-targeted drugs.
17. Spectroscopic verification: IR (loss of C=C peak; new functional group peak); NMR (loss of vinyl 5-6 ppm signal; new sp³ peaks).

Master the toolbox. Pick the right reagent for the desired regio-, stereo-, and chemoselectivity. Use asymmetric variants for chiral targets. Chapter 17 turns to alkynes; the chemistry is similar but with a triple bond.