Chapter 15 — Alkenes: Structure, Stability, and Electrophilic Addition

"If alkyl halides + nucleophile are the substrate-electrophile pair of Part III, then alkenes + electrophile are the pair of Part IV. The π electrons of the alkene are the nucleophile. Now you understand the symmetry." — organic chemistry teaching aphorism

"An alkene is just two electrons sitting above and below a plane, looking for an electrophile to attack."

Part III had alkyl halides as the substrate — electrophilic carbon being attacked by a nucleophile. Part IV flips roles: the alkene is now the nucleophile, and the electrophile attacks it. This inversion is one of the most beautiful symmetries in organic chemistry.

An alkene has a C=C double bond consisting of one σ bond + one π bond. The two electrons of the π bond sit in a π orbital above and below the plane of the C=C. That pair of electrons is the nucleophile. Any electrophile (H⁺ from HCl, Br⁺ from Br₂, O³⁺-style from peroxides) can grab them.

By the end of this chapter you should be able to: - Describe alkene structure (sp² hybridization, restricted rotation, geometric isomers). - Predict alkene stability (degree of substitution, hyperconjugation). - Assign E/Z to disubstituted alkenes using CIP priorities. - Draw the mechanism of electrophilic addition (HX to alkene as the archetype). - Apply Markovnikov's rule and explain why it works (carbocation stability). - Predict regiochemistry, stereochemistry, and the possibility of rearrangement. - Recognize when bromonium-style mechanisms apply.

15.1 Alkene structure and bonding

An alkene has a $C=C$ double bond — one σ + one π. Both carbons are sp² hybridized: - Three sp² hybrid orbitals form three σ bonds (to two substituents + the other carbon). - One unhybridized p orbital forms the π bond with the other carbon's p orbital.

Geometry: - Bond angles around each sp² carbon: ~120°. - The four substituents + two carbons of the C=C are coplanar. - Bond length: C=C is ~1.34 Å (shorter than C-C single bond at ~1.54 Å). - Bond strength: C=C ~146 kcal/mol (vs. C-C ~80 kcal/mol). The π bond contributes ~66 kcal/mol.

Restricted rotation: rotating around the C=C breaks the π bond, costing ~60 kcal/mol. So substituents on the C=C are locked in their geometric positions — alkenes can have cis/trans (or E/Z) isomers.

E/Z nomenclature

For a disubstituted alkene with four different substituents, the geometric isomers are named E (entgegen, "opposite") or Z (zusammen, "together") based on CIP priority:

• E: higher-priority substituents on opposite sides of the C=C.
• Z: higher-priority substituents on the same side of the C=C.

CIP priority: atomic number of the first atom in each substituent (higher Z = higher priority); ties broken by atoms further from C.

For cis/trans (the older nomenclature): - cis: same side (often Z, but not always). - trans: opposite sides (often E).

The E/Z system is preferred in modern IUPAC nomenclature.

Stereochemistry impact on properties

Cis-alkenes have larger substituents on the same side → more steric strain. Trans-alkenes have substituents apart → less strain.

Examples: - cis-2-butene: m.p. -139 °C, b.p. 4 °C. - trans-2-butene: m.p. -106 °C, b.p. 1 °C. - The cis isomer is slightly less stable (due to steric strain) but has a higher b.p. (more polar).

For larger alkenes, the cis/trans difference in stability becomes more pronounced.

15.2 Alkene stability

Alkene stability increases with substitution. More alkyl groups attached to the C=C → more stable alkene. Why?

Hyperconjugation

Alkyl C-H bonds adjacent to the C=C donate electron density into the π* (antibonding) orbital. This stabilizes the alkene by reducing the antibonding character.

The more alkyl groups on the C=C, the more hyperconjugation, the more stable the alkene.

Stability order (most to least stable)

Approximate energy differences: ~3 kcal/mol per substitution. Trans-disubstituted is ~1 kcal/mol more stable than cis-disubstituted (sterics).

Why this matters

• Zaitsev's rule (Ch 12): E2 elimination preferentially forms the more-substituted (more-stable) alkene.
• Hydrogenation (Ch 16): more-substituted alkenes have lower heats of hydrogenation.
• Markovnikov addition (Section 15.3): goes through the more-stable carbocation, which gives the more-substituted alkene-derived product.

Hyperconjugation in alkene stability ↔ hyperconjugation in carbocation stability ↔ Markovnikov rule. They are all the same chemistry.

Worked Problem 15.1: Rank these alkenes by stability: (a) 2-methyl-2-butene, (b) 1-pentene, (c) trans-2-pentene, (d) cis-2-pentene.

Solution: (a) trisubstituted — most stable. (c) disubstituted, trans — second. (d) disubstituted, cis — third (sterically hindered). (b) monosubstituted (terminal) — least stable.

15.3 Electrophilic addition: the archetype reaction

The simplest electrophilic addition to an alkene is addition of HCl (or HBr, HI): $$R_2C=CR_2 + HCl \to R_2CH-CR_2Cl$$

The alkene's π bond donates its two electrons to HCl, attacking the H. The H-Cl bond breaks heterolytically — Cl leaves with both bonding electrons. Net result: a carbocation intermediate.

Mechanism (two steps)

Step 1 (slow, rate-determining): the π bond attacks HCl. Proton adds to one carbon; chloride dissociates. A carbocation forms on the other carbon.

Step 2 (fast): chloride attacks the carbocation. The C-Cl bond forms. Product is the alkyl chloride.

Mechanism Map 15.1: Electrophilic addition of HCl to propene.

Step 1: π bond of propene attacks HCl's H. Proton adds to the terminal CH₂. Carbocation forms at the central CH (a 2° carbocation). Chloride is released.

Step 2: Chloride attacks the 2° carbocation. New C-Cl bond forms. Product: 2-chloropropane.

The 2-chloropropane is the Markovnikov product (H ends up on the less-substituted C; Cl on the more-substituted C).

15.4 Markovnikov's rule

Vladimir Markovnikov (1869) observed: when HX adds to an unsymmetrical alkene, H goes to the carbon with more H's; X goes to the carbon with fewer H's (the more-substituted one).

Why Markovnikov works (carbocation stability)

In step 1 of electrophilic addition, the proton can in principle add to either carbon. The resulting carbocation is on the OTHER carbon. The more-stable carbocation forms preferentially (Hammond postulate: lower-energy intermediate goes through lower-energy TS).

For propene + HCl: - If H adds to terminal CH₂ → 2° carbocation at the central CH. Stable (alkyl groups stabilize via hyperconjugation). - If H adds to central CH → 1° carbocation at the terminal CH. Unstable.

The 2° pathway dominates. Halide attacks the 2° cation. Result: 2-chloropropane (Markovnikov product).

Carbocation stability ranking

$$3° > 2° > 1° > methyl$$

Each alkyl group adjacent to the carbocation stabilizes it through hyperconjugation. (Allyl and benzyl are also stabilized by resonance.)

Markovnikov vs. anti-Markovnikov

Markovnikov product (the major one for HCl, HBr, HI on alkene): more-substituted carbon gets the X.

Anti-Markovnikov product: less-substituted carbon gets the X. Two main scenarios: 1. HBr + peroxides (radical addition, Ch 18): radical chain mechanism gives the more-stable radical at the less-substituted C. 2. Hydroboration-oxidation (Section 16.5): uses B-H instead of H-X; the boron adds to the less-substituted C due to steric considerations.

Worked example

Worked Problem 15.2: Predict the major product of (a) 2-methyl-2-butene + HCl, (b) (Z)-3-hexene + HBr.

Solution: (a) 2-methyl-2-butene: trisubstituted. With HCl, protonation gives the more-stable 3° carbocation: 2-chloro-2-methylbutane. (b) (Z)-3-hexene: symmetric internal alkene. Both carbons of the C=C are equivalent (each with 1 H + 1 ethyl). Product is 3-bromohexane (Markovnikov gives the same result either way; both carbons are 2°).

15.5 Stereochemistry of electrophilic addition

When HX adds to a substituted alkene, the carbocation is sp² (planar). The halide can attack from either face. So: - HX addition often gives mixtures of stereochemistry — both syn (same face) and anti (opposite faces) addition products. - For a specific stereo-defined alkene, you typically get a 1:1 mix of cis and trans products of the addition.

This is in contrast to: - Br₂ or I₂ addition (Section 15.6): goes through a halonium ion, giving anti exclusively. - Hydroboration (Section 16.5): syn addition exclusively. - OsO₄ (Section 16.7): syn addition.

15.6 Halogenation: Br₂ and I₂ via halonium ion

Bromine (Br₂) addition to alkenes produces anti addition — the two bromines end up on opposite faces. This is mechanistically distinct from HX addition.

The bromonium ion mechanism

When Br₂ approaches an alkene: 1. The alkene's π electrons attack one Br atom. The Br-Br bond breaks; the other Br leaves as Br⁻. 2. A three-membered bromonium ion forms — a cyclic intermediate where one Br bridges the two carbons of what was the C=C. Both carbons are partially bonded to Br. 3. Br⁻ attacks the bromonium ion from the opposite face, opening the ring. The new C-Br σ bond forms with backside-attack stereochemistry. 4. Net result: anti-1,2-dibromide. The two Br atoms are on opposite faces of the original alkene.

This is one of the most stereospecific reactions in organic chemistry. The bromonium ion's geometry dictates the anti product directly.

Mechanism Map: Bromonium ion formation

$$\text{alkene} + Br_2 \to \text{bromonium ion}^+ + Br^-$$

Bromonium ion structure: a cyclic 3-membered ring with the Br at the apex and two C atoms forming the base. Both C atoms are partially bonded to Br (the bonds are weaker than typical σ bonds, but cyclic).

When Br⁻ attacks one C from the opposite face of the Br, the C-Br bond on the other side breaks, giving a planar C=C bond again — but now with two Br atoms attached, anti.

Iodonium and chloronium ions

I₂ behaves similarly (iodonium ion intermediate). Cl₂ also gives a chloronium ion but is less commonly used (Cl₂ also does radical reactions; selectivity issues).

Why anti?

The bromonium ion is a 3-membered ring with strain. Opening the ring requires backside attack on the C atom. This is the same chemistry as SN2 on a 3-membered ring (epoxide opening, etc., Ch 16). The stereochemistry: anti.

Specifically: cis-2-butene + Br₂ → meso-2,3-dibromobutane (the two Br atoms anti, with the methyls also anti). trans-2-butene + Br₂ → racemic 2,3-dibromobutane (the two Br anti, methyls cis).

This is a textbook stereospecific reaction.

15.7 Hydration: H₂O addition

In aqueous acid, alkenes undergo acid-catalyzed hydration to give alcohols: $$R_2C=CR_2 + H_2O + H^+ \to R_2C(OH)-CR_2H$$

Mechanism: 1. Acid (H⁺ from H₂SO₄) protonates the alkene; carbocation forms at the more-substituted C (Markovnikov). 2. Water attacks the carbocation (with its lone pair, becomes positively charged on O). 3. Deprotonation gives the alcohol.

Markovnikov-selective (more-substituted alcohol). Used industrially to make ethanol from ethylene, sec-butanol from 1-butene, etc.

Anti-Markovnikov hydration uses hydroboration-oxidation (Section 16.5).

15.8 Carbocation rearrangements

Because electrophilic addition goes through a carbocation, rearrangements can occur. A 2° carbocation formed initially might migrate via 1,2-hydride or 1,2-methyl shift to a more-stable 3° cation before the halide attacks.

1,2-hydride shift

A H atom adjacent to the cation migrates with its electrons to give a more-stable cation: $$R-CH(+)-CH_2-CH_3 \to R-CH_2-CH(+)-CH_3$$

If the new cation is more stable (e.g., 3° instead of 2°), the rearrangement is favored.

1,2-methyl shift (and 1,2-alkyl shift)

A CH₃ (or other alkyl) group can migrate similarly: $$R-C(CH_3)_2-CH_2-CH_2(+) \to R-C(CH_3)_2(+)-CH_2-CH_3$$

The methyl moves with its bonding electrons, creating a new cation at the former methyl-bearing carbon.

Watching for rearrangements

Watch for: - 2° carbocation adjacent to a 3° carbon (potential 1,2-H shift to give 3°). - 1° carbocation adjacent to a 2° or 3° carbon (will rearrange spontaneously; 1° cations rarely persist). - Strained ring systems that can open via cation migration.

If a rearrangement is possible, the product reflects the rearranged cation, not the original one.

Common Mistake 15.1: Forgetting to consider rearrangement when predicting the product. If your predicted product involves a 1° or unstable 2° cation, check if a 1,2-shift could give a more-stable 3° cation. The product is often surprisingly different from the initial expectation.

15.9 Spectroscopy of alkenes

Alkenes have characteristic spectroscopy:

IR

• C=C stretch: 1620–1680 cm⁻¹ (medium intensity).
• =C-H stretch (vinyl H): 3000–3100 cm⁻¹ (slightly higher than alkane C-H).
• =C-H bend: 700–1000 cm⁻¹ (out-of-plane bending).

For trans alkenes: characteristic absorption at 970 cm⁻¹. For cis: at 700–730 cm⁻¹.

¹H NMR

• Vinyl H: δ 4.5–6.5 ppm (deshielded by C=C π).
• For monosubstituted alkene (R-CH=CH₂): three signals — terminal CH₂ (~5.0), vinyl CH (~5.7).
• Coupling between vinyl Hs:
• cis (Z): J ≈ 6–12 Hz.
• trans (E): J ≈ 12–18 Hz.
• geminal: J ≈ 0–3 Hz.

The coupling pattern can identify cis vs trans and even unambiguously assign E/Z for some substrates.

¹³C NMR

Vinyl C atoms at δ 100–145 ppm. Substituted vinyl C: ~110–140; unsubstituted: ~115–125.

Mass spec

Alkene fragmentation: cleavage of allylic C-H or C-C bonds. The allylic cation (resonance-stabilized) is a common fragment.

15.10 Industrial alkenes

Alkenes are central to industrial chemistry:

• Ethylene (C₂H₄): 200+ million tons/year. Made from steam cracking of ethane or naphtha. Used to make polyethylene, ethylene oxide (then ethylene glycol), styrene, vinyl chloride, ethanol.
• Propylene (C₃H₆): 100+ million tons/year. Used to make polypropylene, acrylonitrile (ACN for Spandex), cumene (for phenol/acetone).
• Butadiene (CH₂=CH-CH=CH₂): 10+ million tons/year. Used to make synthetic rubber (SBR, polybutadiene), neoprene, ABS plastic.
• Styrene (PhCH=CH₂): 25+ million tons/year. Polymerized to polystyrene (foam, packaging).

The alkene functional group is the input to most polymer chemistry.

15.11 Why this chapter matters

Alkenes are the central electrophile-acceptor (or electrophile-donor, more accurately — the alkene is the nucleophile here) of organic synthesis. They are: - Substrates for electrophilic addition (Ch 15-16). - Substrates for radical chemistry (Ch 18). - Substrates for Diels-Alder (Ch 19). - Building blocks for polymers (industrial use). - Intermediates in many natural product syntheses.

Mastering alkene chemistry is essential for the rest of Part IV and for understanding much of organic synthesis.

15.12 Alkene structure in detail

Bond properties of C=C

The C=C double bond has: - 1 σ bond (sp² + sp²): bond length ~1.34 Å. - 1 π bond (p_z + p_z perpendicular to molecular plane): weaker than σ. - Total bond strength ~146 kcal/mol. - C-H bonds (sp²-1s): bond length ~1.09 Å; bond strength ~110 kcal/mol.

The π bond is weaker than the σ but stronger than alkene's σ. Approximate breakdown: σ ~85 kcal/mol; π ~60 kcal/mol.

Restricted rotation

Rotation around the C=C requires breaking the π bond — ~60 kcal/mol. This is far above thermal energy at room T (RT ~ 0.6 kcal/mol). So: - The π bond doesn't rotate at room T. - E and Z isomers don't interconvert thermally. - They are distinct stereoisomers (configurational isomers).

This contrasts with single C-C bonds (Ch 5), which rotate freely (3 kcal/mol barrier).

Geometry

The two sp² carbons of a C=C sit at the corners of a planar geometry. Each carbon has: - 3 substituents at ~120° angles. - Trigonal planar.

For a 1,2-disubstituted alkene: the two substituents on each C lie in the alkene plane; one above and one below. The cis/trans relationship is set.

Examples of alkene shapes

• Ethylene (HC=CH₂): planar, 4 H's all in plane.
• (E)-2-butene: methyls on opposite sides of C=C; planar; methyls equatorial.
• (Z)-2-butene: methyls on same side; planar; minor steric clash between methyls.
• Propene (CH₂=CHCH₃): mono-substituted; planar.
• Cyclohexene: 6-membered ring with C=C; the C=C is in a planar half-chair.

15.13 Stability of alkenes

The stability of an alkene depends on substitution:

Hierarchical stability

• Tetrasubstituted (R₂C=CR₂) > trisubstituted (R₂C=CHR or RCH=CR₂) > disubstituted (RCH=CHR or R₂C=CH₂) > monosubstituted (RCH=CH₂) > unsubstituted (CH₂=CH₂).

The energy difference is ~2-3 kcal/mol per substituent.

Why?

Three factors:

Hyperconjugation: alkyl substituents donate electron density into the π* (alkene's antibonding orbital), stabilizing the alkene. More substituents = more donation.

Inductive: alkyl groups slightly donate; weak effect.

Steric: 1,1-disubstituted is slightly more stable than 1,2-disubstituted; cis isomer is slightly less stable than trans (steric clash between cis substituents).

E/Z relative stability

For 2-butene: - (E)-2-butene: trans methyls; lower energy. - (Z)-2-butene: cis methyls; ~1 kcal/mol higher (steric clash).

For larger substituents, the cis/trans difference grows. (E)-stilbene (PhCH=CHPh) is much more stable than (Z)-stilbene (where the bulky phenyls clash).

Stability of strained alkenes

Cyclic alkenes have additional considerations: - Cyclohexene: nearly strain-free (planar half-chair). - Cyclopentene: slight angle strain. - Cyclobutene: significant angle strain. - Cyclopropene: enormous strain (60° internal angles vs ideal 120°).

trans-Cyclooctene is the smallest stable trans-cycloalkene; ~10 kcal/mol of ring strain.

15.14 The π bond and π* orbital

The π bond of an alkene is formed from two p orbitals (one on each sp² C) overlapping side-by-side.

Bonding π orbital (HOMO)

The bonding π combines the two p orbitals constructively (in phase). Has a node along the C=C axis. The electrons in the π bond are above and below the plane of the alkene.

Antibonding π* orbital (LUMO)

The antibonding π* has a node perpendicular to the C=C axis (between the two carbons, in addition to the node along the axis). Higher energy than π.

Reactivity

• The HOMO (π bonding) is the nucleophile in electrophilic addition: it attacks electrophiles.
• The LUMO (π*) is the acceptor in conjugate addition (Ch 29) or in metal coordination (Ch 37).

MO diagram

For ethylene: - 2 carbon p orbitals → 1 π (lower) + 1 π (higher). - Energy gap π → π: ~7 eV (UV absorption around 165 nm).

For conjugated dienes (Ch 19), the MO picture is richer; HOMO is higher in energy and the gap is smaller.

15.15 The mechanism of electrophilic addition

The general mechanism for electrophilic addition to an alkene:

Step 1: π bond attacks electrophile

The alkene π electrons attack the electrophile (e.g., H⁺ from HBr, Br from Br₂, etc.). This forms: - A carbocation if the electrophile is small (H⁺): the alkene's π electrons make a new σ to H; the other C bears the positive charge. - A bridged 3-membered cation if the electrophile is large/halogen (Br, Cl, Hg, etc.): the electrophile bridges both carbons.

Step 2: nucleophile attacks

The nucleophile (e.g., Br⁻ from HBr) attacks the cation: - For an open carbocation: from either face; gives mostly Markovnikov product but can rearrange. - For a bridged 3-membered cation: anti-attack; gives anti dibromide (or anti halohydrin if H₂O is present).

Markovnikov regiochemistry

For an open carbocation, the more-stable cation forms preferentially. The H ends up on the less-substituted C; the X ends up on the more-substituted C.

Stereochemistry

For a bridged ion, stereochemistry is anti. For an open cation, stereochemistry is partly random (modest preference for inversion via "ion pair" effect).

15.16 Examples of electrophilic addition

HBr addition to propene

Propene + HBr → 2-bromopropane (Markovnikov; major) + 1-bromopropane (anti-Markov; minor).

Major product: -Br on the more-substituted C (the central C, which was the more-stabilized cation).

In presence of peroxides: anti-Markov; -Br on the less-substituted C (radical mechanism, Ch 18).

Bromine addition to cyclohexene

Cyclohexene + Br₂ → trans-1,2-dibromocyclohexane (anti addition).

Bromonium ion intermediate; back-side attack by Br⁻; anti dibromide.

HCl addition to 2-methyl-2-butene

2-methyl-2-butene + HCl → 2-chloro-2-methylbutane (Markov; very stable 3° cation).

Hydration of 1-butene

1-butene + H₂O + H₂SO₄ → 2-butanol (Markov).

Hydration of 2-methyl-2-butene

2-methyl-2-butene + H₂O + H₂SO₄ → 2-methyl-2-butanol (3° alcohol; the most-stable cation gives the major product).

Halohydrin formation

Cyclohexene + Br₂/H₂O → trans-2-bromocyclohexan-1-ol.

Bromonium ion; water attacks the more-substituted C (more cation-like); Br ends up at the less-substituted C, OH at the more.

15.17 Markovnikov vs anti-Markovnikov

The Markovnikov rule emerges from carbocation stability: - More substituted C = more stable cation. - Cation stability: 3° > 2° > 1° (hyperconjugation + inductive). - Markov: H on less-substituted; nucleophile on more-substituted.

When Markov holds

• HX (HBr, HCl, HI) addition (without peroxides).
• H₂O hydration (acid-catalyzed).
• Halohydrin formation (in aqueous solution).
• Most cation-mediated electrophilic additions.

When anti-Markov holds

• HBr + peroxide (radical mechanism; Ch 18).
• Hydroboration-oxidation (concerted; Ch 16).
• The peroxide effect is specific to HBr; doesn't work for HCl or HI.

Why are Markov rules sometimes broken?

Anti-Markov radical addition: the radical mechanism doesn't go through a cation. Br• adds to the less-hindered C; the resulting radical is more stable; H from HBr adds. Result: anti-Markov.

Hydroboration: the concerted mechanism puts B on the less-substituted C (steric) and H on the more-substituted C; the BH then becomes BO-H upon oxidation. Result: anti-Markov alcohol.

These mechanism-specific exceptions confirm the rule's dependence on cation chemistry.

15.18 Carbocation rearrangements: deeper analysis

1,2-hydride shift

A hydrogen atom adjacent to a cation migrates with its bonding electrons:

R₃C⁺-CR₂H → R₃C-H + R₂C⁺

The original cation gets the H; the C that lost the H now has the positive charge.

Why: if the new cation is more stable than the original.

Common examples: - 2° → 3° cation: 1,2-H shift to a 3° C-H produces 3° cation. Strongly favored. - 1° → 2° (or 3°): 1° cation almost never persists; rearranges to a 2° or 3° cation immediately.

1,2-methyl shift (or alkyl shift)

A CH₃ (or other R) adjacent to the cation migrates:

R₂C(CH₃)-CR₂⁺ → R₂C⁺-CR₂(CH₃)

The methyl moves with its bonding electrons. The original cation gets the methyl; the new cation is on the C that had the methyl.

Common when: - 2° → 3° via methyl shift (e.g., neopentyl rearrangement).

Watch for rearrangements

Rearrangement is likely when: - A 2° cation could be promoted to a 3° by 1,2-H shift. - A 1° cation is impossible; it rearranges immediately. - A strained ring system can open via rearrangement.

The product reflects the rearranged cation, not the original.

Famous rearrangement examples

• Wagner-Meerwein rearrangement: 1,2-shift in pinacol/pinacolone rearrangement.
• Beckmann rearrangement: ketoxime → amide (aryl/alkyl shift).
• Pinacol rearrangement: 1,2-diol → ketone (via carbocation rearrangement).
• Norbornyl cation rearrangement: classical study of non-classical cation chemistry.

These are all mechanism studies rich with cation chemistry.

15.19 Industrial applications

Polymerization

Alkenes polymerize via radical addition (Ch 18) or coordination polymerization (Ziegler-Natta, Ch 37).

Industrial polymer scale: - Polyethylene: ~110 million tons/year (HDPE + LDPE + LLDPE). - Polypropylene: ~75 million tons/year. - Polystyrene: ~25 million tons/year. - Poly(vinyl chloride): ~50 million tons/year.

Combined: ~260 million tons of polymers per year, all from alkene chemistry.

Petrochemicals

• Ethylene + Cl₂ → ethylene dichloride → vinyl chloride → PVC.
• Ethylene + O₂ → ethylene oxide → ethylene glycol (antifreeze).
• Propene + NH₃ + O₂ → acrylonitrile (Spandex precursor).
• Propene + benzene + cat → cumene → phenol + acetone.

Specialty chemicals

• Synthetic rubber (SBR, polybutadiene): from butadiene.
• Surfactants from olefins.
• Alpha-olefins for detergent production.

The alkene functional group is at the heart of modern petrochemistry.

15.20 Biological alkenes

Alkenes are common in biology:

Terpenoids

Plant essential oils (limonene, menthol, pinene) are terpenoids built from isoprene units. Each isoprene has 2 C=C; terpenes can have many alkenes.

Examples: - Limonene (orange/lemon scent): 2 C=C; chiral. - Menthol: contains a cyclohexane with a chair conformation; the chiral menthol is the cooling-flavor isomer. - β-carotene: 11 conjugated C=C; orange color of carrots. - Vitamin A: 5 conjugated C=C from oxidation/cleavage of β-carotene.

Fatty acids

Many fatty acids have alkenes (cis usually): - Oleic acid (olive oil): 18:1 ω-9; one cis C=C. - Linoleic acid (vegetable oils): 18:2 ω-6; two cis C=C. - Arachidonic acid: 20:4 ω-6; four cis C=C. - DHA, EPA (fish oil): polyunsaturated; multiple cis C=C.

The cis configuration of biological alkenes is critical: it gives membrane fluidity and specific receptor interactions.

Pheromones

Insect pheromones are often specific alkenes with specific geometry: - (Z)-9-tricosene: aphrodisiac in some flies. - (E,Z)-2,4-hexadienol: specific cabbage moth pheromone.

Synthesizing these requires specific alkene geometry; alkyne-to-cis (Lindlar) or alkyne-to-trans (Na/NH₃) chemistry is critical.

Steroids

The cholesterol scaffold has one C=C (at C5-C6 position). This alkene is the substrate for many steroid hormones' biosynthesis.

Plant hormones

• Ethylene (CH₂=CH₂) is a plant hormone! Triggers fruit ripening, abscission. Used commercially to ripen bananas.
• Auxin (IAA): indole acetic acid; aromatic with no alkene.

Implication

Biological alkene chemistry is rich. Mastery of alkene reactions enables understanding of metabolism, drug design, and natural product synthesis.

15.21 The Hammond postulate applied to alkene addition

The Hammond postulate (Ch 5) explains Markovnikov regiochemistry:

For HBr addition to propene: - TS resembles the carbocation intermediate (since the carbocation is high-energy on the path). - More stable carbocation = lower TS = faster reaction. - Markovnikov direction: H on less-substituted (forms 2° cation) gives a TS with 2° cation character. - Anti-Markov: H on more-substituted gives a TS with 1° cation character. - 2° > 1° in stability; Markov is preferred.

This is the rigorous origin of Markovnikov's rule: it's not just empirical; it's derived from cation stability + Hammond postulate.

For other reactions: - Hydroboration: TS resembles the boron-bonded product (concerted; no cation). The boron prefers the less-substituted C (steric); anti-Markov. - Bromination: TS resembles the bromonium ion (cation-like at the more-substituted C); anti-attack by Br⁻ at the more-substituted C (where cation character is higher).

The Hammond postulate explains regiochemistry across all alkene additions.

15.22 Common reactions of alkenes (preview)

Beyond Chapter 15's electrophilic additions, alkenes participate in:

• Hydroboration-oxidation (Ch 16): anti-Markov hydration.
• Halogenation (Ch 16): Br₂, Cl₂ → vicinal dihalides.
• Halohydrin formation (Ch 16): with H₂O.
• Epoxidation (Ch 16): mCPBA.
• Dihydroxylation (Ch 16): OsO₄/NMO.
• Ozonolysis (Ch 16): O₃ → carbonyls.
• Catalytic hydrogenation (Ch 16): H₂/Pd.
• Radical addition (Ch 18): HBr + peroxide.
• Diels-Alder (Ch 19): with conjugated diene.
• Polymerization (Ch 18): radical or coordination.
• Olefin metathesis (Ch 37): exchange substituents.
• Pd cross-coupling (Ch 37): Heck, etc.

These extensions are covered in detail in subsequent chapters.

15.23 Synthesis of alkenes

Alkenes can be synthesized by:

From alkyl halides

• E2 elimination: R-CHX-CHR' + base → R-CH=CR'
• Standard synthesis route.

From alcohols

• Acid-catalyzed dehydration (E1).
• Tosylation + base (E2).

From alkynes

• Lindlar Pd: alkyne → cis alkene.
• Na in NH₃: alkyne → trans alkene.

From carbonyls (Wittig, etc.)

• Wittig reaction: aldehyde/ketone + phosphorus ylide → alkene.
• Horner-Wadsworth-Emmons: similar; gives E preferentially.
• Julia-Kocienski olefination: similar.
• Peterson olefination: silicon-based variant.

From other alkenes

• Olefin metathesis: cross-metathesis or RCM.
• Heck reaction: aryl alkene synthesis.

From alkanes (cracking)

• Industrial: ethylene from steam cracking of ethane/naphtha.

Each method has its own conditions, scope, and stereochemistry.

15.24 Practical considerations for alkene chemistry

Storage

Alkenes can polymerize or oxidize: - Polymerization: most alkenes polymerize over time (or with light, heat). Use stabilizers (BHT, hydroquinone). - Oxidation: with air and light, alkenes can form peroxides. Store in dark, sealed containers.

Common stable alkenes: cyclohexene, styrene (with stabilizer). Less stable: terminal alkenes; smaller alkenes.

Handling

• Volatile alkenes (ethylene, propylene): gas at room T.
• Liquid alkenes (cyclohexene, 1-octene): handle in fume hood; flammable.
• Solid alkenes (cinnamic acid; β-carotene): standard solid handling.

Reaction conditions

• Most alkene reactions work at room T to 100 °C.
• Cyclic systems (e.g., cyclohexene): standard chair geometry.
• Strained alkenes: more reactive (e.g., norbornene very reactive in Diels-Alder).

15.25 Worked problem 1: Predict products

A: cis-2-butene + HBr

Markovnikov addition; -Br on more-substituted C. But 2-butene is symmetric (both vinyl Cs are equally substituted = both 2°). So either Markov direction is equivalent.

Product: 2-bromobutane (single product); racemic mix at the new stereocenter.

B: 2-methyl-2-butene + HBr

Markovnikov: -H on less-substituted (the CH side); -Br on more-substituted (the C(CH₃)₂ side).

Product: 2-bromo-2-methylbutane (3° bromide; major).

C: 2-methyl-2-butene + HBr + peroxide

Anti-Markov; radical mechanism.

Product: 2-bromo-3-methylbutane (the -Br on less-substituted side; radical adds H to more-substituted; final isolated cation is more stable).

Wait, that's wrong. Let me re-think.

Anti-Markov: H goes to more-substituted C; X goes to less-substituted C.

For 2-methyl-2-butene = (CH₃)₂C=CHCH₃: - More substituted: (CH₃)₂C side. - Less substituted: CH side (with CH₃).

Anti-Markov: H goes to (CH₃)₂C side; Br goes to CH side.

Product: 2-bromo-3-methylbutane (CH₃)₂CH-CHBr-CH₃.

D: cyclopentene + Br₂

Anti addition via bromonium intermediate.

Product: trans-1,2-dibromocyclopentane (single diastereomer; chiral; racemic).

E: 1-methylcyclohex-1-ene + H₂O/H₂SO₄

Markovnikov: -OH on more-substituted (3°); -H on less-substituted.

Product: 1-methylcyclohex-1-ol (3° alcohol).

These exemplify the application of Markovnikov and stereoelectronic rules.

15.26 Computational chemistry of alkenes

DFT calculations

DFT calculations of alkenes can: - Compute alkene relative stabilities (cis vs trans, branched vs linear). - Calculate carbocation energies (TS energy estimates). - Show the Markovnikov preference quantitatively. - Visualize MOs (HOMO π, LUMO π*).

Hammond postulate by computation

DFT can compute the TS for alkene + electrophile addition. The TS geometry resembles either: - Reactant (early TS; small Hammond shift). - Product (late TS; large Hammond shift).

For exothermic additions: early TS (reactant-like). For endothermic: late TS (product-like).

The cation stability differences (Markov vs anti-Markov) translate to TS energy differences via Hammond.

Visualization

• The π HOMO of an ethylene shows electron density above and below the plane (the π electrons).
• The π* LUMO has a node along the C=C axis (in addition to the π node above and below).
• Carbocations show empty p orbital perpendicular to the planar three-substituent geometry.

Computational Exercise 15.1 — Build ethylene in Avogadro. Optimize. View the MOs. Identify HOMO (π) and LUMO (π*). Note the geometry: planar, ~120° angles.

Computational Exercise 15.2 — Build 2-bromopropane (Markov product of HBr + propene). Compare its geometry to 1-bromopropane (anti-Markov). Both are stable; the difference is in the TS energies leading to them.

15.27 Stereochemistry deep dive

Stereospecific addition

When the mechanism is concerted or via a fixed-geometry intermediate, the alkene's stereochemistry is preserved in the product.

For Br₂ addition (anti via bromonium): - (E)-2-butene → (2R,3R)- and (2S,3S)-2,3-dibromobutane (racemic chiral). - (Z)-2-butene → meso-2,3-dibromobutane.

Stereospecific syn addition (e.g., OsO₄)

For OsO₄ dihydroxylation (syn): - (E)-2-butene → (2R,3R)- and (2S,3S)-2,3-butanediol (racemic chiral). - (Z)-2-butene → meso-2,3-butanediol.

Note: the same starting alkene gives different products with different reagents! Br₂ + (E) gives chiral; OsO₄ + (E) also gives chiral. But Br₂ + (Z) gives meso; OsO₄ + (Z) also gives meso. The pattern is preserved because both are stereospecific (one cis, one trans, one anti, one syn — they map differently from substrate to product).

Stereospecificity vs stereoselectivity

• Br₂ is stereospecific: substrate stereochemistry → product stereochemistry, predictable.
• Catalytic hydrogenation is stereoselective: gives mostly one stereoisomer (the more stable cis-product on the same face) but not always 100%.
• HX addition gives partial stereospecificity (anti via bromonium-like intermediate; mostly anti).

These distinctions matter for synthesis design: you choose reagents based on the desired stereo outcome.

15.28 Common mistakes

Common Mistake 15.2 — Forgetting the regiochemistry. Markovnikov: nucleophile (H of HBr; OH of water) on more-substituted C. Anti-Markov: opposite.

Common Mistake 15.3 — Predicting cis when anti is correct (or vice versa). Br₂ goes through bromonium → anti. OsO₄ goes through cyclic ester → syn.

Common Mistake 15.4 — Forgetting carbocation rearrangements. If the initial cation could rearrange to a more stable one (e.g., 2° → 3° via 1,2-H shift), the product reflects the rearranged cation.

Common Mistake 15.5 — Thinking electrophilic addition is always cation-mediated. Hydroboration is concerted (no cation; no rearrangement); radical HBr is radical (no cation; anti-Markov).

Common Mistake 15.6 — Confusing E1 (Ch 12, alkene → from alcohol or alkyl halide) with electrophilic addition (Ch 15, alkene + reagent → addition product). Different starting materials, different products.

15.29 The chemistry of dienes (preview to Ch 19)

When two alkenes are conjugated (separated by one single bond), they form a conjugated diene:

$$CH_2=CH-CH=CH_2 \text{ (1,3-butadiene)}$$

Conjugated dienes have: - Lower energy than two isolated alkenes (~3-7 kcal/mol stabilization). - Specific UV absorption at λ_max ~217 nm. - Different addition chemistry: 1,2-addition (Markov) vs 1,4-addition (terminal addition).

The 1,4-addition is unique to conjugated dienes; in 1,3-butadiene + HBr, you get both 1,2-bromobutene (kinetic) and 1,4-bromobutene (thermodynamic).

Diels-Alder cycloaddition (Ch 19) is the most important reaction of conjugated dienes: a [4+2] cycloaddition with a dienophile.

We'll cover dienes in detail in Chapter 19.

15.30 Cumulated dienes (allenes)

When two C=C bonds share a central carbon: an allene (R₂C=C=CR'₂).

The central carbon is sp hybridized; the two C=C bonds are perpendicular to each other.

This means: - The terminal substituents are in perpendicular planes. - The molecule can be chiral if the substituents differ.

Allene chemistry: - HX addition: typically goes Markovnikov on the more reactive C=C. - Hydration: gives an enol or ketone after tautomerization. - Cycloaddition: with a partner alkene, gives a 5-membered ring.

Allenes are present in some natural products (mycomycin, certain pheromones).

Cumulated trienes and longer cumulenes are unusual; only short cumulenes are stable.

15.31 The Markovnikov-Vladimir story

Vladimir Markovnikov (1838-1904) was a Russian chemist who, in 1870, observed empirically that HX addition to alkenes gave one product preferentially. He published his rule:

"In the addition of HX to an asymmetrical alkene, the H goes to the C with more H's already attached."

This was empirical; the mechanistic basis (carbocation stability + Hammond postulate) was developed later by Whitmore (1932) and others.

The Hughes-Ingold framework

In the 1930s, Hughes and Ingold showed that: - HBr + alkene goes through a cation intermediate. - The more-stable cation gives the major product. - This explains Markov regiochemistry.

Modern understanding

Today, we understand: - Carbocation stability is the key. - The Hammond postulate says the TS resembles the cation. - More stable cation = lower TS = faster reaction. - Therefore Markov regiochemistry.

For radical addition, the analogous analysis with radical stability gives anti-Markov.

For concerted hydroboration, steric factors give anti-Markov.

The Markov rule is one of the cornerstones of organic chemistry; understanding its mechanistic basis is the key to predicting alkene reactions.

15.32 Connections to other chapters

• Chapter 5: thermodynamics, Hammond postulate (foundation for Markov rule).
• Chapter 7: stereochemistry (E/Z; chirality of products).
• Chapter 8: stereochemistry of reactions (syn/anti).
• Chapter 12: alkene synthesis by E1/E2.
• Chapter 13: SN/E vs alkene addition decisions.
• Chapter 16: full alkene addition toolkit.
• Chapter 17: alkynes (similar chemistry).
• Chapter 18: radical alkene chemistry.
• Chapter 19: conjugated dienes; Diels-Alder.
• Chapter 21: Friedel-Crafts alkylation (alkene as electrophile to aromatic).
• Chapter 25: carbonyl addition (analogous mechanism).
• Chapter 27: enolate alkylation (similar).
• Chapter 34: lipids (fatty acid alkenes).
• Chapter 37: Pd cross-coupling (alkene + Pd; metathesis).
• Chapter 39: pericyclic reactions (alkene + diene = Diels-Alder).
• Chapter 40: green alkene chemistry (continuous flow, biocatalysis).

15.33 The Wagner-Meerwein rearrangement

The classic 1,2-shift in carbocation chemistry. Discovered by Wagner (1899) in terpene chemistry; refined by Meerwein (1922).

A 2° (or 1°) carbocation rearranges by 1,2-migration of an alkyl or hydride to give a more stable cation:

$$C^+_{\text{less stable}} \to C_{\text{more stable}}^+$$

Examples: - Norbornyl cation: classical Wagner-Meerwein; 1,2-alkyl shift. - Pinacol → pinacolone: protonation; 1,2-shift; loss of water. - Camphor isomerization. - Many terpene biosynthesis steps.

The rearrangement is fast (often faster than nucleophile attack); the product reflects the rearranged cation.

Implication

When you see a 2° cation in an electrophilic addition, check: - Is there an adjacent 3° H? Possible 1,2-H shift. - Is there an adjacent quaternary C with -CH₃? Possible 1,2-methyl shift. - Is the resulting cation more stable?

If yes, the rearrangement happens. The product reflects the new cation.

Norbornyl rearrangement (classical)

The norbornyl cation has been a subject of debate (classical vs non-classical cation). The non-classical view: the cation has a 3-center, 2-electron bridging structure. Decades of experiment and theory finally resolved in favor of the non-classical view (Olah, 1980s).

This is a famous example of carbocation chemistry's subtleties.

15.34 Stereochemistry of bromonium ion opening

The bromonium ion is a 3-membered ring with Br bridging both C atoms. Opening: - Backside attack on one C by Br⁻ (or H₂O, etc.). - The other C is freed. - The resulting product has the two substituents (Br and Br, or Br and OH) on opposite faces (anti).

For a chiral substrate, the bromonium opening gives a specific diastereomer: - (Z)-2-butene → bromonium → meso-2,3-dibromobutane. - (E)-2-butene → bromonium → (R,R)+(S,S) racemic chiral.

The mechanism predicts the product directly from the substrate stereochemistry. Stereospecific.

Asymmetric bromonium opening

Some chiral catalysts can intercept the bromonium intermediate, giving asymmetric products. Modern catalytic asymmetric halogenation is an active research area.

Halohydrin formation

In aqueous solution: water (more abundant nucleophile) attacks the bromonium. Water attacks the more substituted C (more cation-like). The product has -OH on more substituted C, -Br on less substituted; anti diastereoselectivity.

This gives a specific halohydrin; useful precursor to epoxides.

15.35 Spectroscopy of alkene reactions

After alkene addition, products are verified by spectroscopy:

IR

• Loss of C=C peak at 1640-1680 cm⁻¹.
• Loss of vinyl C-H peaks at 3000-3100 cm⁻¹.
• New peaks for the added groups (OH, Br, etc.).

NMR

• Loss of vinyl H at 5-6 ppm.
• New signals for the new sp³ C-H bonds.
• Coupling patterns from the new substituents.

MS

• New molecular formula reflecting the added atoms.
• Characteristic isotope patterns for halogenated products (M:M+2 = 1:1 for Br; 3:1 for Cl).
• New fragmentation patterns.

Specific examples

For propene + HBr → 2-bromopropane: - IR: loss of C=C (1645); new C-Br (560-700); broadened CH at 1450. - ¹H NMR: loss of vinyl 5-6 peaks; new CHBr at 4.0; new CH₃ at 1.7 (doublet). - MS: M = 122 + 124 (1:1 Br pattern); base peak from -Br loss = 43.

These spectroscopic shifts confirm the reaction.

15.36 Take-home insights

Alkenes are central to organic synthesis. From them you can install:

1. Alcohols (Markov or anti-Markov; via hydration or hydroboration).
2. Halides (Markov; via HX).
3. Vicinal dihalides (anti; via X₂).
4. Halohydrins (anti, Markov-like for OH; via X₂/H₂O).
5. Diols (cis, syn; via OsO₄).
6. Diols (trans, anti; via mCPBA + H₂O).
7. Epoxides (via mCPBA).
8. Carbonyls (via O₃/Zn).
9. Alkanes (via H₂/Pd).
10. Polymers (via radical or Ziegler-Natta).
11. Cyclic compounds (via Diels-Alder, RCM).

The toolbox of alkene chemistry is the foundation for organic synthesis.

The Markovnikov rule and its mechanistic basis (cation stability + Hammond) are central. Stereospecific addition (Br₂ anti, OsO₄ syn) is the diagnostic.

By Chapter 16, you'll have the full toolkit.

15.37 Real-world applications

Polyethylene production

Ethylene + Ziegler-Natta or metallocene catalyst → polyethylene. Industrial scale: ~110 million tons/year.

The chemistry: alkene addition (Ch 15) + chain propagation. Each alkene adds via insertion into the metal-alkyl bond.

Polyethylene types

• LDPE (low density): branched chains; from radical polymerization (Ch 18).
• HDPE (high density): linear chains; from Ziegler-Natta.
• LLDPE (linear low density): linear with short branches; copolymer of ethylene + 1-butene or 1-octene.

These different types have different properties; used for plastic bags, bottles, films, etc.

Margarine production

Plant oils (with cis alkenes) + H₂/Pd → margarine (saturated or partially saturated; trans fats can form during partial hydrogenation).

This is industrial alkene hydrogenation. The trans fats issue led to changes in the food industry; now most margarine uses different methods to achieve solid texture without partial hydrogenation.

Synthetic rubber

Butadiene polymerization → polybutadiene (with various microstructures depending on conditions). Used in tires.

Styrene-butadiene rubber (SBR): copolymer; widely used in tires.

Adhesives and sealants

Many polyurethanes start from alkene-containing diols + isocyanates.

These are alkene chemistry at industrial scale.

15.38 Modern alkene chemistry: the future

The alkene functional group is at the center of modern chemistry. Future directions include:

Asymmetric alkene chemistry

• Asymmetric Heck reactions (Ch 37).
• Asymmetric hydroboration (chiral boranes).
• Asymmetric hydrogenation (Knowles, Noyori).
• Asymmetric epoxidation (Sharpless, Jacobsen).

These methods give chiral products with high ee.

Earth-abundant catalysis

Replacing precious metals with Fe, Co, Ni, Cu, Mn for alkene transformations: - Fe-catalyzed hydroboration. - Ni-catalyzed alkene functionalization. - Mn-catalyzed asymmetric epoxidation (Jacobsen).

Photoredox alkene chemistry

Visible light + photocatalysts → radical chemistry. New transformations: - Anti-Markov hydration. - Anti-Markov hydroamination. - Trifluoromethylation.

Electrochemistry

Anodic oxidation of alkenes to give cation radicals; new C-C bond formations.

Flow chemistry

Continuous flow alkene additions: better thermal control, scalability, automation.

Biocatalysis

Engineered enzymes for asymmetric alkene transformations: - Hydratases: H₂O addition. - Halohydrinases: halohydrin formation with high ee. - Engineered P450s: epoxidation.

Looking ahead

Alkene chemistry is one of the most active areas of organic chemistry research. Each year brings new methods, new mechanisms, new applications.

The chemistry of Chapter 15 (and Ch 16) is the foundation. The future is being built on it.

15.39 Looking ahead

Chapter 16 expands the alkene addition toolbox: hydration, hydroboration, halohydrin, epoxidation, dihydroxylation, ozonolysis, hydrogenation. The chemistry of Chapter 15 is the foundation; Chapter 16 fills in the rest.

After Chapter 16, you'll have the complete alkene addition toolkit. Combined with Chapter 12's elimination chemistry and Chapter 13's decision framework, you have the tools to build alkene-containing molecules.

Chapter 17 covers alkynes (similar but with two π bonds). Chapters 18-19 cover radical and conjugated alkene chemistry. By the end of Part IV, the alkene/alkyne chemistry is comprehensive.

15.40 The mechanism-first thesis applied

Chapter 15 illustrates the mechanism-first thesis: - Markovnikov rule emerges from cation stability + Hammond. - Anti vs syn addition comes from intermediate type. - Rearrangement potential comes from cation chemistry. - Stereochemistry follows from mechanism geometry.

Master the mechanism; predict the products. This is the mechanism-first approach to alkene chemistry.

By understanding the carbocation pathway, the bromonium pathway, the concerted hydroboration pathway, and the radical pathway, you can predict any alkene addition's regiochemistry, stereochemistry, and rearrangement behavior.

15.41 Closing thoughts

The chemistry of alkenes is at the heart of organic chemistry. From the simplest ethylene to complex natural products, the same principles apply.

The Markovnikov rule, the carbocation stability arguments, the bromonium intermediate — these are foundational. The variations (radical mechanism, concerted hydroboration, syn addition) build on the foundation.

By Chapter 16, the alkene addition toolkit will be complete. You'll have everything you need to design alkene-based syntheses.

The Markovnikov rule (1870) was empirical; the mechanistic understanding came in the 1930s; modern computational confirmation is from the 1980s onward. The chemistry has been refined for over a century, and it remains central to modern organic chemistry.

15.42 The chemistry of cyclohexene

Cyclohexene is a useful test substrate: - 6-membered ring + 1 C=C. - The C=C is in a half-chair conformation; the four sp³ C's are slightly puckered. - Reactions of cyclohexene give cyclohexane derivatives.

Common cyclohexene reactions: - + Br₂ → trans-1,2-dibromocyclohexane (anti). - + OsO₄ → cis-1,2-cyclohexanediol (syn). - + mCPBA → cyclohexene oxide. - + H₂/Pd → cyclohexane. - + HCl → chlorocyclohexane (Markov). - + BH₃ then H₂O₂/NaOH → cyclohexan-1-ol (anti-Markov; for cyclohexene the product is the same as Markov since both vinyl Cs are equivalent).

Cyclohexene is widely used in teaching as it gives clean stereochemistry.

Industrial cyclohexene

Cyclohexene is industrial intermediate: - Cyclohexene + air + Co catalyst → cyclohexanone + cyclohexanol → adipic acid → nylon-6,6. - Annual scale: ~3 million tons cyclohexanol/cyclohexanone for nylon production.

Cyclohexene in natural products

Many natural products have cyclohexene rings: - Carvone (from caraway/spearmint): cyclohexenone. - Limonene: cyclohexene with isopropyl chain. - Cholesterol: contains a cyclohexene-related ring. - Many terpenes.

The chemistry of cyclohexene is a microcosm of alkene chemistry.

15.43 More worked problems

Problem A: 2-methyl-2-butene + HCl

Markovnikov: -H to less-substituted (the CH₃-CH side); -Cl to more-substituted (the (CH₃)₂C side).

Product: 2-chloro-2-methylbutane (3° chloride).

Problem B: trans-2-butene + Br₂

Bromonium intermediate; anti opening.

For trans-2-butene (the methyls are on opposite sides initially), the anti addition gives a chiral product: (2R,3R)- and (2S,3S)-2,3-dibromobutane (racemic).

Problem C: cis-2-butene + Br₂

Bromonium intermediate; anti opening.

For cis-2-butene (methyls on same side), the anti addition gives meso-2,3-dibromobutane.

This is the textbook example of stereospecific addition.

Problem D: 1-pentene + HBr (no peroxide)

Markovnikov: -H to less-substituted (C1 = CH₂); -Br to more-substituted (C2 = CH).

Product: 2-bromopentane.

Problem E: 1-pentene + HBr + peroxide

Anti-Markov; radical mechanism.

Product: 1-bromopentane.

These exemplify the Markov vs anti-Markov decision.

15.44 The carbocation in detail

The carbocation is the key intermediate in many alkene additions. Properties:

Geometry

Carbocations are sp² hybridized: - 3 substituents in a plane. - Empty p orbital perpendicular. - ~120° bond angles.

Stability ordering

3° (most stable) > 2° > 1° > methyl > vinyl/aryl (very unstable).

Hyperconjugation: σ C-H electrons donate into the empty p orbital of the cation. More substituents = more stabilization.

Cation stability: - t-butyl cation (3°): pKa of conjugate acid ~ -10 (very stable cation). - Isopropyl cation (2°): less stable. - Ethyl cation (1°): unstable; rarely observed. - Vinyl cation: very unstable; not formed in solution.

Stabilization by adjacent π systems

• Allyl cation (CH₂=CH-CH₂⁺): resonance-stabilized; comparable to 2°.
• Benzyl cation (PhCH₂⁺): resonance-stabilized; comparable to 2°.
• Tropylium cation (cyclic 7-mem aromatic): aromatic; very stable.

These resonance-stabilized cations are accessible even from primary positions if the system is allylic or benzylic.

Stabilization by heteroatoms

• Oxocarbenium (R₂C=OR⁺): O lone pair donates; very stable.
• Iminium (R₂C=NR₂⁺): N lone pair donates.

Examples in alkene chemistry

• Propene + H⁺ → 2° cation (preferred Markov).
• 2-methyl-2-butene + H⁺ → 3° cation (very stable).
• Vinyl chloride + H⁺ → would give 2° cation (Markov; accessible).

The cation stability hierarchy explains all alkene addition regiochemistry.

15.45 Final overview

Alkenes (Chapter 15) are the workhorse functional group of organic chemistry:

• C=C bond: 1σ + 1π; sp² hybridization.
• E/Z geometric isomerism (restricted rotation).
• Stability: more substituted = more stable.
• Electrophilic addition follows Markovnikov via the more-stable cation.
• Br₂ addition is anti via bromonium intermediate.
• Hydration is Markovnikov (with possible rearrangement via cation).
• Carbocation rearrangements (1,2-H, 1,2-methyl shifts) can occur.
• Industrial polymers, fuels, and chemicals are based on alkene chemistry.
• Biological alkenes are central in lipids, terpenes, pheromones, vitamins.

Chapter 16 expands the toolkit. Mastery of Chapter 15 enables understanding of all subsequent alkene chemistry.

15.46 Summary

1. Alkenes have a C=C double bond (σ + π); sp² hybridization; ~120° bond angles; restricted rotation gives cis/trans (E/Z) isomers.
2. Alkene stability: more substituted = more stable. Hyperconjugation explains it.
3. E/Z nomenclature: based on CIP priority of substituents.
4. Electrophilic addition: π bond attacks electrophile. Two-step mechanism via carbocation.
5. Markovnikov's rule: H adds to less-substituted C; X adds to more-substituted C. Reason: more-stable carbocation forms.
6. Br₂ addition goes through a bromonium ion; anti addition (stereospecific).
7. Acid-catalyzed hydration: alkene + H₂O + H⁺ → alcohol (Markovnikov).
8. Carbocation rearrangements (1,2-hydride, 1,2-methyl shift) can occur; check for them when predicting products.
9. Anti-Markovnikov hydration: hydroboration-oxidation (Ch 16).

Chapter 16 covers the full suite of alkene addition reactions: HX, X₂, H₂O, hydroboration, OsO₄, ozonolysis, and more.