Alkenes: properties, reactions, mechanisms

Alkenes – Overview (Cambridge AS & A‑Level Chemistry 9701)

Alkenes are unsaturated hydrocarbons that contain at least one carbon–carbon double bond (C=C). They are examined for their preparation, physical properties, characteristic reactions, mechanisms, stereochemistry and industrial importance.

1. General Features

• Molecular formula (acyclic): C_nH_2n
• Hybridisation: sp² carbon – trigonal‑planar, bond angles ≈120°.
• Isomerism: cis–trans (geometric) when each carbon of the double bond bears two different substituents.
• Reactivity centre: the π‑bond, which is more accessible to electrophiles than the σ‑bond.

2. IUPAC Nomenclature

1. Identify the longest carbon chain that contains the double bond.
2. Number the chain so that the double bond receives the lowest possible locant.
3. Replace the “‑ane” suffix with “‑ene”.
4. If required, give the locant (e.g., 2‑butene).
5. For more than one double bond use “‑diene”, “‑triene”, etc., with appropriate locants (e.g., 1,3‑butadiene).

3. Physical Properties & Trends

4. Industrial Production of Alkenes

The syllabus expects you to know the four main laboratory‑scale routes and the industrial context in which they are applied.

Environmental & Economic Considerations

• Energy efficiency: Heat‑integrated reactors and waste‑heat recovery are used to reduce the high energy demand of cracking.
• Product recovery: Ethylene is separated by cryogenic distillation; modern plants recycle unreacted feed and capture flue gases to minimise loss.
• Emissions control: Catalytic incinerators and scrubbers remove SOₓ, NOₓ and volatile organic compounds, satisfying environmental legislation.
• Scale‑up issues: Rapid quench prevents polymerisation of the newly formed alkenes; coke deposition on the SiO₂/Al₂O₃ catalyst is monitored and removed by steam‑air oxidation.

5. Core Reactions of Alkenes

All reactions listed involve addition to the C=C bond unless a cleavage step is specified (ozonolysis, oxidative cleavage).

5.1 General Pattern of Electrophilic Addition

1. Generation of the electrophile (often by protonation of a reagent).
2. π‑Bond attack – the double bond donates electrons to the electrophile, forming a carbocation or a cyclic halonium ion.
3. Nucleophilic capture – a nucleophile (counter‑ion, solvent, or added nucleophile) attacks the positively‑charged centre.

5.2 Halogenation (X₂, X = Cl, Br)

• Intermediate: cyclic halonium ion.
• Outcome: anti‑addition → trans‑1,2‑dihalide.
• Example: CH₂=CH₂ + Br₂ → CH₂Br‑CH₂Br

5.3 Hydrohalogenation (HX)

• Regiochemistry follows Markovnikov’s rule (H adds to the carbon bearing more H atoms).
• Carbocation stability (3° > 2° > 1° > methyl) dictates the product.
• Example: CH₃‑CH=CH₂ + HCl → CH₃‑CHCl‑CH₃

5.4 Anti‑Markovnikov Hydrohalogenation (HBr + peroxides)

• Radical chain mechanism.
• Initiation: RO‑OR → 2 RO·
• Propagation: RO· + HBr → ROH + Br· then Br· + CH₂=CH‑R → CH₂‑CH·‑R‑Br
• Result: Br adds to the less substituted carbon.
• Example: CH₃‑CH=CH₂ + HBr (peroxides) → CH₃‑CH₂‑CH₂Br

5.5 Acid‑Catalysed Hydration

• Reagents: H₂SO₄ (or other strong acid) + water.
• Markovnikov addition of –OH to the more substituted carbon.
• Overall: CH₂=CH‑CH₃ + H₂O → CH₃‑CH(OH)‑CH₃ (2‑propanol).

5.6 Hydroboration–Oxidation (Anti‑Markovnikov Hydration)

1. Hydroboration – BH₃·THF adds syn; boron attaches to the less substituted carbon.
2. Oxidation – H₂O₂ / NaOH converts the C–B bond to C–OH.

• Overall: anti‑Markovnikov alcohol.
• Example: CH₂=CH‑CH₃ →[BH₃] CH₃‑CH₂‑CH₂‑BH₂ →[H₂O₂/NaOH] CH₃‑CH₂‑CH₂‑OH (1‑propanol).

5.7 Ozonolysis (Oxidative Cleavage)

• Reagents: O₃ at –78 °C, followed by reductive work‑up (Zn/H₂O) or oxidative work‑up (H₂O₂).
• Cleaves the C=C bond to give carbonyl fragments.
• Symmetrical alkene → two identical carbonyl compounds.
• Unsymmetrical alkene → mixture of aldehydes/ketones according to substitution.
• Example (reductive work‑up): CH₃‑CH=CH‑CH₃ + O₃ → CH₃‑CHO + CH₃‑CHO (acetaldehyde from 2‑butene).

5.8 Oxidative Cleavage with Potassium Permanganate (KMnO₄)

• Cold, dilute KMnO₄ (0 °C, < 0.1 M) → syn‑dihydroxylation → vicinal diol.
• Hot, concentrated KMnO₄ (≥ 150 °C) → oxidative cleavage → aldehydes/ketones or carboxylic acids if the carbon is fully substituted.
• Example (cold): CH₂=CH‑CH₃ + KMnO₄/H₂O → CH₂(OH)‑CH(OH)‑CH₃ (1,2‑propane‑diol).
• Example (hot): CH₂=CH‑CH₃ + KMnO₄ → CH₃‑COOH + CO₂ (oxidative cleavage of a terminal alkene).

5.9 Catalytic Hydrogenation

• Metal catalysts: Pt, Pd, Ni (often supported on carbon).
• Syn addition – both H atoms add to the same face of the double bond.
• Example: CH₂=CH₂ + H₂ →[Pt] CH₃‑CH₃ (ethane).

5.10 Addition Polymerisation

• Monomers must contain at least one unsubstituted carbon of the double bond (e.g., ethene, propene).
• Initiation: radical (peroxide) or cationic (Lewis acid) species adds to the double bond forming a chain‑carrying radical/cation.
• Propagation: repeated addition of monomer units.
• Termination: combination or disproportionation of chain radicals.
• Industrial examples: polyethylene (from ethene), polypropylene (from propene), poly‑butene (from 1‑butene).

6. Mechanistic Details & Stereochemistry

6.1 Carbocation Stability (Regiochemistry)

6.2 Stereochemical Outcomes

• Anti‑addition (e.g., halogenation, bromination) – products are trans because the halonium ion is opened from the opposite side.
• Syn‑addition (e.g., catalytic hydrogenation, hydroboration) – both new groups add to the same face of the double bond.
• Radical addition (anti‑Markovnikov HBr) – generally gives a mixture of stereoisomers because the planar radical intermediate can be attacked from either side.

7. Summary of Typical Reaction Conditions