Alkenes: Structure, Isomerism, and Addition Reactions

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Topic 3 — Key Facts for Kenyatta University (Kenya) Core concept: Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (C=C); they undergo addition reactions because the π bond is relatively weak and breakable High-yield point: Markovnikov’s rule states that in electrophilic addition of HX to an unsymmetrical alkene, the hydrogen adds to the carbon with more hydrogens; anti-Markovnikov addition occurs with peroxides ⚡ Exam tip: Kenyatta University exams frequently test Markovnikov addition, cis-trans isomerism around C=C bonds, and polymerisation mechanisms — be precise with these

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Alkenes: Unsaturated Hydrocarbons

Alkenes are hydrocarbons containing at least one carbon-carbon double bond. Their general formula is CₙH₂ₙ for acyclic alkenes (acyclic means open-chain). Each double bond reduces the hydrogen count by two compared to the corresponding alkane.

Nomenclature of Alkenes

IUPAC rules for naming alkenes:

1. Identify the longest chain containing the C=C bond
2. Number from the end that gives the C=C bond the lowest possible number
3. Use the suffix -ene and specify the position (e.g., propene, but-1-ene)
4. If geometric isomers exist, use cis/trans or (E)/(Z) notation

First few alkene names:

• Ethene (C₂H₄): CH₂=CH₂
• Propene (C₃H₆): CH₃–CH=CH₂
• But-1-ene (C₄H₈): CH₂=CH–CH₂–CH₃
• But-2-ene (C₄H₈): CH₃–CH=CH–CH₃

Geometric Isomerism in Alkenes

The C=C double bond cannot rotate freely (unlike C–C single bonds), which gives rise to geometric isomerism.

Conditions for Geometric Isomerism:

• There must be a C=C double bond
• Each carbon of the double bond must have two different groups attached

Cis-Trans Isomerism:

• Cis isomer: Identical/highest priority groups are on the same side of the double bond
• Trans isomer: Identical/highest priority groups are on opposite sides of the double bond

Example — But-2-ene:

• Cis-but-2-ene: Both CH₃ groups on same side
• Trans-but-2-ene: CH₃ groups on opposite sides

Property	Cis-but-2-ene	Trans-but-2-ene
Melting point (°C)	−139	−106
Boiling point (°C)	0.9	0.3
Polarity	Dipole moment (0.33 D)	Zero dipole moment

⚡ Exam Tip: The cis isomer is generally less stable than the trans isomer due to steric repulsion between the two substituents on the same side. This stability difference is reflected in physical properties — trans alkenes typically have higher melting points.

The E/Z System for Geometric Isomerism

For more complex alkenes where cis/trans notation is ambiguous, the Cahn-Ingold-Prelog (CIP) priority rules are used:

1. Assign priorities to groups on each double bond carbon (by atomic number of atoms directly attached)
2. If the two highest-priority groups are on the same side → Z (zusammen, German for “together”)
3. If the two highest-priority groups are on opposite sides → E (entgegen, German for “opposite”)

Example: 1-bromo-1-chloroethene

• Carbon 1 has Br (Z=35) and H (Z=1) attached
• Carbon 2 has Cl (Z=17) and H (Z=1) attached
• On each carbon, Br > H and Cl > H
• If Br and Cl are on the same side → Z; if opposite → E

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Addition Reactions of Alkenes

Mechanism of Electrophilic Addition

The π bond of an alkene is weaker than the σ bond and is therefore the site of chemical reactivity. The π electrons are exposed above and below the plane of the molecule, making them susceptible to attack by electrophiles (electron-deficient species).

General mechanism for addition of HX:

1. Protonation of the π bond: The π electrons attack H⁺ from HX, forming a carbocation intermediate on the more substituted carbon
2. Nucleophilic attack: The halide ion (X⁻) attacks the carbocation to form the alkyl halide product

Why the More Substituted Carbocation is Preferred: The carbocation intermediate is stabilised by nearby alkyl groups via the hyperconjugation effect. More substituted carbocations are more stable: Stability order: 3° carbocation > 2° carbocation > 1° carbocation > methyl cation

1. Addition of Hydrogen Halides (HX)

Reaction: Alkene + HX → Haloalkane

Markovnikov’s Rule: In electrophilic addition of HX to an alkene, the hydrogen adds to the carbon of the double bond that already has more hydrogen atoms (the less substituted carbon), and the halogen adds to the more substituted carbon.

Examples:

CH₃–CH=CH₂ + HBr → CH₃–CH(Br)–CH₃   (2-bromopropane)
CH₂=CH–CH₃ + HCl → CH₃–CH(Cl)–CH₃   (2-chloropropane)

Peroxide Effect (Anti-Markovnikov Addition): In the presence of peroxides (R–O–O–R), HBr adds contrary to Markovnikov’s rule:

CH₃–CH=CH₂ + HBr (peroxides) → CH₃–CH₂–CH₂Br   (1-bromopropane)

Why? Peroxides generate bromine radicals (Br•) which add to the less substituted carbon, and hydrogen then adds to the more substituted carbon. The radical intermediate is more stable at the more substituted carbon (but in radical addition, the step where Br• attacks is rate-determining).

⚡ Exam Tip: The presence or absence of peroxides is a common examination question. If the question does not mention peroxides, apply Markovnikov’s rule. If peroxides are mentioned, apply anti-Markovnikov addition.

2. Addition of Water (Hydration)

Reaction: Alkene + H₂O → Alcohol (Markovnikov addition)

Conditions: Acidic catalyst (H₂SO₄) at 300°C and 60–70 atm pressure

Mechanism:

1. H⁺ from the acid catalyst adds to the double bond (forming the more stable carbocation)
2. Water (a nucleophile) attacks the carbocation
3. A proton is lost from the oxonium ion to give the alcohol

Example:

CH₂=CH₂ + H₂O → CH₃–CH₂OH   (ethanol)
CH₃–CH=CH₂ + H₂O → CH₃–CH(OH)–CH₃   (propan-2-ol)

3. Addition of Halogens (X₂)

Reaction: Alkene + X₂ → vicinal dihalide (1,2-dihalide)

Example:

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br   (1,2-dibromoethane)
CH₃–CH=CH₂ + Br₂ → CH₃–CHBr–CH₂Br   (1,2-dibromopropane)

Key Characteristics:

• Reaction is decolorised (brown bromine solution becomes colourless) — a test for unsaturation
• Proceeds via a cyclic bromonium ion intermediate (three-membered ring with Br⁺)
• Anti addition: Br atoms add to opposite faces of the double bond
• Stereospecific: cis-alkene gives racemic mixture of enantiomers; trans-alkene gives meso compound or pair of enantiomers

4. Catalytic Hydrogenation

Reaction: Alkene + H₂ → Alkane

Conditions: Nickel (Ni), palladium (Pd), or platinum (Pt) catalyst at 150–300°C

Hydrogenation is a reduction reaction — it adds hydrogen across the double bond, converting the alkene to an alkane. This is an exothermic reaction (ΔH ≈ −120 kJ/mol per double bond).

⚡ Exam Tip: Hydrogenation is used industrially to convert unsaturated vegetable oils (liquid) to saturated fats (solid) in food processing. The “partially hydrogenated oils” in margarine are a direct consequence of this reaction.

Oxidation Reactions of Alkenes

Mild Oxidation (Cold dilute KMnO₄ or OsO₄): Converts C=C to a diol (vicinal diol / glycol):

CH₂=CH₂ + [O] + H₂O → HO–CH₂–CH₂–OH   (ethane-1,2-diol)
CH₃–CH=CH₂ + [O] + H₂O → CH₃–CH(OH)–CH₂OH   (propane-1,2-diol)

Strong Oxidation (Hot concentrated KMnO₄ or CrO₃): Cleaves the C=C bond completely:

• Disubstituted carbon → carboxylic acid (RCOOH)
• Monosubstituted carbon → CO₂ (or HCOOH if末端)
• Terminal alkene → CO₂

Example:

CH₃–CH=CH–CH₃ + [O] → 2CH₃COOH   (acetic acid)

⚡ Exam Tip: If the alkene has two different groups on each carbon, strong oxidation produces different carboxylic acids. If either carbon has two H atoms, CO₂ is produced from that carbon.

Polymerisation of Alkenes

Polymerisation: Alkenes can undergo addition polymerisation where many small alkene molecules (monomers) join to form a long-chain polymer.

Examples:

• Polyethene (PE): n CH₂=CH₂ → (−CH₂–CH₂−)ₙ
• Polypropene (PP): n CH₂=CH–CH₃ → (−CH₂–CH(CH₃)−)ₙ
• Polystyrene (PS): n CH₂=CH(C₆H₅) → (−CH₂–CH(C₆H₅)−)ₙ

Polymer Properties and Uses:

Polymer	Monomer	Properties	Uses
LDPE	Ethene	Flexible, transparent	Plastic bags, film
HDPE	Ethene	Rigid, stronger	Containers, pipes
PVC	Vinyl chloride	Strong, versatile	Pipes, window frames
PS	Styrene	Transparent, brittle	Insulation, cups

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