Alkenes

Full Notes

Alkenes and their bonding have been outlined in more detail here and here
This page is just what you need to know for CIE A-level Chemistry :)

Production of Alkenes

Elimination of HX from Halogenoalkanes

• Reagent: Ethanolic NaOH
• Conditions: Heat under reflux
• Reaction: Base removes H and halide leaves, C=C forms

Example Elimination to form ethene

CH₃CH₂Br + NaOH (ethanol) → CH₂=CH₂ + NaBr + H₂O

Dehydration of Alcohols

• Reagents:
• Al₂O₃ catalyst, heat
• OR conc. H₂SO₄, heat
• Produces alkene + water

Example Dehydration to form ethene

CH₃CH₂OH → CH₂=CH₂ + H₂O

Cracking of Alkanes

See here for more detail on cracking.

• Conditions: High temp, catalyst (e.g. Al₂O₃ or zeolite)
• Produces alkenes + short-chain alkanes

Example Thermal/catalytic cracking

C₁₀H₂₂ → C₂H₄ + C₈H₁₈

Electrophilic Addition Reactions of Alkenes

Alkenes react with electrophiles in electrophilic addition reactions. The high electron density within a carbon-carbon double bond attracts electrophiles.

Hydrogenation – Addition of H₂

• Reagents: H₂ gas, catalyst: Ni or Pt
• Conditions: 150–200°C
• Converts C=C to C–C

Example Ethene to ethane

CH₂=CH₂ + H₂ → CH₃CH₃

Addition of Steam – Hydration

• Reagents: H₂O(g), catalyst: H₃PO₄
• Conditions: 300°C, 60 atm
• Forms alcohol

Example Ethene to ethanol

CH₂=CH₂ + H₂O → CH₃CH₂OH

Addition of Hydrogen Halide – HX

• Reagent: HX (e.g. HBr)
• Conditions: Room temperature
• Forms halogenoalkanes

Example Propene + HBr

CH₂=CHCH₃ + HBr → CH₃CHBrCH₃ (major) — Markovnikov’s Rule applies (see below)

Addition of Halogen – X₂

• Reagent: Br₂ or Cl₂
• Conditions: Room temperature
• Forms dihalogenoalkane

Example Ethene + bromine

CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br

Oxidation with Cold Dilute KMnO₄

• Reagent: Cold dilute acidified KMnO₄
• Forms diol
• Purple to colourless change occurs
  Mn gets reduced from +7 in KMnO₄ to 2+ (Mn²⁺)

Example Ethene to ethane-1,2-diol

CH₂=CH₂ → CH₂OH–CH₂OH

Oxidation with Hot Conc. KMnO₄

• Reagent: Hot, concentrated acidified KMnO₄
• C=C bond is broken
• Products depend on alkene structure:
• –CH=CH₂ → CO₂ + H₂O
• –CH=R → carboxylic acid
• –R=R → ketone

Example Propene cleavage

CH₃CH=CH₂ → CH₃COOH + CO₂

Addition Polymerisation

• C=C bonds open up and link to form long chains
• Conditions: heat, high pressure, catalyst
• No by-products

Example Ethene and propene

n CH₂=CH₂ → –[CH₂–CH₂]–_n

n CH₂=CHCH₃ → poly(propene)

Test for Alkenes – Bromine Water

• Reagent: Br₂(aq) (orange)
• Positive test: orange to colourless colour change
• Indicates presence of C=C double bond

Mechanism of Electrophilic Addition

The high electron density within a carbon-carbon double bond attracts electrophiles and the reaction mechanism follows three basic steps:

Step 1: Electrophile Attraction

• The C=C bond has high electron density and attracts electrophiles (electron pair acceptors).
• Electrophiles include H⁺, Br₂, and H₂SO₄ (see below).

Step 2: Formation of Carbocation Intermediate

• The double bond breaks as one carbon gains a new bond with the electrophile.
• The other carbon becomes positively charged (carbocation).

Step 3: Forming final product

• A negative species (e.g., Br⁻, HSO₄⁻) bonds to the carbocation, forming the final product.

For Example: Electrophilic addition mechanism for Bromine + Ethene

• Br₂ molecule approaches C=C (polarised by electron density)
• Double bond breaks, Br⁺ forms bond → Carbocation intermediate
• Br⁻ ion attacks carbocation → Forms CH₂Br–CH₂Br

For Example: Electrophilic addition mechanism for HBr + Ethene

• HBr is polar (Hδ⁺—Brδ⁻), and H⁺ acts as the electrophile.
• H⁺ bonds to one carbon of the C=C bond, forming a carbocation.
• Br⁻ attacks the carbocation, forming a halogenoalkane.

Inductive Effect and Carbocation Stability

When adding HX to an unsymmetrical alkene, two possible products can form.

• The two possible products won’t be formed in equal amounts. The product formed most is called the major product and the one formed the least is the minor product.
• We can predict the major product based on the carbocation intermediate formed in the reaction.

• The major product forms from the most stable carbocation.
• Alkyl (carbon) groups bonded to the positively charged carbon in the intermediate stabilise the positive charge by ‘giving’ electron density to the positively charged carbon. This is called the positive inductive effect. The more alkyl groups there are bonded to the positively charged carbon, the more stable the carbocation is and the more likely it is to form.



• Carbocation stability order: Tertiary > Secondary > Primary

More stable carbocation = major product

Example Propene + HBr

Secondary carbocation → 2-bromopropane (major)

Primary carbocation → 1-bromopropane (minor)

This explains Markovnikov’s rule (Major product will be the one where H from HX bonds to carbon in C=C that is bonded to the most hydrogens).

Summary

• Alkenes are unsaturated hydrocarbons containing a C=C bond.
• Prepared by elimination (ethanolic NaOH), dehydration of alcohols (Al₂O₃ or conc. H₂SO₄), and cracking.
• Undergo electrophilic addition with H₂, steam/H₃PO₄, HX and X₂; also oxidation (cold dilute KMnO₄ → diols; hot conc. KMnO₄ → oxidative cleavage).
• Bromine water is decolourised by alkenes.
• Mechanism proceeds via a carbocation; Markovnikov’s rule explained by the positive inductive effect and carbocation stability (3° > 2° > 1°).
• Addition polymerisation forms long chains with no by-products.