Structure and reactivity of alkenes

· Alkenes are unsaturated hydrocarbons containing at least one C=C double bond.
· The C=C bond contains one σ bond and one π bond.
· The π bond is electron-rich and attracts electrophiles, so alkenes mainly undergo electrophilic addition.
· Around each carbon in C=C: sp² hybridised, trigonal planar, bond angle about 120°.
· The C=C bond has restricted rotation, which links to cis/trans isomerism from Topic 13.4.

Producing alkenes

· Elimination of HX from halogenoalkanes:
· Reagents/conditions: ethanolic NaOH, heat.
· Example: bromoethane → ethene + HBr overall, with base removing H and Br from adjacent carbons.
· Dehydration of alcohols:
· Reagents/conditions: heated Al₂O₃ catalyst or concentrated H₂SO₄, heat.
· Example: ethanol → ethene + H₂O.
· Cracking of longer-chain alkanes:
· Conditions: heat, often Al₂O₃ catalyst.
· Produces shorter alkanes and alkenes.
· Exam tip: production reactions often ask for reagent + condition + organic product.

Electrophilic addition reactions of alkenes

· Hydrogenation:
· Alkene + H₂(g) → alkane.
· Conditions: Pt/Ni catalyst, heat.
· Hydration with steam:
· Alkene + H₂O(g) → alcohol.
· Conditions: H₃PO₄ catalyst.
· Example: ethene + steam → ethanol.
· Addition of hydrogen halides:
· Alkene + HX(g) → halogenoalkane.
· Conditions: room temperature.
· Example: propene + HBr → mainly 2-bromopropane.
· Addition of halogens:
· Alkene + X₂ → dihalogenoalkane.
· Example: ethene + Br₂ → 1,2-dibromoethane.
· In all these reactions, the C=C double bond opens and atoms add across the double bond.

This reaction shows halogen addition across a C=C bond. It is directly linked to the bromine water test and the electrophilic addition mechanism. Source

Bromine water test for C=C

· Test for alkene: add aqueous bromine / bromine water.
· Positive result: orange/brown bromine water decolourises.
· Reason: bromine undergoes addition across the C=C bond.
· Alkanes do not decolourise bromine water rapidly without UV light, so this distinguishes alkenes from alkanes.
· Exam wording: “presence of a C=C bond” must be linked to decolourisation of aqueous bromine.

Electrophilic addition mechanism: bromine and ethene

· The π bond in ethene has high electron density.
· It polarises Br₂, creating Brδ+–Brδ−.
· Curly arrow 1: from C=C π bond to Brδ+.
· Curly arrow 2: from Br–Br bond to the other Br atom, forming Br⁻.
· Intermediate: a carbocation forms.
· Curly arrow 3: Br⁻ attacks the carbocation.
· Product: 1,2-dibromoethane.
· Key exam phrase: curly arrows show movement of electron pairs, so arrows must start at a bond or lone pair.

This mechanism shows how bromine is added across ethene’s C=C bond. It is useful for practising curly arrows, polarisation of Br₂, and formation of the dibromo product. Source

Electrophilic addition mechanism: HBr and propene

· Propene is unsymmetrical, so HBr can add in two possible ways.
· Step 1: C=C attacks Hδ+ in HBr; the H–Br bond breaks to form Br⁻.
· Two possible carbocations can form:
· Primary carbocation → less stable → minor product.
· Secondary carbocation → more stable → major product.
· Alkyl groups donate electron density by a positive inductive effect.
· Carbocation stability order: tertiary > secondary > primary.
· Therefore propene + HBr gives mainly 2-bromopropane.
· This is Markovnikov addition: H adds to the carbon already bonded to more H atoms, forming the more stable carbocation.

Oxidation of alkenes

· Cold dilute acidified KMnO₄:
· Alkene → diol.
· Example: ethene → ethane-1,2-diol.
· Observation: purple KMnO₄ decolourises.
· Hot concentrated acidified KMnO₄:
· Causes oxidative cleavage / rupture of the C=C bond.
· Products help identify the position of the C=C bond in a larger molecule.
· Product rules for hot concentrated acidified KMnO₄:
· C of C=C with two H atoms → CO₂.
· C of C=C with one H atom → carboxylic acid.
· C of C=C with no H atoms → ketone.
· Exam tip: use cleavage products to work backwards and locate the original alkene linkage.

Addition polymerisation

· Addition polymerisation occurs when many alkene monomers join to form a polymer.
· The C=C double bond opens; no small molecule is lost.
· Ethene forms poly(ethene):
· monomer: CH₂=CH₂
· repeat unit: –CH₂–CH₂–
· Propene forms poly(propene):
· monomer: CH₂=CHCH₃
· repeat unit: –CH₂–CH(CH₃)–
· Exam tip: when drawing repeat units, place brackets around the repeat unit and write n outside.
· To identify the monomer from a polymer, locate the repeat unit and re-form the C=C bond between the two backbone carbons.

This diagram shows addition polymerisation of ethene. It clearly links the alkene monomer to the polymer repeat unit. Source

High-yield reaction summary

· Alkene → alkane: H₂(g), Pt/Ni catalyst, heat; reaction type = hydrogenation / electrophilic addition.
· Alkene → alcohol: H₂O(g), H₃PO₄ catalyst; reaction type = hydration / electrophilic addition.
· Alkene → halogenoalkane: HX(g), room temperature; reaction type = electrophilic addition.
· Alkene → dihalogenoalkane: X₂; reaction type = electrophilic addition.
· Alkene → diol: cold dilute acidified KMnO₄; reaction type = oxidation.
· Alkene → cleavage products: hot concentrated acidified KMnO₄; reaction type = oxidative cleavage.
· Alkene → polymer: addition polymerisation; C=C opens to form saturated polymer chain.

Common exam traps

· Do not write substitution for alkene reactions; alkenes usually undergo addition.
· Do not forget conditions: hydrogenation needs Pt/Ni + heat; hydration needs steam + H₃PO₄.
· For bromine water, the observation is orange/brown to colourless, not precipitate formation.
· For Markovnikov addition, explain using carbocation stability, not just “the major product forms”.
· For hot KMnO₄ cleavage, identify whether each C=C carbon has two H, one H, or no H.
· For polymerisation, the repeat unit must show single bonds in the polymer backbone, not C=C.