Hydrocarbon

Hydrogenation

Reagents: H₂ (g)
Conditions: Pt/Ni catalyst and heat

alkene + hydrogen gas → alkane

Cracking

Reagents: Longer chain alkane
Conditions: Al₂O₃ catalyst and heat

decane → octane + ethene

→ Necessary as cracking produce shorter chains hydrocarbon which are more useful and higher in demands

Complete combustion of alkanes

alkane + O₂ in excess → CO₂ + H₂O

Incomplete combustion of alkanes

alkane + O₂ in limited → CO + H₂O

Free radical substitution

Reagents: Alkanes + X₂

Conditions: UV light

1. Initiation

Cl₂ → (in UV light) 2Cl•

2. Propagation

Cl• + C₂H₆ → HCl + •C₂H₅

•C₂H₅ + Cl₂ → C₂H₅Cl + Cl•

3. Termination

Cl• + Cl• → Cl₂

•C₂H₅ + Cl• → C₂H₅Cl

•C₂H_{5 +} •C₂H₅ → C₄H₁₀

General unreactivity of alkanes

• same electronegative between C and H

• strong C-H bond

Consequences of incomplete combustion of hydrocarbons in car exhaust engines

• CO is a toxic, binds to haemoglobin so blood cannot transport O₂

• N₂ reacts with O₂ to form nitrogen oxides

• Nitrogen oxides react with water to form nitric acid → acid rain

• Nitrogen oxides react with unburnt hydrocarbon → PAN (photochemical smog)

Eliminition of HX from halogenoalkane

Reagents: NaOH (in ethanol)

Conditions: heat

bromoethane + NaOH (in ethanol) → ethene + water + sodium bromide

Dehydration of an alcohol

Reagents: Alcohol

Conditions: Al₂O₃ catalyst and heat

ethanol → ethene + water

Electrophilic addition of steam to alkene

Reagents: H₂O (g)

Condition: H₃PO₄ catalyst

Electrophilic addition of hydrogen halide to alkene

Reagents: HX (g)

Conditions: room temp

Electrophilic addition of halogens to alkene

Reagents: X₂

Condition: no condition stated

→ produces dihalogenoalkane

oxidation of alkene by cold dilute acidified KMnO₄

• colour change from pale purple solution of KMnO₄ to colourless solution

• produces diol

oxidation of alkene by hot concentrated acidified KMnO₄

Predicting the position of double bonds formed

addition polymerisation of alkene

aqueous bromine test

→ shows C=C presence

• electrophilic addition of Br₂ to alkene

• Br₂ has brownish colour

• when added to alkene, brown → colourless

→ Br₂ is decolourised by alkene

inductive effects of alkyl groups

• alkyl groups push e^- density towards carbocation making it energentically more stable

• more alkyl group → more stable → more likely to form on its own