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Reactions of Organic Compounds

13 min read

Reactions of hydrocarbons

Reactions of organic compoundsA map of reactions. Alkanes burn to give carbon dioxide and water, and react with halogens by substitution in sunlight. Alkenes react by addition with hydrogen, halogens and water. Alcohols burn, are oxidised to carboxylic acids, and react with carboxylic acids to make esters.AlkaneAlkeneAlcoholHalogenoalkaneCarboxylic acidCO₂ + H₂Ocracking+ water (steam)combustion+ Cl₂ / Br₂ (addition)oxidation
ReactionAlkanesAlkenes
CombustionBurn in plenty of air to give CO₂ and water. In limited air, carbon monoxide or sootBurn with a smoky flame
SubstitutionHalogen atoms replace hydrogen atoms in sunlight: CH₄ + Cl₂ → CH₃Cl + HClNo
AdditionNo (saturated)Atoms add across the double bond
Bromine water testStays orange-brownTurns colourless
Complete combustion of methane: the main gas in natural gas and biogas.
The bonds in methane and oxygen break and the atoms form carbon dioxide and water.
Addition reaction of etheneProductUse
With hydrogen (nickel catalyst, 150 °C)EthaneMaking margarine from vegetable oils
With bromine or chlorine1,2-dibromoethaneTest for unsaturation
With steam (phosphoric acid catalyst, 300 °C)EthanolIndustrial ethanol
With itself (polymerisation)Poly(ethene)Plastic bags and pipes

Reactions of alcohols

ReactionEquation or description
CombustionC₂H₅OH + 3O₂ → 2CO₂ + 3H₂O (clean fuel)
OxidationEthanol is oxidised (for example by acidified potassium dichromate) to ethanoic acid. The orange solution turns green.
Reaction with a carboxylic acid (esterification)Ethanol + ethanoic acid ⇌ ethyl ethanoate + water (sweet-smelling ester; acid catalyst)
DehydrationHeated with concentrated sulphuric acid, ethanol loses water to give ethene
With sodiumHydrogen gas and sodium ethoxide

Local and industrial preparation

CompoundLocal methodIndustrial method
EthanolFermentation: yeast converts sugar (from sugar cane, maize, bananas or millet) to ethanol and carbon dioxide at 25–35 °C. C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂Hydration of ethene from crude oil with steam and a phosphoric acid catalyst
Alkanes and alkenesBiogas (methane) from the fermentation of cow dung and wasteFractional distillation of crude oil, then cracking large molecules to alkenes
SoapHeating oils with the alkali from wood ashHeating oils and fats with sodium hydroxide
Try itBurning an alkane

Choose the number of carbon atoms n in an alkane. Each molecule needs (3n + 1) / 2 molecules of oxygen and gives n of CO₂ and (n + 1) of water. This is also true of gas volumes.

Check yourself Which test distinguishes ethene from ethane?
Check yourself Which type of reaction do alkenes undergo?
Check yourself What is made when ethanol reacts with ethanoic acid?

Key terms

Addition reaction
A reaction in which atoms add across a double bond.
Substitution reaction
A reaction in which an atom or group replaces a hydrogen atom.
Fermentation
Breakdown of sugar by yeast to make ethanol and carbon dioxide.
Ester
A compound made from an alcohol and a carboxylic acid; often has a sweet smell.

Summary

  • Alkanes burn and undergo substitution; alkenes burn and undergo addition and decolourise bromine water.
  • Alcohols burn, are oxidised to acids, form esters and dehydrate to alkenes.
  • Ethanol is made locally by fermentation and industrially from ethene and steam.
  • Crude oil is separated by fractional distillation and cracked to make alkenes.
Organic Chemistry