LEARNING OBJECTIVES
By the end of this chapter, you should be able to:
- use displayed, structural and general formulae
- name compounds and recognise structural isomers
- explain petroleum fractional distillation and cracking
- describe alkane and alkene reactions
- compare ethanol manufacture
- describe carboxylic acids and esterification
- compare addition and condensation polymers
THE BIG IDEA
Recognise, name and explain the reactions of hydrocarbons, alcohols, acids, esters and polymers.
Organic chemistry studies carbon compounds. Functional groups create families with similar reactions, while carbon-chain length controls many physical properties.
Displayed formulae show every bond; structural formulae show an unambiguous atom arrangement in a compact form.
SECTION 01
Formulae, homologous series and naming
A homologous series shares a functional group and general formula, has similar chemical properties, differs by CH₂ between neighbours and shows gradual physical-property trends.
The required general formulae are alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alcohols CₙH₂ₙ₊₁OH and carboxylic acids CₙH₂ₙ₊₁COOH.
Prefixes meth-, eth-, prop- and but- show one to four carbon atoms. Endings -ane, -ene, -ol and -oic acid identify the series.
KEY IDEAS
- Saturated compounds have only carbon–carbon single bonds.
- Unsaturated compounds contain at least one non-single carbon–carbon bond.
- Structural isomers share molecular formula but have different structural formulae.
SECTION 02
Fuels and fractional distillation
Coal, natural gas and petroleum are fossil fuels. Methane is the main component of natural gas; petroleum is a mixture of hydrocarbons.
Fractional distillation separates petroleum by boiling range. Long-chain fractions condense low in the column; short-chain fractions rise higher.
Up the column, chain length, boiling point and viscosity decrease while volatility increases.
KEY IDEAS
- Refinery gas: heating and cooking.
- Petrol: cars; naphtha: chemical feedstock; kerosene: aircraft.
- Diesel: diesel engines; fuel oil: ships and heating; lubricating fraction: lubricants and waxes; bitumen: roads.
SECTION 03
Alkanes
Alkanes are saturated hydrocarbons with single covalent bonds. They are relatively unreactive but burn and react with chlorine.
Complete combustion forms carbon dioxide and water. Limited oxygen produces carbon monoxide and/or carbon particles.
Ultraviolet light supplies activation energy for chlorine substitution, where one atom is replaced. Monosubstitution of methane forms chloromethane and hydrogen chloride.
SECTION 04
Alkenes and cracking
Alkenes are unsaturated hydrocarbons containing C=C. Large alkanes are cracked at high temperature with a catalyst to make smaller useful alkanes, alkenes and hydrogen.
Alkenes decolourise aqueous bromine from orange-brown to colourless. They undergo addition with bromine, hydrogen using nickel, and steam using an acid catalyst.
Addition opens the double bond and forms one product.
KEY IDEAS
- Cracking matches supply to demand and produces monomers.
- Hydrogenation forms an alkane.
- Hydration with steam forms an alcohol.
SECTION 05
Alcohols
Ethanol is manufactured by fermenting aqueous glucose with yeast at 25–35 °C without oxygen, or by reacting ethene with steam at 300 °C, 6000 kPa and an acid catalyst.
Fermentation uses renewable sugar and mild conditions but is slow, produces dilute ethanol and needs purification. Hydration is fast and continuous with a pure product but uses non-renewable ethene and demanding conditions.
Ethanol burns, acts as a solvent and can be used as a fuel.
SECTION 06
Carboxylic acids and esters
Ethanoic acid reacts like other acids with metals, bases and carbonates to form ethanoate salts.
Ethanol oxidises to ethanoic acid using acidified potassium manganate(VII) or by bacterial oxidation in vinegar production.
A carboxylic acid reacts reversibly with an alcohol using an acid catalyst to make an ester and water. Ester names combine the alcohol-derived alkyl name followed by the acid-derived alkanoate name.
Naming an ester
- Ethanol supplies the ethyl part.
- Ethanoic acid supplies the ethanoate part.
- Write the alcohol-derived name first.
Answer: Ethanol and ethanoic acid form ethyl ethanoate.
SECTION 07
Polymers and proteins
Polymers are large molecules made from monomers. Addition polymerisation joins alkene monomers without forming a small by-product.
Condensation polymerisation forms a linkage and a small molecule. Diols with dicarboxylic acids form polyesters; diamines with dicarboxylic acids form polyamides.
Nylon is a polyamide, PET is a polyester and proteins are natural polyamides made from amino acids.
Plastics accumulate in landfill and oceans and may form toxic gases when burned. PET can be returned to monomers and re-polymerised.
KEY IDEAS
- An addition-polymer repeat unit has the same atoms as the alkene monomer.
- Polyester linkage: –COO–; polyamide linkage: –CONH–.
- Condensation monomers need two functional groups.
QUICK CHAPTER SUMMARY
The ideas to carry forward
- Functional groups define homologous series.
- Petroleum fractions differ through chain length.
- Alkenes undergo addition and decolourise bromine water.
- Ethanol has fermentation and hydration routes.
- Acids and alcohols form esters.
- Addition and condensation polymers form differently.