Chemistry · Lesson 15

Hydrocarbons in depth: alkanes, alkenes, alkynes, benzene, petroleum

JAMBWAECNECO

What the syllabus asks

JAMB: Hydrocarbons: alkanes, alkenes, alkynes, aromatics; petroleum refining and petrochemicals

WAEC: Hydrocarbons: alkanes, alkenes, alkynes and aromatic hydrocarbons (benzene): general formulae, nomenclature (IUPAC), isomerism, preparation, physical and chemical properties, uses; petroleum: composition, fractional distillation, cracking, reforming, octane number; natural gas and coal

NECO: Hydrocarbons: alkanes, alkenes, alkynes and aromatic hydrocarbons (benzene): general formulae, nomenclature (IUPAC), isomerism, preparation, physical and chemical properties, uses; petroleum: composition, fractional distillation, cracking, reforming, octane number; natural gas and coal

What you will learn

State the general formulae of alkanes, alkenes and alkynes and name members by IUPAC rules.
Draw and name structural isomers.
Describe the preparation, properties and uses of methane, ethene and ethyne.
Distinguish saturated from unsaturated hydrocarbons by chemical tests.
Describe the structure and main reactions of benzene.
Describe fractional distillation of crude oil, cracking and reforming, and explain octane number.
Solve calculations on hydrocarbon formulae and combustion.

Key points

Hydrocarbons contain C and H only; carbon forms 4 bonds and chains (catenation).
Alkanes CₙH₂ₙ₊₂: saturated; combustion and substitution (with Cl₂ in light).
Alkenes CₙH₂ₙ: C=C; addition (Br₂, H₂/Ni, H₂O, HX) and polymerisation; ethene from ethanol + conc. H₂SO₄ at 170 °C.
Alkynes CₙH₂ₙ₋₂: C≡C; ethyne from CaC₂ + water; oxy-acetylene flame; precipitate with ammoniacal Cu(I).
IUPAC: longest chain, lowest locants, alphabetical branches.
Isomers: same molecular formula, different structures; branching lowers boiling point.
Benzene C₆H₆: delocalised, stable, substitution (nitration, bromination with FeBr₃).
Crude oil is separated by fractional distillation; cracking makes petrol and alkenes; reforming raises octane number.

Start here: What hydrocarbons are and why carbon is special

Cooking gas, petrol, kerosene, candle wax and the black bitumen on Nigerian roads all have one thing in common: they are made mainly of hydrocarbons, compounds containing only carbon and hydrogen.

Carbon is special for three reasons:

It forms four covalent bonds (it has four valence electrons).
It can bond to other carbon atoms to make long chains, branches and rings. This ability is called catenation.
It can form single, double and triple bonds.

So millions of organic compounds exist. To study them, chemists group them into families called homologous series.

A homologous series has: the same general formula; members differing by CH₂; similar chemical properties; a gradual change in physical properties (boiling point rises with chain length); and similar methods of preparation.

Classifying hydrocarbons

Aliphatic (open-chain or non-benzene ring): alkanes, alkenes, alkynes.
Aromatic: containing the benzene ring.
Saturated: only single bonds (alkanes). Unsaturated: at least one double or triple bond.

Naming: the first part tells the number of carbons

C atoms	1	2	3	4	5	6	7	8
Stem	meth-	eth-	prop-	but-	pent-	hex-	hept-	oct-

The ending tells the family: -ane (alkane), -ene (alkene), -yne (alkyne).

Try it yourself. Name the alkane with 5 carbon atoms and the alkene with 3.

Show solution

Pentane; propene.

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Practice questions

1. C₅H₁₂ belongs to the homologous series of

alkanes
alkenes
alkynes
alkanols

Show answer

A: alkanes
It fits CₙH₂ₙ₊₂.

2. The general formula of the alkenes is

CₙH₂ₙ₊₂
CₙH₂ₙ₋₂
CₙHₙ
CₙH₂ₙ

Show answer

D: CₙH₂ₙ
They contain C=C.

3. Bromine water is decolourised by

ethane
methane
propene
propane

Show answer

C: propene
Alkenes add bromine across the C=C.

4. Alkanes typically undergo

neutralisation
substitution reactions
addition reactions
polymerisation

Show answer

B: substitution reactions
They are saturated.

5. Ethyne has the formula

C₂H₂
C₂H₄
C₂H₆
CH₄

Show answer

A: C₂H₂
A triple bond.

Key words

Hydrocarbon: Compound of carbon and hydrogen only.
Homologous series: Family of compounds with the same general formula, similar chemical properties, and successive members differing by CH₂.
Saturated: Having only single C–C bonds.
Unsaturated: Having at least one C=C or C≡C bond.
Isomerism: Existence of compounds with the same molecular formula but different structures.
Substitution reaction: An atom in a molecule is replaced by another atom or group.
Addition reaction: Atoms add across a multiple bond.
Polymerisation: Joining of many small molecules (monomers) to form a large molecule.
Cracking: Breaking large hydrocarbon molecules into smaller ones using heat and/or catalysts.
Reforming: Converting straight-chain hydrocarbons into branched or ring compounds to improve petrol.
Octane number: Measure of a petrol's resistance to knocking.
Fraction: A mixture of hydrocarbons boiling within a range.

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