Chemistry · Lesson 14

Organic chemistry families

JAMBWAECNECO

What the syllabus asks

JAMB: hydrocarbons, homologous series, isomerism, functional groups, alkanols, alkanoic acids, esters, petroleum, polymers, fats and soaps

WAEC: tetravalency of carbon, classification of organic compounds, homologous series and functional groups, IUPAC nomenclature, isomerism, determination of empirical and molecular formulae, alkanes, alkenes, alkynes, benzene, alkanols, alkanoic acids, alkanoates (esters), fats and oils, soaps and detergents, petroleum (fractional distillation, cracking, octane rating), polymers, carbohydrates and…

NECO: tetravalency of carbon, classification of organic compounds, homologous series and functional groups, IUPAC nomenclature, isomerism, determination of empirical and molecular formulae, alkanes, alkenes, alkynes, benzene, alkanols, alkanoic acids, alkanoates (esters), fats and oils, soaps and detergents, petroleum (fractional distillation, cracking, octane rating), polymers, carbohydrates and…

What you will learn

Explain why carbon forms so many compounds and classify organic compounds.
Identify functional groups and homologous series, and name compounds by IUPAC rules.
Draw and name structural isomers.
Determine empirical and molecular formulae of organic compounds.
Describe the preparation, properties and reactions of alkanes, alkenes, alkynes and benzene.
Describe the reactions of alkanols, alkanoic acids and esters, including fermentation, esterification and saponification.
Explain fractional distillation and cracking of petroleum, octane rating and polymerisation.

Key points

Carbon is tetravalent and catenates, forming chains, rings and multiple bonds.
Homologous series: same general formula and functional group, differ by CH₂, gradual change in physical properties.
IUPAC: longest chain stem (meth, eth, prop, but…) + suffix (-ane, -ene, -yne, -ol, -oic acid, -oate) + lowest numbers.
Isomers: same molecular formula, different structures (chain, position, functional group, cis–trans).
Empirical formula from % mass or combustion data; molecular formula = n × empirical formula.
Alkanes: saturated, combustion and substitution (Cl₂, UV light); incomplete combustion gives CO.
Alkenes: addition with Br₂ (decolorises), H₂ (Ni), H₂O, HX; addition polymerisation.
Ethyne from CaC₂ + water; oxy-acetylene welding; benzene is stable and prefers substitution.
Ethanol by fermentation (zymase) or hydration of ethene; oxidised to ethanoic acid; dehydrated to ethene.
Acid + alkanol ⇌ ester + water (conc. H₂SO₄); esters smell fruity; fats + NaOH → soap + glycerol.
Crude oil is separated by fractional distillation; cracking makes petrol and alkenes; addition vs condensation polymers.

Start here: Why carbon is special

Organic chemistry was once defined as the chemistry of things made by living organisms: sugar from cane, oil from palm fruit, alcohol from palm wine. In 1828, Friedrich Wöhler made urea, a substance found in urine, from an inorganic salt, showing that no "life force" is needed. Today we define organic chemistry as the chemistry of carbon compounds (except simple ones like CO, CO₂ and carbonates).

More than ten million organic compounds are known, far more than for all other elements combined. Why?

Tetravalency: carbon has four valence electrons and forms four strong covalent bonds, arranged tetrahedrally (109.5° apart) when all are single.
Catenation: carbon atoms bond strongly to each other, forming long chains, branched chains and rings.
Multiple bonds: carbon can form single, double (C=C) and triple (C≡C) bonds.
Bonding with other elements: carbon bonds readily with H, O, N, S and halogens.

Ways of writing formulas

Molecular formula: actual numbers of atoms, e.g. C₂H₆O.
Structural formula: shows how atoms are joined, e.g. CH₃CH₂OH.
Displayed formula: every bond drawn.
Empirical formula: simplest whole-number ratio, e.g. CH₂ for C₂H₄.

Organic compounds are mainly covalent, so they usually have low melting and boiling points, are often insoluble in water but soluble in organic solvents, are flammable, and react more slowly than ionic compounds.

Worked example 14.1

Classify each as organic or inorganic: CH₄, CaCO₃, C₆H₁₂O₆, CO₂, C₂H₅OH.

Carbonates and oxides of carbon are treated as inorganic.
The others contain carbon chains bonded to hydrogen.

Organic: CH₄, C₆H₁₂O₆, C₂H₅OH; inorganic: CaCO₃, CO₂

⚠️ Common mistake: drawing a carbon with 3 or 5 bonds. Count: every carbon must have exactly four bonds.

Try it yourself. What is the empirical formula of C₄H₁₀?

Show solution

C₂H₅.

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Practice questions

1. The general formula of the alkanes is

CₙH₂ₙ₊₂
CₙH₂ₙ
CₙH₂ₙ₋₂
CₙH₂ₙ₊₁OH

Show answer

A: CₙH₂ₙ₊₂
Alkanes are saturated.

2. Which compound decolourises bromine water?

Ethane
Methane
Ethanol
Ethene

Show answer

D: Ethene
Alkenes undergo addition across C=C.

3. The reaction between ethanoic acid and ethanol produces

an alkene
an aldehyde only
an ester
a soap

Show answer

C: an ester
Esterification gives ethyl ethanoate + water.

4. The IUPAC name of CH₃CH₂CH₂OH is

propan-2-ol
propan-1-ol
propanal
propanoic acid

Show answer

B: propan-1-ol
Three carbons, OH on carbon 1.

5. Soap is made by heating fats or oils with

sodium hydroxide
sulphuric acid
ethanol
chlorine

Show answer

A: sodium hydroxide
Saponification.

Key words

Organic chemistry: The chemistry of carbon compounds (except oxides of carbon, carbonates and a few others).
Hydrocarbon: A compound of carbon and hydrogen only.
Catenation: The ability of atoms of an element to bond to each other forming chains and rings.
Homologous series: A family of organic compounds with the same general formula and functional group, differing by CH₂.
Functional group: The atom or group of atoms that determines the characteristic reactions of a compound.
Saturated: Containing only single carbon–carbon bonds.
Unsaturated: Containing at least one C=C or C≡C bond.
Isomerism: Existence of compounds with the same molecular formula but different structures.
Substitution reaction: A reaction in which an atom or group is replaced by another.
Addition reaction: A reaction in which atoms add across a multiple bond to form a single product.
Cracking: Breaking large hydrocarbon molecules into smaller, more useful ones.
Polymerisation: Joining of many small molecules (monomers) to form a large molecule (polymer).

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