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IB Diploma Chemistry HL · Structure 3.2 — functional groups
Mini-Lesson

Classifying organic compounds

This mini-lesson covers Structure 3.2: functional groups, homologous series, naming organic compounds, and structural isomers. This HL lesson also builds in the Additional Higher Level (AHL) material.

homologous seriesfunctional groupsisomers Structure 3.2 — classifying organic compounds

Work through each screen, answer the questions as you go (some are wordy, some are calculations) and collect ⭐ stars. Press Start when you're ready.

Structure 3.2

Organic chemistry basics

Organic compounds are built on carbon chains. A homologous series is a family sharing the same general formula and functional group, with each member differing by CH₂.

Stems name the chain length: meth- (1C), eth- (2C), prop- (3C), but- (4C), pent- (5C).

Structure 3.2

Alkanes, alkenes and alkynes

Alkanes (CₙH₂ₙ₊₂) are saturated — only single C–C bonds. Alkenes (CₙH₂ₙ) contain a C=C double bond and are unsaturated. Alkynes contain a C≡C triple bond.

Unsaturation (a C=C bond) is what lets alkenes undergo addition reactions and decolourise bromine water.

Quick check

Name that molecule

?A saturated hydrocarbon with three carbons is called:
Structure 3.2

Functional groups

A functional group is the reactive part that defines a family:

  • Alcohol –OH (suffix -ol)
  • Aldehyde –CHO (-al)
  • Ketone C=O in chain (-one)
  • Carboxylic acid –COOH (-oic acid)
  • Halogenoalkane –X (prefix chloro-/bromo-)
  • Amine –NH₂
Quick check

Spot the group

?Ethanoic acid (CH₃COOH) belongs to which family?
Structure 3.2

Structural isomers

Structural isomers share a molecular formula but differ in how atoms are joined:

  • Chain isomers — branched vs straight
  • Position isomers — functional group in a different place
  • Functional-group isomers — a different family (e.g. an alcohol vs an ether)
Calculate

Counting isomers (1)

1How many structural isomers does C₄H₁₀ have?
Butane (straight chain) and methylpropane (branched) — two isomers.
Calculate

Counting isomers (2)

2How many structural isomers does C₅H₁₂ have?
Pentane, 2-methylbutane and 2,2-dimethylpropane — three isomers.
Calculate

One rung of the ladder

3By what relative mass do successive members of a homologous series differ (one CH₂ unit)?
CH₂ = 12.01 + 2(1.01) ≈ 14.
AHL — Structure 3.2

More functional groups & unsaturation

AHL adds esters (–COO–), amides (–CONH–) and nitriles (–C≡N), and the degree of unsaturation (rings + multiple bonds) calculated from a molecular formula. It also treats the delocalised structure of benzene and how spectroscopic data help deduce structures.

AHL check

Spot the ester

?Which functional group is present in an ester such as ethyl ethanoate?
Sort it

Which family?

Tap a compound, then its functional-group family.

🍺 Alcohols

🧪 Carboxylic acids

🧂 Halogenoalkanes

Match it

Match group to ending

Tap an item on the left, then its match on the right.

Functional group
Name ending
Recap

The big ideas to know

Homologous series: same general formula & functional group; differ by CH₂

Hydrocarbons: alkanes (saturated), alkenes (C=C, unsaturated)

Functional groups: –OH alcohol, –COOH acid, –CHO aldehyde, C=O ketone, –X halogenoalkane

Isomers: chain, position and functional-group structural isomers

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