PAPER 2 · UNIT 1 · NAMING, ISOMERS & PROPERTIESORGANIC MOLECULES
Learning outcomes · you must be able to
Identify the functional group in a structure and classify the compound into its correct homologous series.
Apply the IUPAC naming procedure to name a compound from a structural formula, and draw a structural formula from an IUPAC name.
Identify and distinguish chain, positional and functional isomers that share the same molecular formula.
Classify an alcohol as primary, secondary or tertiary from its structure.
Explain trends in boiling point, melting point and vapour pressure using intermolecular forces.
Compare the physical properties of two named compounds by reasoning about the intermolecular forces present in each.
📖 KEY DEFINITIONOrganic compound: a compound built on a carbon backbone, almost always containing only carbon, hydrogen, oxygen and, in haloalkanes, one halogen. Hydrocarbon: an organic compound containing carbon and hydrogen only. Functional group: the atom, bond or small group of atoms responsible for a family of compounds' characteristic chemical and physical behaviour. Homologous series: a family of compounds sharing one general formula and one functional group, each member differing from the next by one - CH2 - unit. Structural isomers: compounds sharing the same molecular formula but differing in how their atoms are connected.
Core content · bonding rules and ways of representing a molecule
Carbon always forms exactly 4 bonds; oxygen always forms exactly 2; hydrogen and a halogen (F, Cl, Br, I) each form exactly 1 - every structural formula you draw must obey these three rules.
Saturated compounds contain only single C - C bonds. Unsaturated compounds contain at least one C=C or C≡C bond.
Representation
What it shows
Molecular formula
Element symbols and atom counts only, e.g. C3H8 - tells you nothing about connectivity.
Structural formula
Every atom and every bond drawn out individually.
Condensed structural formula
Atoms grouped by the carbon they attach to, most bond lines dropped, e.g. CH3CH2CH3.
General formula
The algebraic pattern for a whole homologous series, e.g. CnH2n+2 for alkanes.
Core content · the homologous series in scope
Series
General formula
Functional group
Suffix
Example
Alkane
CnH2n+2
none (all single bonds)
-ane
propane
Alkene
CnH2n
one C=C
-ene
propene
Alkyne
CnH2n-2
one C≡C
-yne
propyne
Haloalkane
CnH2n+1X
one halogen (X)
named as prefix
1-chloropropane
Alcohol
CnH2n+1OH
hydroxyl, - OH
-ol
propan-1-ol
Aldehyde
CnH2nO, n≥1
- CHO on end carbon
-al
butanal
Ketone
CnH2nO, n≥3
>C=O on internal carbon
-one
butan-2-one
Carboxylic acid
CnH2nO2, n≥1
carboxyl, - COOH
-oic acid
propanoic acid
Ester
CnH2nO2, n≥2
- COO - linkage
-oate
ethyl ethanoate
⚡ MUST MEMORISE
Bonding rule: C forms 4 bonds, O forms 2, H/halogen forms 1. Aldehydes and ketones share CnH2nO and are functional isomers of each other; carboxylic acids and esters share CnH2nO2 and are functional isomers of each other. A ketone needs at least 3 carbons, since its carbonyl carbon must sit between two others; an ester needs at least 2 carbons in total.
Core content · the IUPAC naming procedure
Find the functional group present - it fixes the suffix (a halogen is always named as a prefix, never a suffix).
Find the longest continuous carbon chain that contains the functional group, wherever it runs on the page.
Number the chain to give the functional group the lowest possible locant (for alkanes/haloalkanes, number from the end nearest the first substituent instead).
Name every substituent, using di-/tri-/tetra- for repeats - but ignore these multiplying prefixes when alphabetising.
Order multiple substituents alphabetically in the name.
Punctuate correctly: a hyphen between a number and a letter, a comma between two numbers.
Carboxylic acids and aldehydes never take a locant number - the functional group can only ever sit on carbon 1 (e.g. "ethanoic acid," never "1-ethanoic acid").
⭐ EXAM FAVOURITE
Naming a structure from its formula, or drawing a structure from an IUPAC name, is tested in almost every paper - usually as several short sub-questions spanning different homologous series in one go. Expect at least one structure with a branch that forces you to choose the longest chain carefully, and at least one isomer-identification question.
Core content · structural isomers
Isomer type
What stays the same
What differs
Example pair
Chain
Homologous series, functional group
How the carbon skeleton branches
butane vs 2-methylpropane
Positional
Homologous series, functional group
Which carbon the group sits on
propan-1-ol vs propan-2-ol
Functional
Molecular formula only
Which functional group is present
propanal vs propanone
⚠️ COMMON MISTAKE
Naming CH3CH(CH3)CH3 as "1-methylpropane" is not valid IUPAC - a methyl group on a terminal carbon simply makes the chain one carbon longer, so the compound must be renamed on its new, longer chain. The correct name is 2-methylpropane. Also common: adding a stray locant number to a carboxylic acid or aldehyde name (there is no "1-" in "ethanoic acid" or "pentanal").
Core content · intermolecular forces and physical property trends
Physical properties (boiling point, melting point, vapour pressure, solubility) depend on the intermolecular forces (IMF) between neighbouring molecules, not the strong covalent bonds inside a molecule.
IMF, weakest → strongest
Present in
Requires
Induced dipole (dispersion)
Every organic molecule; the ONLY force in alkanes, alkenes, alkynes
Nothing special - momentary shifts in electron distribution
Dipole-dipole
Haloalkanes, aldehydes, ketones, esters
A permanent partial-positive/partial-negative end, without an O - H bond
Hydrogen bonding
Alcohols, carboxylic acids
An O - H bond on one molecule and a lone pair on a neighbouring molecule's O
Factor increases →
IMF strength
Boiling/melting point
Vapour pressure
Chain length
Stronger
Higher
Lower
Branching
Weaker (less surface contact)
Lower
Higher
Molecular mass
Stronger
Higher
Lower
Polarity of functional group
Stronger
Higher
Lower
🔥 FREQUENTLY TESTED
A question asking you to explain why compound X has a higher boiling point than compound Y appears in almost every sitting, usually comparing two compounds of similar chain length but different functional groups, or the same series with different branching. Always name the strongest IMF present in EACH compound before comparing - never just say "X is heavier."
💡 EASY MARK
"Name the strongest type of intermolecular force present in this compound" or "is this compound likely to dissolve in water" are 1-2 mark gifts once you can recognise the functional group - no calculation needed.
Worked example 1 · Naming a branched alkene
Q: Give the IUPAC name for CH3 - CH(CH3) - CH2 - CH=CH - CH3.
Functional group: one C=C → alkene, suffix -ene. Longest chain containing the double bond: 6 carbons → hex-.
Numbering from the left: the methyl branch is on C2, and the double bond starts at C4 (locant 4).
Numbering from the right: the double bond starts at C2 (locant 2), and the methyl branch is on C5.
The functional group (the double bond) gets the lower locant, so number from the right: locant 2 beats locant 4.
∴ 5-methylhex-2-ene (molecular formula C7H14)
Worked example 2 · Drawing a structure and classifying it
Q: Draw the structural formula of 3-methylbutan-2-ol and classify it as primary, secondary or tertiary.
Root but- = 4-carbon main chain. Suffix -2-ol = hydroxyl on C2. Substituent 3-methyl = methyl branch on C3.
Chain: C1(CH3) - C2(CHOH) - C3(CH(CH3)) - C4(CH3)
Structural formula: CH3 - CHOH - CH(CH3) - CH3
The carbon bonded to - OH (C2) is itself bonded to two other carbons (C1 and C3).
∴ secondary alcohol
Worked example 3 · Classifying isomer pairs
Q: State whether each pair is a chain, positional or functional isomer pair: (a) CH3CH2CH2CH2CH3 and CH3CH(CH3)CH2CH3; (b) CH3CH2CH2COOH and CH3CH(CH3)COOH; (c) CH3CH2CH2CHO and CH3COCH2CH3.
(a) Both C5H12 alkanes, same functional group (none), different carbon skeleton → chain isomers.
(b) Both C4H8O2 carboxylic acids - the - COOH group is fixed on C1 in both, so only the skeleton differs → chain isomers.
(c) Both C4H8O, but one is an aldehyde and the other a ketone, so they belong to different homologous series → functional isomers.
∴ (a) chain, (b) chain, (c) functional. Note: carboxylic acids and aldehydes can only ever produce chain isomers among themselves, never positional, since naming rule 7 fixes their functional group on C1.
Worked example 4 · Boiling point comparison (the recurring exam pattern)
Q: Compound P is pentan-1-ol, CH3CH2CH2CH2CH2OH (M ≈ 88 g·mol-1). Compound Q is hexane, CH3CH2CH2CH2CH2CH3 (M ≈ 86 g·mol-1). Predict, with reasons, which compound has the higher boiling point.
Compound P has an - OH group, so its molecules form hydrogen bonds with each other, on top of weaker induced-dipole forces.
Compound Q is a non-polar alkane - the ONLY force between its molecules is the induced-dipole (dispersion) force.
The two compounds have almost identical molecular mass, so mass cannot explain a difference - the TYPE of intermolecular force present decides the answer.
∴ Compound P (pentan-1-ol) has the higher boiling point, because hydrogen bonding between its molecules requires more energy to overcome than the induced-dipole forces between hexane molecules.
Every molecule is a sentence written in carbon - learn the grammar of functional groups and every name reads itself.- on organic naming
Why it matters: every question in this topic starts by finding the functional group, because that one step fixes the suffix, the general formula and the strongest intermolecular force at the same time. Skip it and you name the wrong chain, then explain a boiling point difference with 'it is heavier', which earns almost nothing.
🧠 ACTIVE RECALL
Which compound below is a hydrocarbon?(2)
Which of these four compounds is classified as a hydrocarbon?(2)
A hydrocarbon contains no elements besides carbon and hydrogen. Which compound below fits that description?(2)
Four compounds are listed. Three each contain at least one atom of oxygen or a halogen. Which is the exception?(2)
Select the compound that is classified purely as a hydrocarbon.(2)
The general formula CnH2n+2 describes which homologous series?(2)
A compound has 8 hydrogen atoms for every 3 carbon atoms it contains. Which homologous series does it belong to?(2)
Which general formula below matches the alkane homologous series?(2)
A saturated open-chain hydrocarbon containing only single C-C bonds follows which general formula?(2)
Butane, C4H10, belongs to a homologous series whose general formula is:(2)
Which functional group corresponds to the suffix "-oic acid"?(2)
The suffix "-oate" in an IUPAC name signals which functional group?(2)
A compound named "butanoic acid" must contain which functional group?(2)
Which suffix would you attach to a compound whose functional group is -COOH?(2)
Identify the functional group present in a compound ending in "-anoic acid":(2)
Propan-1-ol and propan-2-ol are an example of:(2)
Butan-1-ol and butan-2-ol are both C4H10O alcohols but the hydroxyl group sits on a different carbon in each. What relationship do they have?(2)
But-1-ene and but-2-ene share the molecular formula C4H8 and the same functional group, but the double bond starts on a different carbon in each. This makes them:(2)
Pentan-2-ol and pentan-3-ol have the same molecular formula and the same functional group, but the -OH sits on C2 in one and C3 in the other. This pair is best classified as:(2)
Two alcohols share the formula C5H12O and the hydroxyl group, but differ in which carbon carries it. This is an example of:(2)
Which of these compounds can hydrogen-bond with other molecules of itself?(2)
Which compound below has an O-H bond that allows it to hydrogen-bond with a neighbouring molecule of the same substance?(2)
Molecules of which compound can form hydrogen bonds with each other?(2)
Which of these four compounds lacks the O-H bond needed to hydrogen-bond with its own kind?(2)
Self-hydrogen-bonding between identical molecules requires an O-H bond. Which compound below has one?(2)
As branching increases in an alkane of the same molecular formula, its boiling point generally:(2)
Pentane and 2,2-dimethylpropane share the molecular formula C5H12, but the second is far more branched. Which one has the lower boiling point?(2)
Two alkane isomers share a molecular formula. The more branched isomer will generally have:(2)
Why does a highly branched alkane generally boil at a lower temperature than its straight-chain isomer?(2)
Comparing hexane with its most branched isomer, 2,2-dimethylbutane, which statement is correct?(2)
Explain, using intermolecular forces, why butan-1-ol has a considerably higher boiling point than butane, even though the two molecules have a similar number of carbon atoms.(4)
Butan-1-ol forms hydrogen bonds via its -OH groupThe examiner needs the specific force named for butan-1-ol; without an -OH group there is no hydrogen bonding, so this identifies the stronger force correctly.
Butane has only induced-dipole (dispersion) forcesButane has no polar functional group, so the reader must name the weaker force actually present rather than just saying the forces are weaker.
Butan-1-ol has an -OH group, so its molecules can form hydrogen bonds with each other.Butane is non-polar, so the only force between its molecules is the induced-dipole (dispersion) force. Hydrogen bonds require more energy to break than induced-dipole forces, so more heat is needed to separate the butan-1-ol molecules, giving butan-1-ol the higher boiling point.
Give the IUPAC name for CH3CH2CH2CH2OH, then write the condensed structural formula and IUPAC name of ONE positional isomer of this compound.(5)
Correct IUPAC name for the given compoundThe parent compound must be named correctly, with the hydroxyl group placed on C1, before any isomer of it can be produced.
Valid positional isomer namedA positional isomer of butan-1-ol must move the -OH group to a different carbon on the same chain; butan-2-ol is the only other position available on a straight 4-carbon chain.
States the isomer shares the same molecular formulaStating explicitly that the formula is unchanged is what distinguishes a positional isomer from a functional isomer, where the group itself changes.
The compound CH3CH2CH2CH2OH is named butan-1-ol, with the hydroxyl group fixed on carbon 1 of the four-carbon chain.A valid positional isomer is butan-2-ol, CH3CH(OH)CH2CH3, in which the hydroxyl group has moved to carbon 2.Both compounds share the same molecular formula, C4H10O, and only the position of the -OH group along the chain has changed.
Explain this for a Grade 8: why does a carboxylic acid usually have an even higher boiling point than an alcohol with a similar number of carbons?
Alcohol offers only one hydrogen-bonding siteThe Grade 8 explanation hinges on the alcohol only having a single -OH spot, so it can only link to a neighbour once.
Carboxylic acid offers two hydrogen-bonding sitesThe reader must show the carboxyl group gives two places to hydrogen-bond, needing more heat energy to separate the molecules, which is why the boiling point is even higher.
An alcohol molecule only has one hydroxyl spot, so it only has one hydrogen bond it can make with a neighbour.A carboxylic acid molecule has two separate spots that can each form a hydrogen bond with a neighbour, almost like grabbing on with both hands at once, so more heat energy is needed to pull the molecules apart.
Explain to a Grade 8 why chemists always work out a compound's functional group before they try to name it.
Functional group fixes the naming suffixA Grade 8 needs to see that the ending of a chemical name is not arbitrary, it is chosen because of the functional group present.
Functional group also decides the strongest intermolecular forceThis links naming directly back to physical properties, showing the functional group decides how the compound behaves, not just what it is called.
The functional group is what decides the suffix at the end of the compound's name, like -ol for an alcohol or -ene for an alkene.It also decides which intermolecular force is the strongest one between its molecules, which is why the functional group has to be spotted first.
Explain to a Grade 8 the difference between a chain isomer and a positional isomer.
Chain isomers differ in how the carbon skeleton branchesA Grade 8 must grasp that a chain isomer is a completely different shaped frame, not just a moved feature.
Positional isomers keep the same skeleton but move the functional groupThis is the key distinction: the frame stays identical, only the special feature's location changes.
A chain isomer keeps the same family and functional group, but its carbon skeleton has a different branch pattern, like a straight wall versus an L-shaped one.A positional isomer keeps exactly the same skeleton shape, but its functional group moves to a different position along that skeleton, like moving the chimney to another spot on the same wall.
Explain to a Grade 8 why you cannot just find a compound's longest carbon chain by reading the drawing from left to right.
The chain can branch off in any direction, not just left to rightA Grade 8 needs to see the drawing is not a fixed reading direction; the actual chain can turn any way on the page.
You must pick the longest path that still contains the functional groupChoosing a shorter, easier-to-read path is the classic mistake; the rule requires checking every path for the true longest one.
A carbon chain does not have to run neatly left to right; it can bend and branch off in any direction across the page.The rule is to trace every path through the structure and choose whichever one is the longest path that still contains the functional group, however it bends.
Explain to a Grade 8 why a liquid's boiling point depends on the pull between separate molecules, not the strong bonds holding one molecule's own atoms together.
Boiling separates molecules from their neighbours, it does not break bonds inside one moleculeA Grade 8 must distinguish the unbreakable internal bonds from the much weaker attraction being overcome during boiling.
The strength of the pull between neighbouring molecules sets the boiling pointThis links the abstract idea of intermolecular force directly to the observable boiling point.
Boiling pulls molecules apart from their neighbouring molecules; the strong bonds inside a single molecule hold its own shape together and are never broken.A liquid with a stronger pull between neighbours needs more heat before it boils, while a liquid with only a weaker pull boils at a much lower temperature.
ONE-MINUTE SUMMARY
Find the functional group first - it fixes the suffix, the general formula and the strongest intermolecular force present. Name the longest chain containing the group, number for the lowest locant, then alphabetise substituents. Chain isomers differ in skeleton, positional isomers differ in group position, functional isomers belong to different series entirely. Boiling point rises with chain length and mass, falls with branching, and jumps sharply once hydrogen bonding becomes possible in alcohols and carboxylic acids.
IN THE EXAM
Organic molecules is worth 35 marks on Paper 2, the largest single topic outside Chemical Change, and it is almost always tested as two separate structured questions: one on naming/isomers/drawing structures, and a second on physical properties, often built around a boiling-point comparison table.
Expect at least one structure where you must find the longest chain containing the functional group carefully, not just read left to right.
Marks are awarded for identifying the correct functional group AND naming the actual type of intermolecular force - an answer that only says "it's heavier" earns little to no credit.
🔒 ORGANIC REACTIONS
🔒 RATE OF REACTION
🔒 CHEMICAL EQUILIBRIUM
🔒 ACIDS AND BASES
🔒 ELECTROCHEMISTRY
🔒
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