IB Chemistry HLTopic 3 — Classification of MatterPaper 1 & 2Trends~12 min read
Homologous Series
Carbon bonds to itself over and over, which is why there are millions of organic compounds instead of a few dozen. To keep that manageable, chemists file compounds into families where each member is just the last one plus one more CH2. Learn one member properly and you have a very good idea how the rest behave.
📘 What you need to know
Catenation is the ability of an element to bond to itself in chains. Carbon is exceptionally good at it.
A homologous series is a family of compounds with the same functional group and the same general formula, whose members differ by CH2.
Members have similar chemical properties, because the reacting group is identical.
Members have gradually changing physical properties — boiling point, melting point, density and viscosity all trend upwards as the chain lengthens.
You must be able to use a general formula such as CnH2n+2 for alkanes or CnH2n+1OH for alcohols.
Each CH2 adds 14.03 to the relative molecular mass. Two consecutive members always differ by 14.
Longer chains have larger surface area, so stronger London dispersion forces, so higher boiling points.
Why carbon can do this at all
Carbon forms four strong covalent bonds, and crucially it forms strong bonds to other carbon atoms. That means chains, branches and rings can be built to almost any length without falling apart. Silicon is directly below carbon and can also catenate, but Si–Si bonds are much weaker, which is why there is no silicon-based equivalent of organic chemistry.
Once you accept that the chain can be any length, you need a filing system. That system is the homologous series.
What makes a homologous series
Definition worth learning word for word
A family of compounds with the same functional group and the same general formula, whose successive members differ by CH2
Four things follow from that definition, and exam questions test all four:
Same functional group, so the same reactions happen.
Same general formula, so you can predict any member from its carbon count.
Successive members differ by CH2 — often called the homologous increment.
Physical properties change gradually, in a predictable direction.
Teal is the part that never changes — that is the functional group, and it is why all three react the same way. Amber is what gets added each step, and it is why the boiling points climb.
General formulas you should recognise
Homologous series
General formula
Example with 3 carbons
Formula of that example
alkanes
CnH2n+2
propane
C3H8
alkenes
CnH2n
propene
C3H6
alkynes
CnH2n-2
propyne
C3H4
halogenoalkanes
CnH2n+1X
1-chloropropane
C3H7Cl
alcohols
CnH2n+1OH
propan-1-ol
C3H8O
aldehydes
CnH2nO, written RCHO
propanal
C3H6O
ketones
CnH2nO, written RCOR
propanone
C3H6O
carboxylic acids
CnH2n+1COOH
propanoic acid
C3H6O2
ethers
CnH2n+2O, written ROR
methoxyethane
C3H8O
amines
CnH2n+1NH2
propan-1-amine
C3H9N
esters
CnH2nO2, written RCOOR
methyl ethanoate
C3H6O2
One inconsistency to watch. In CnH2n+1OH the letter n is the total number of carbons, so n = 3 gives propan-1-ol. But in CnH2n+1COOH the n only counts the alkyl part, because the acid’s own carbon is already written in the COOH. So n = 2 gives propanoic acid, not ethanoic acid. Whenever you use a general formula, substitute a small n and check you get the compound you expected before you trust it.
Notice that aldehydes and ketones share CnH2nO, and that alcohols and ethers share CnH2n+2O. A general formula alone cannot tell you the series — you still have to look at where the oxygen sits. This trips people up in multiple-choice questions constantly.
The physical trend, and why it happens
Every homologous series shows the same pattern: as you go up the series, boiling and melting points rise. The reasoning chain is short and you should be able to write it out in three steps.
🧩 The three-step explanation examiners want
More carbons means a bigger molecule with more electrons and a larger surface area.
Bigger surface area means stronger London dispersion forces between neighbouring molecules, because there is more area over which temporary dipoles can attract each other.
Stronger intermolecular forces need more energy to overcome, so the boiling point is higher.
Say “intermolecular forces”, never “bonds”. Boiling does not break covalent bonds — it only pulls molecules away from each other. Writing that you are “breaking bonds between the molecules” is the single most common way to lose this mark.
Two separate effects are on display here. Each line rises because chains get longer and London forces get stronger. The lines are stacked because of hydrogen bonding: alkanes have none, alcohols have one OH, and carboxylic acids pair up through two hydrogen bonds at once.
Watch the gaps narrow as you move right. Going from one carbon to two changes the molecule enormously; going from nine to ten barely matters, because the CH2 you added is a small fraction of what is already there. That is why these graphs flatten out rather than staying straight.
Worked examples
WORKED EXAMPLE
An alkane has seven carbon atoms. Deduce its molecular formula and its relative molecular mass. Use Ar(C) = 12.01 and Ar(H) = 1.01.
Step 1: pick the right general formulaAlkanes are CnH2n+2, and here n = 7.Step 2: substitutehydrogens = (2 × 7) + 2 = 16molecular formula = C7H16Step 3: work out MrMr = (7 × 12.01) + (16 × 1.01)= 84.07 + 16.16 = 100.23C7H16, Mr = 100.23This is heptane. If you had got C7H14 you have used the alkene formula by mistake.
WORKED EXAMPLE
Two compounds are next to each other in the same homologous series. Show that their relative molecular masses must differ by 14.
Step 1: state what separates successive membersBy definition, going up one place in a homologous series adds one CH2 unit.Step 2: find the mass of that CH2mass of CH2 = 12.01 + (2 × 1.01)= 12.01 + 2.02 = 14.03Step 3: check it on a real pairpropane C3H8: Mr = 36.03 + 8.08 = 44.11butane C4H10: Mr = 48.04 + 10.10 = 58.1458.14 − 44.11 = 14.03the difference is always 14.03, so 14 to the nearest whole numberKeep this number in your head — a gap of 14 in a mass spectrum is a very strong hint that a CH2 has been lost.
WORKED EXAMPLE
Butane, butan-1-ol and butanoic acid all have four carbon atoms. Put them in order of increasing boiling point and explain your order.
Step 1: chain length is the same, so compare the groups insteadAll three have four carbons, so London dispersion forces are broadly similar. The difference has to come from the functional group.Step 2: identify the strongest intermolecular force in eachButane is non-polar, so London forces only. Butan-1-ol has an OH, so it can hydrogen bond. Butanoic acid has a COOH, so it can form two hydrogen bonds at once and pair up with another molecule.Step 3: order them, weakest force firstbutane (−0.5 °C) < butan-1-ol (118 °C) < butanoic acid (164 °C)butane < butan-1-ol < butanoic acidThe explanation is what earns the marks, not the order. Name the force in each case and say which is strongest.
WORKED EXAMPLE
A compound has the molecular formula C4H8O. State two homologous series it could belong to, and explain how you would tell them apart.
Step 1: match the formula to a general formulaC4H8O fits CnH2nO with n = 4Check: 2n = 8, and there is one oxygen. It fits.Step 2: recall which series share that general formulaBoth aldehydes and ketones are CnH2nO, because both contain one C=O and no other oxygen.Step 3: say how to distinguish themLook at where the C=O sits. At a chain end with a hydrogen on it, it is an aldehyde (butanal). Inside the chain with carbons on both sides, it is a ketone (butanone).aldehydes or ketones — check whether the carbonyl carbon carries a hydrogenPractically, you would oxidise it. Aldehydes are oxidised to carboxylic acids; ketones resist oxidation.
💡 Exam tip
Learn the general formulas as a set, not one at a time. Alkane 2n+2, alkene 2n, alkyne 2n−2 — every double or triple bond costs you two hydrogens.
Always check a general formula by substituting a small n. Put n = 2 into CnH2n+2 and you should get ethane, C2H6.
Explain trends with the full chain of reasoning. More carbons, larger surface area, stronger London forces, more energy needed, higher boiling point. Missing links lose marks.
Never say “breaking bonds” when you mean boiling. Say overcoming intermolecular forces.
When two compounds have the same chain length, the boiling point difference must be down to the functional group and the type of intermolecular force it allows.
Remember 14. It is the mass of a CH2 and it appears in general formula questions, mass spectra and homologous series questions alike.
⚠ Common mix-up
Confusing a homologous series with a class of compound. A class is defined by the functional group alone. A homologous series also needs the same general formula and the CH2 difference.
Assuming a general formula identifies a series. CnH2nO covers aldehydes and ketones; CnH2n+2O covers alcohols and ethers; CnH2n covers alkenes and cycloalkanes.
Saying chemical properties change up a series. They stay similar — it is the physical properties that trend.
Explaining boiling point with “the molecule is heavier”. Mass is not the mechanism. Surface area and the strength of the intermolecular forces are.
Using CH2 as the increment but forgetting it goes into the chain. The extra CH2 is inserted into the carbon skeleton; it is not stuck onto the functional group.
Getting the alkyne formula wrong. It is CnH2n-2. A triple bond removes four hydrogens compared with the alkane, not two.
Expecting the boiling point graph to be a straight line. The steps get smaller as the chain grows, so the line curves and flattens.
Up next: IUPAC Naming — you can spot the group and place it in a series, so now you can build the name that tells a chemist all of it at once.
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