Homologous Series and Naming

The definition of a homologous series is one of the most predictable marks in 0620 organic chemistry, and one of the most fumbled. Examiners want four specific features, and a typical answer gives two. Add the general formulae and the meth/eth/prop/but naming stems, and this subtopic underwrites every other page of organic chemistry.

The four-part definition

A homologous series is a family of compounds in which all members have:

1. The same general formula: every alkane fits CnH2n+2, every alkene fits CnH2n.
2. The same functional group: the atom or group of atoms responsible for the chemical reactions of the compound (C=C in alkenes, −OH in alcohols, −COOH in carboxylic acids).
3. Similar chemical properties: because the functional group is the same, the reactions are the same. Propene decolourises bromine water exactly as ethene does.
4. A trend in physical properties: boiling point rises steadily as the carbon chain lengthens, so volatility falls down the series.

Each member differs from the next by CH2. A 3-mark question expects three of these features; learn all four and pick the clearest three.

The four general formulae

Series	General formula	Functional group	First named member
Alkanes	CnH2n+2	none (C−C and C−H single bonds only)	methane, CH4
Alkenes	CnH2n	C=C double bond	ethene, C2H4
Alcohols	CnH2n+1OH	−OH (hydroxyl)	methanol, CH3OH
Carboxylic acids	CnH2n+1COOH	−COOH (carboxyl)	methanoic acid, HCOOH

These formulae do double duty in exams. Forwards: write the formula of pentane (n = 5, so C5H12). Backwards: identify the series of C3H7OH (fits CnH2n+1OH, so an alcohol, propanol). Watch the carboxylic acid count: in CnH2n+1COOH, the n counts only the carbons outside the −COOH group, so ethanoic acid (two carbons total) is CH3COOH with n = 1.

Naming to C4

The stem gives the carbon count; the ending gives the series:

Carbons	Stem	Alkane	Alkene	Alcohol	Carboxylic acid
1	meth-	methane	none	methanol	methanoic acid
2	eth-	ethane	ethene	ethanol	ethanoic acid
3	prop-	propane	propene	propanol	propanoic acid
4	but-	butane	butene	butanol	butanoic acid

There is no methene, as a C=C needs two carbons. On the Extended paper you also name branched isomers (methylpropane) and positional isomers (but-1-ene, but-2-ene); the detail lives in isomerism.

Displayed formulae

A displayed formula shows every atom and every bond. The instruction “draw the displayed formula” is a command to draw all the C−H bonds. Writing CH3CH2OH scores nothing. The two checks before moving on: count the hydrogens against the general formula, and make sure the O−H bond in an alcohol or acid is drawn as a bond, not squashed into “OH”. Saturated compounds (alkanes) contain only single bonds; unsaturated compounds (alkenes) contain the C=C.

Worked exam question

Compound Y is the third member of the alcohol homologous series. (a) Name Y and give its molecular formula. [2] (b) State two features of a homologous series. [2]

Model answer, mark by mark:

• (a) M1: propanol (propan-1-ol accepted).
• (a) M2: C3H7OH, from CnH2n+1OH with n = 3. C3H8O earns the mark, but C3H7OH shows the examiner you know the functional group.
• (b) M3: same general formula (or: same functional group).
• (b) M4: similar chemical properties (or: physical properties show a trend, e.g. boiling point increases with chain length; or: consecutive members differ by CH2).

The classic error in (a) is counting methanol as “member zero” and answering butanol. Methanol is the first member; count from there.

The mistakes that cost marks

1. Two-feature definitions. “Same general formula and similar properties” leaves a mark on the table. Specify chemical properties similar, physical properties trending: they are separate points.
2. Wrong n in carboxylic acids. Writing propanoic acid as C3H7COOH (that is butanoic acid). The COOH carbon counts in the name but not in the n of the general formula.
3. Structural formulae offered as displayed formulae. CH3CH3 is not a displayed formula of ethane. Draw all eight bonds.
4. “Methene” or “methanoic” confusion. No one-carbon alkene exists, and methanoic acid is HCOOH: a hydrogen, not a CH3, on the COOH.

How examiners want it phrased

Student wording	Mark-scheme wording
”They’re in the same family so they act the same"	"Same functional group, so similar chemical properties"
"The formula follows a pattern"	"Same general formula; consecutive members differ by CH2"
"Bigger ones boil higher"	"Boiling point increases as the number of carbon atoms increases"
"It’s got an OH on it"	"It contains the −OH functional group, so it is an alcohol”

Mark schemes reward the syllabus vocabulary: general formula, functional group, trend. If the general formulae refuse to stick, a free trial lesson takes about twenty minutes to build the naming grid above from scratch. Most students never lose those marks again.

Test yourself

Try these three from memory. The answers stay hidden until you click.

Q1 (3 marks). State three features shared by all members of a homologous series.

Show answer

Any three of: • same general formula [1] • same functional group [1] • similar chemical properties [1] • trend in physical properties, e.g. boiling point increases with chain length (or: consecutive members differ by CH2) [1]

Q2 (2 marks). A hydrocarbon has the molecular formula C5H10. Name its homologous series and justify your answer.

Show answer

• alkene [1] • C5H10 fits the general formula CnH2n with n = 5 [1]

Q3 (2 marks). Give the molecular formula of butanoic acid and name its functional group.

Show answer

• C3H7COOH (the COOH carbon is the fourth carbon, so n = 3 in CnH2n+1COOH) [1] • carboxyl group, −COOH [1]