Homologous series and nomenclature

This page covers content standard 10.2 of the Form 5 Carbon Compounds chapter: homologous series and nomenclature. Every family you study later, alkanes, alkenes, alcohols, carboxylic acids and esters, is a homologous series, and every compound in the chapter is named by the same IUPAC rules. Master this standard and the whole chapter becomes a set of patterns rather than a list to memorise.

What is a homologous series?

A homologous series is a family of organic compounds that share the same general formula and the same functional group. Members are called homologues.

Key definition. A homologous series is a series of organic compounds with the same general formula and the same functional group, in which each member differs from the next by a –CH₂– unit.

The five characteristics you must be able to state are:

1. All members can be represented by one general formula (for example, alkanes are CₙH₂ₙ₊₂).
2. Consecutive members differ by CH₂ (a difference in relative molecular mass of 14).
3. Members have the same chemical properties, because they have the same functional group.
4. Members show a gradual change in physical properties as molecular size increases, for example, melting and boiling points rise, and density rises, down the series.
5. Members can be prepared by similar methods.

General formulae you must know

Homologous series	Functional group	General formula
Alkanes	C–C single bonds only	CₙH₂ₙ₊₂
Alkenes	C=C double bond	CₙH₂ₙ
Alcohols	–OH (hydroxyl)	CₙH₂ₙ₊₁OH
Carboxylic acids	–COOH (carboxyl)	CₙH₂ₙ₊₁COOH

You should be able to write the formula of any member by substituting the value of n.

Why physical properties change gradually

As you go down a series, the molecules get larger and have more electrons, so the van der Waals forces between molecules become stronger. More energy is needed to separate the molecules, so boiling and melting points increase. The first members are gases, the middle members are liquids, and the later members are solids. Because the functional group is unchanged, the chemical properties stay the same even though the physical ones drift.

IUPAC nomenclature

A compound’s name is built from three parts:

1. Stem, shows the number of carbon atoms in the longest chain: meth- (1), eth- (2), prop- (3), but- (4), pent- (5).
2. Suffix, shows the family: -ane (alkane), -ene (alkene), -ol (alcohol), -oic acid (carboxylic acid).
3. Position numbers and prefixes, show where a double bond, a functional group, or a branch (side chain) is attached.

To name a branched or substituted compound:

• Find the longest continuous carbon chain, this gives the stem.
• Number the chain from the end that gives the functional group or double bond (and then any branch) the lowest number.
• Name each branch as a substituent (a –CH₃ branch is methyl, a –C₂H₅ branch is ethyl) and place its position number in front.

Worked example

Question. Name the compound below and give its molecular formula.

CH₃, CH(CH₃), CH₂, CH₃

Step 1, Find the longest chain. The longest continuous chain has four carbon atoms, so the stem is but-. All bonds between carbons are single, so the family suffix is -ane → the base name is butane.

Step 2, Identify the branch. There is a –CH₃ (methyl) group attached as a side chain.

Step 3, Number for the lowest locant. Numbering from the left puts the methyl branch on carbon 2; from the right it would be carbon 3. We choose the lower number, so the methyl is on carbon 2.

Step 4, Assemble the name. Substituent (with position) + base name = 2-methylbutane.

Step 5, Molecular formula. Counting all atoms: 5 carbons and 12 hydrogens → C₅H₁₂.

Answer. The compound is 2-methylbutane, molecular formula C₅H₁₂.

Practice question

Question. The third member of the alcohol series has the general formula CₙH₂ₙ₊₁OH with n = 3. Write its molecular formula and name, and explain how it differs from the second member in the series.

Answer. With n = 3, the formula is C₃H₇OH, named propanol. The second member is ethanol, C₂H₅OH. Propanol differs from ethanol by exactly one –CH₂– unit (a relative molecular mass difference of 14), which is the defining feature of consecutive members of a homologous series. Because both have the same –OH functional group, they show the same chemical properties, while propanol has a slightly higher boiling point because its larger molecule has stronger forces of attraction between molecules.

Exam tip

The word “differ by CH₂” must appear when you describe a homologous series, leaving it out costs a mark. Keep the two ideas separate and correct: the same functional group gives the same chemical properties, while increasing molecular size gives a gradual change in physical properties. In naming, two mistakes lose marks most often: choosing a chain that is not the longest, and numbering from the wrong end so the locant is not the lowest. Always find the longest chain first, then number to give the functional group (or double bond) the lowest possible number.

How this connects

This standard is the toolkit for the rest of the chapter. When you name an alkene as but-2-ene, or an acid as ethanoic acid, or draw the next member of a series, you are applying exactly these rules. Because the same naming logic and the CₙH₂ₙ₊₂/CₙH₂ₙ patterns run through every question in SPM Chemistry, time spent here is repaid throughout Form 5.

Where this fits

This is content standard 10.2 of the Carbon Compounds chapter, coming straight after the introduction and before the individual families. Reinforce the definitions with the chapter revision notes and lock down the vocabulary, homologue, functional group, general formula, substituent, with the key terms list. In our online 1-to-1 SPM Chemistry lessons, taught in English, from RM50/hr, our teachers drill the naming steps on real structures until finding the longest chain and the lowest locant becomes automatic.

Quick recap

• A homologous series has one general formula, one functional group, and members that differ by CH₂.
• Same functional group → same chemical properties; larger molecules → gradual change in physical properties (higher boiling point).
• Name = stem (meth/eth/prop/but/pent) + suffix (-ane/-ene/-ol/-oic acid) + position numbers for branches and functional groups.
• Always pick the longest chain and number for the lowest locant.