Homologous Series

Introduction

There are millions of organic compounds, far too many to learn one at a time. Fortunately, chemists noticed that many of them fall into neat families in which each member looks like the one before it with one extra carbon atom. These families are called homologous series . Once you know how one member of a series behaves, you can predict the behaviour of the others, which makes organic chemistry far more manageable.

Core explanation

What makes a homologous series? A homologous series is a family of organic compounds whose members share five features:

1. The same general formula. For example, every alkane fits CₙH₂ₙ₊₂ and every alkene fits CₙH₂ₙ, where n is the number of carbon atoms. 2. Each member differs from the next by CH₂. Methane is CH₄, ethane is C₂H₆, propane is C₃H₈ — each time one carbon and two hydrogens are added. 3. The same functional group. The functional group is the part of the molecule that reacts. In alkenes it is the C=C double bond; in alcohols it is the –OH group. Alkanes have no functional group beyond C–C and C–H single bonds. 4. Similar chemical properties. Because the functional group is the same, members react in the same ways. All alkanes burn and are otherwise fairly unreactive; all alkenes decolourise bromine water. 5. A gradual trend in physical properties. As the chain gets longer, boiling point, melting point, density and viscosity change steadily rather than jumping around.

Using the general formula. Substituting a value of n gives the formula of that member. For an alkane with 5 carbons, n = 5 and 2n + 2 = 12, giving C₅H₁₂. For an alkene with 5 carbons: C₅H₁₀.

Trends in physical properties. Consider the first few alkanes:

Alkane Formula Boiling point (°C) State at 25 °C --- --- --- --- methane CH₄ −162 gas ethane C₂H₆ −89 gas propane C₃H₈ −42 gas butane C₄H₁₀ −1 gas pentane C₅H₁₂ 36 liquid hexane C₆H₁₄ 69 liquid

Boiling point rises steadily with chain length. Larger molecules have stronger forces between them, so more energy is needed to separate them. Longer-chain members are also more viscous (thicker and slower to flow) and harder to ignite.

Series you will meet. In this unit: alkanes, alkenes and alkynes. Later in organic chemistry: alcohols, carboxylic acids, esters and more. Each is its own homologous series with its own general formula.

Step-by-step reasoning

To predict the formula and state of a member of a series:

1. Identify the series and its general formula. 2. Substitute the number of carbon atoms for n. 3. Simplify to get the molecular formula. 4. Use the trend: the more carbons, the higher the boiling point, so compare with known members to predict the state.

Visual explanation

Picture a staircase in which each step is one CH₂ unit. Methane stands on the bottom step, ethane on the next, propane on the next. Each step up adds the same small piece, and each step up raises the boiling point a little. The overall shape of the family stays the same all the way up.

Real-world analogy

A homologous series is like a set of Russian dolls of increasing size. Every doll has the same design and opens in the same way (similar chemical properties), but each one is a little bigger than the last (one more CH₂), and the bigger ones are heavier and harder to move (higher boiling points).

Real-world example

Bottled camping gas contains propane and butane, which are gases; petrol contains alkanes with roughly 5 to 10 carbons, which are runny liquids; lubricating oil contains much longer alkanes and is thick; paraffin wax contains alkanes with more than about 20 carbons and is solid. All are members of the same series, showing the trend in state with chain length.

Why?

Why do members of a series have similar chemical properties? Chemical reactions happen at the functional group. Adding extra CH₂ units lengthens the carbon chain but does not change the functional group, so the reactions stay essentially the same.

Common misconception

"Members of a homologous series have identical properties." Their chemical properties are similar, but their physical properties — boiling point, viscosity, flammability — change gradually along the series. Methane and octane react alike but behave very differently physically.

Worked example

Question: The general formula of the alkenes is CₙH₂ₙ. Write the formula of the alkene with 6 carbon atoms, and state its relative formula mass (C = 12, H = 1).

Reasoning: n = 6, so 2n = 12, giving C₆H₁₂. Relative formula mass = (6 × 12) + (12 × 1) = 72 + 12 = 84.

Answer: C₆H₁₂; relative formula mass 84.

Quick check

1. By what group of atoms does each member of a homologous series differ from the next? Answer: By a CH₂ unit.

Exam focus

Be ready to list the features of a homologous series: same general formula, differ by CH₂, same functional group, similar chemical properties, and a gradual trend in physical properties. A common question gives the general formula and asks for a formula or relative formula mass — show your substitution.

Advanced insight

Successive members of a series differ in relative formula mass by 14, the mass of one CH₂ unit. The trend in boiling point is steepest for the smallest molecules: going from methane to ethane raises the boiling point by over 70 °C, but going from decane to undecane raises it by only about 20 °C, because each extra CH₂ makes a smaller fractional difference to a long chain.

Summary

A homologous series is a family of organic compounds with the same general formula, the same functional group and similar chemical properties. Each member differs from the next by CH₂. Physical properties such as boiling point and viscosity change gradually as the chain gets longer. General formulas let you write the formula of any member.

Practice questions

1. State two features that all members of a homologous series share. Answer: Any two of: the same general formula; the same functional group; similar chemical properties; each differs from the next by CH₂; a gradual trend in physical properties. 2. Use the general formula CₙH₂ₙ₊₂ to write the formula of the alkane with 8 carbon atoms. Answer: C₈H₁₈. 3. Heptane boils at 98 °C and nonane boils at 151 °C. Predict whether octane, the member between them, boils above or below 100 °C, and explain. Answer: Above 100 °C (it is about 126 °C), because boiling point increases gradually with chain length, so octane lies between heptane and nonane. 4. Explain why ethene and propene react in similar ways. Answer: They are in the same homologous series and have the same functional group, the C=C double bond, where reactions take place.