Quick Notes

Quick Notes

Chemistry Chapter 2: Carbon Compound
2.2 Homologous Series
2.2 Homologous Series
CHEMISTRY • Form 5 • Chapter 2: Carbon Compounds

2.2Homologous Series

There are millions of organic compounds known today. To systematically study their physical properties and chemical reactions, organic compounds are classified into specific families known as homologous series. Members of each series share common structural features, a specific functional group, and follow distinct trends in their physical and chemical behaviour.

Learning Objectives

  • Explain the meaning and characteristics of a homologous series.
  • Construct molecular formulae and structural formulae, and name the members of the homologous series according to IUPAC nomenclature.
  • Describe the physical properties of compounds in a homologous series (alkanes, alkenes, alkynes, alcohols, and carboxylic acids).

Summary of Homologous Series Classification and Functional Groups

Compare the general formula, functional group structure, IUPAC suffix, and classification for each homologous series.

Homologous series General formula Functional group IUPAC suffix Classification
Alkane \(\text{C}_n\text{H}_{2n+2}\) \(-\text{C}-\text{C}-\) -ane Saturated hydrocarbon
Alkene \(\text{C}_n\text{H}_{2n}\) \(>\text{C}=\text{C}<\) -ene Unsaturated hydrocarbon
Alkyne \(\text{C}_n\text{H}_{2n-2}\) \(-\text{C}\equiv\text{C}-\) -yne Unsaturated hydrocarbon
Alcohol \(\text{C}_n\text{H}_{2n+1}\text{OH}\) \(-\text{OH}\) -ol Non-hydrocarbon
Carboxylic acid \(\text{C}_n\text{H}_{2n+1}\text{COOH}\) \(-\text{COOH}\) -oic acid Non-hydrocarbon
Ester \(\text{C}_m\text{H}_{2m+1}\text{COOC}_n\text{H}_{2n+1}\) \(-\text{COO}-\) -oate Non-hydrocarbon

Brief Explanation

Understand the key ideas of a homologous series.

A homologous series is an organic family where all members have the same functional group and fit into the same general formula. As you go down a series, each successive compound has one more \(-\text{CH}_2-\) unit (\(+14\) in molar mass). Because their molecular size grows, the intermolecular van der Waals forces strengthen, resulting in progressively higher melting and boiling points.

1. Characteristics of a Homologous Series

Members of a homologous series:

  • are represented by the same general formula.
  • have the same functional group.
  • show the same chemical properties.
  • differ by one carbon atom and two hydrogen atoms (\(-\text{CH}_2-\) group, equivalent to a relative molecular mass difference of 14).
  • show a gradual transition in physical properties from one member to the next.

2. Functional Groups and Classification

A functional group is a group of atoms bonded to an organic molecular compound. It determines the chemical properties of the homologous series, and chemical reactions occur at this functional group.

Organic compounds are classified as hydrocarbons (alkanes, alkenes, alkynes) or non-hydrocarbons (alcohols, carboxylic acids, esters).

Root name Number of carbon atoms Root name Number of carbon atoms
Meth1Hex6
Eth2Hept7
Prop3Oct8
But4Non9
Pent5Dec10

3. Molecular and Structural Formulae

A molecular formula shows the type and actual number of atoms of each element in a molecule.

A structural formula shows the type of bond and how the atoms in a molecule are bonded to each other.

4. IUPAC Nomenclature

Naming follows standard IUPAC nomenclature rules by combining a root name (representing the carbon chain length) with a specific suffix characteristic of the family.

Example: propane \(\rightarrow\) propan-1-ol; methane \(\rightarrow\) methanoic acid.

SeriesExample molecular-formula constructionCondensed structural formulaIUPAC naming
Alkanen = 3: C3H2(3)+2 = C3H8CH3–CH2–CH3Prop- + -ane = propane
Alkenen = 2: C2H2(2) = C2H4CH2=CH2Eth- + -ene = ethene
Alkynen = 2: C2H2(2)−2 = C2H2CH≡CHEth- + -yne = ethyne
Alcoholn = 3: C3H2(3)+1OH = C3H7OHCH3–CH2–CH2–O–HPropane → propan-1-ol (–OH on carbon 1)
Carboxylic acidn = 0: HCOOH; n = 1: CH3COOHH–C(=O)–O–HMethane → methanoic acid
Bond check: Every carbon atom forms four covalent bonds. In alcohol, carbon bonds to oxygen in C–O–H; in a carboxylic acid, the carboxyl carbon has one C=O double bond and two single bonds, C–O and either C–C or C–H.
Structural formula drawing convention: In drawing the hydroxyl group (\(-\text{O}-\text{H}\)), the single bond from the carbon atom must be linked directly to the oxygen atom (\(\text{C}-\text{O}-\text{H}\)), not directly to the hydrogen atom (\(\text{C}-\text{H}-\text{O}\)), because oxygen forms two covalent bonds while hydrogen can only form one.

5. Physical Properties of Hydrocarbons

Alkanes, alkenes and alkynes consist of neutral covalent molecules.

  • Solubility: Insoluble in water, but soluble in organic solvents.
  • Electrical conductivity: Cannot conduct electricity in all states.
  • Melting and boiling points: Relatively low; increase as the number of carbon atoms per molecule increases.
  • Density: Less dense than water.

6. Physical Properties of Alcohols and Carboxylic Acids

Alcohols and carboxylic acids contain carbon, hydrogen, and oxygen atoms.

  • Boiling points: Low, but relatively higher than their corresponding alkenes; increase with the increasing number of carbon atoms per molecule.
  • Solubility in water: Methanol, ethanol and propanol are miscible in water in all proportions. Methanoic acid, ethanoic acid and propanoic acid are very soluble in water. Solubility decreases as molecular size increases.

Try These Questions First

Answer mentally, then check the explanation.

1

What is the structural difference between two consecutive members in any homologous series?

Answer: Consecutive members differ by one carbon atom and two hydrogen atoms (\(-\text{CH}_2-\) unit), with a difference in relative molecular mass of 14.
2

Why does the value of \(n\) start from 0 (\(n = 0, 1, 2, \dots\)) in the general formula of carboxylic acids (\(\text{C}_n\text{H}_{2n+1}\text{COOH}\))?

Answer: Because the carboxyl functional group (\(-\text{COOH}\)) already contains one carbon atom. When \(n = 0\), the compound has one carbon atom in total, forming methanoic acid (\(\text{HCOOH}\)).
3

Why do the melting and boiling points of alkanes increase as the number of carbon atoms increases?

Answer: As the number of carbon atoms increases, the molecular size increases, causing the intermolecular van der Waals attraction forces to become stronger. More heat energy is needed to overcome these forces.

Common Mistakes

Avoid these errors when working with homologous series.

!
Writing the structural connection of alcohols incorrectly: drawing the \(\text{C}-\text{H}-\text{O}\) bond instead of linking the carbon atom directly to the oxygen atom (\(\text{C}-\text{O}-\text{H}\)).
!
Thinking that alcohols contain hydroxide ions (\(\text{OH}^-\)) like inorganic bases. The hydroxyl group in alcohols is covalently bonded and does not dissociate to produce \(\text{OH}^-\) ions.
!
Starting the formula of carboxylic acids with \(n = 1\) for the first member, resulting in ethanoic acid instead of methanoic acid (\(\text{HCOOH}\), which corresponds to \(n = 0\)).
!
Confusing root names, especially between 3 carbons (Prop-) and 4 carbons (But-).

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

Hydrocarbons can conduct electricity when melted or dissolved.

Correct Fact

Hydrocarbons (alkanes, alkenes, alkynes) consist of neutral covalent molecules and do not conduct electricity in any state due to the absence of free-moving ions or delocalised electrons.

Misconception

All members of alcohols and carboxylic acids dissolve completely in water because they have \(-\text{OH}\) or \(-\text{COOH}\) groups.

Correct Fact

Only the small members (first three members) are completely miscible or very soluble. As the hydrophobic hydrocarbon chain increases in length (molecular size increases), their solubility in water decreases significantly.

Summary

Key facts to remember.

  • A homologous series is defined by a common general formula, same functional group, identical chemical properties, a constant incremental difference of \(-\text{CH}_2-\), and gradual variation in physical properties.
  • Hydrocarbons are divided into saturated (alkanes, containing only \(\text{C}-\text{C}\) single bonds) and unsaturated (alkenes with \(\text{C}=\text{C}\) double bonds, alkynes with \(\text{C}\equiv\text{C}\) triple bonds).
  • Non-hydrocarbons contain heteroatoms such as oxygen (alcohols with \(-\text{OH}\), carboxylic acids with \(-\text{COOH}\), and esters with \(-\text{COO}-\)).
  • Nomenclature combines the chain length root name (Meth- to Dec-) with the homologous suffix (-ane, -ene, -yne, -ol, -oic acid, -oate).
  • Intermolecular van der Waals forces strengthen as molecular size increases, causing melting and boiling points to rise across all series.

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following represents the functional group of an alkene?

2

What is the IUPAC name for an alcohol with the molecular formula \(\text{C}_3\text{H}_7\text{OH}\) where the hydroxyl group is bonded to the terminal carbon atom?

B. Fill in the Blanks

3

Consecutive members in a homologous series differ from each other by a single carbon atom and two hydrogen atoms, represented by the formula ______.

4

In the general formula of carboxylic acids, \(\text{C}_n\text{H}_{2n+1}\text{COOH}\), the value of \(n\) for the first member is ______.

C. Match the Homologous Series with the Functional Group

Drag each series on desktop. On mobile, tap a series and then tap the matching box.

Answer Choices

Alkane Alkyne Alcohol Carboxylic acid

Functional Group Matching

1Single bond between carbon atoms
2Triple bond between carbon atoms
3Hydroxyl group
4Carboxyl group
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Homologous series Functional group Hydrocarbon Saturated hydrocarbon Unsaturated hydrocarbon Molecular formula Structural formula IUPAC nomenclature