Quick Notes

Quick Notes

Chemistry Chapter 2: Carbon Compound
2.3 Chemical Properties and Interchange Between Homologous Series
2.3 Chemical Properties and Interconversion of Compounds Between Homologous Series
CHEMISTRY • Form 5 • Chapter 2: Carbon Compounds

2.3Chemical Properties and Interconversion of Compounds Between Homologous Series

Organic compounds are classified into different homologous series, each characterized by a specific functional group that determines its chemical reactivity. Alkanes, being saturated hydrocarbons, undergo combustion and substitution reactions. Alkenes possess a carbon-carbon double bond, enabling them to undergo addition reactions and addition polymerisation. Alcohols contain the hydroxyl group and take part in combustion, oxidation, and dehydration. Carboxylic acids exhibit typical acidic properties and react with alcohols via esterification to form esters, which are fruity-smelling organic compounds. Understanding these reactions enables the interconversion of one homologous series to another.

Learning Objectives

  • Describe the chemical properties of each homologous series: alkanes, alkenes, alcohols, and carboxylic acids through relevant reactions and activities.
  • Compare and contrast the properties and reactivity between alkanes and alkenes in terms of combustion (sootiness of flame) and reactions with bromine water and acidified potassium manganate(VII) solution.
  • Explain the preparation of ethanol through fermentation of glucose and hydration of ethene.
  • Describe the oxidation and dehydration reactions of alcohols with the required catalysts and conditions.
  • Describe the chemical properties of carboxylic acids in acid reactions and esterification.
  • Understand esters through their preparation, general formula, functional group, naming system, and physical properties.

Summary of Interconversions Between Homologous Series

A single synthetic roadmap linking alkanes, alkenes, alcohols, carboxylic acids, and esters, including required reagents and conditions.

Alkene C=C double bond
+ H2O, H3PO4, 300 °C, 60 atm
Alcohol –OH group
+ 2[O], KMnO4/H+, reflux
Carboxylic acid –COOH group
+ Alcohol, conc. H2SO4, heat
Ester –COO– group
Alkene → Alkane: + H2, Ni, 180 °C
Alcohol → Alkene: porcelain chips, heat

Brief Explanation

How different organic families react based on their functional groups and how they can be converted into one another.

Homologous series are classified by their functional group, which controls how each family reacts.

1. Alkanes

Only contain single bonds. They undergo combustion and substitution, requiring UV light to form halogenated alkanes.

2. Alkenes

Contain a reactive double bond, C=C. They undergo addition reactions with hydrogen, halogens, hydrogen halides, steam, and oxidising agents, or undergo addition polymerisation.

3. Alcohols

Contain the –OH group. They can be made via fermentation or hydration of alkenes. They undergo clean combustion, dehydration, and oxidation to carboxylic acids.

4. Carboxylic Acids

Contain the –COOH group. They act as typical weak acids and undergo esterification when heated with alcohols in the presence of concentrated H2SO4.

5. Esters

Sweet-smelling, neutral compounds formed by esterification. They are named with the alcohol part first (“-yl”) followed by the acid part (“-oate”).

Alkanes: combustion and substitution

Complete combustion: CH4(g) + 2O2(g) → CO2(g) + 2H2O(l). With limited oxygen, carbon monoxide and/or soot and water can form.

Stepwise substitution under UV light:
CH4 + Cl2 → CH3Cl + HCl
CH3Cl + Cl2 → CH2Cl2 + HCl
CH2Cl2 + Cl2 → CHCl3 + HCl
CHCl3 + Cl2 → CCl4 + HCl

Alkenes: combustion, addition and polymerisation

Combustion: C2H4(g) + 3O2(g) → 2CO2(g) + 2H2O(l).

Addition across C=C:
C2H4 + H2 → C2H6 (Ni, 180 °C)
C2H4 + Br2 → CH2Br–CH2Br (room conditions)
C2H4 + HBr → CH3–CH2Br (room conditions)
C2H4 + H2O(g) → C2H5OH (H3PO4, 300 °C, 60 atm)
C2H4 + H2O + [O] → CH2OH–CH2OH (KMnO4/H+)

In each addition, C=C becomes C–C and one new atom/group bonds to each carbon. Addition polymerisation: nCH2=CH2 → (–CH2–CH2–)n at 200 °C and 1200 atm, forming polythene.

Preparation and reactions of alcohol

Anaerobic fermentation: C6H12O6(aq) → 2C2H5OH(l) + 2CO2(g), catalysed by zymase enzyme in yeast. Ethanol is purified by fractional distillation.

Combustion: C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(l).

Oxidation under reflux: CH3CH2OH + 2[O] → CH3COOH + H2O using KMnO4/H+ or K2Cr2O7/H+.

Dehydration: C2H5OH → C2H4 + H2O over strongly heated porcelain chips. –OH and –H are removed from adjacent carbon atoms.

Carboxylic acids and esters

Ethanoic acid ionises partially in water and reacts as a weak acid:
2CH3COOH + Mg → (CH3COO)2Mg + H2
2CH3COOH + CuO → (CH3COO)2Cu + H2O
2CH3COOH + Na2CO3 → 2CH3COONa + H2O + CO2

Esterification: CH3COOH + C2H5OH ⇌ CH3COOC2H5 + H2O (concentrated H2SO4, heat). The product is ethyl ethanoate.

General formula of esters: CmH2m+1COOCnH2n+1, m = 0,1,2,… and n = 1,2,3,…. The general structure R–C(=O)–O–R′ contains an ester group. The name consists of the alkyl part from the alcohol followed by the alkanoate part from the acid, for example ethyl ethanoate.

Simple esters are neutral covalent liquids that are colourless, fruity-smelling, volatile, less dense than water and insoluble in water.

Homologous series Functional group Key reactions
Alkane C–C and C–H single bonds Combustion, substitution with halogens under UV
Alkene C=C double bond Addition reactions, addition polymerisation
Alcohol –OH hydroxyl group Combustion, oxidation, dehydration
Carboxylic acid –COOH carboxyl group Acid reactions, esterification
Ester –COO– ester group Formed by esterification

Try These Questions First

Answer mentally, then check the explanation.

1

Why do alkenes burn with a more sooty flame than alkanes with the same number of carbon atoms?

Answer: Alkenes have a higher percentage of carbon by mass per molecule compared to their corresponding alkanes, resulting in more unburnt carbon particles (soot) during combustion.
2

Explain why methanol cannot undergo a dehydration reaction.

Answer: Dehydration requires the removal of a water molecule (–OH and –H) from two adjacent carbon atoms to form a C=C double bond. Methanol only has one carbon atom, so it cannot form an alkene.
3

State the role of concentrated sulphuric acid in the esterification reaction between ethanoic acid and ethanol.

Answer: It acts as an acid catalyst to speed up the esterification reaction and assists in the removal of water.

Common Mistakes

Avoid these errors when writing reactions and conversions.

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Writing ester names backwards: naming the acid part first instead of the alkyl group from the alcohol, for example writing ethanoic ethyl instead of ethyl ethanoate.
!
Confusing the source of water in esterification: incorrectly assuming the carboxylic acid loses –H and the alcohol loses –OH. In fact, the carboxylic acid contributes the –OH group and the alcohol contributes the –H atom.
!
Omitting reaction conditions such as UV light for substitution, Ni / 180 °C for hydrogenation, and H3PO4 / 300 °C / 60 atm for hydration.
!
Confusing combustion sootiness comparison: assuming alkanes produce more soot because they have more hydrogen atoms, rather than calculating percentage of carbon by mass.

Common Misconceptions

Compare each misconception with the correct fact.

Misconception

Alkanes and alkenes react identically with bromine water.

Correct Fact

Alkenes undergo an addition reaction with bromine water at room condition without sunlight, decolourising it immediately. Alkanes do not react with bromine water in the dark and only undergo slow substitution under UV light.

Misconception

Fermentation can produce pure 100% ethanol directly in the conical flask.

Correct Fact

Fermentation produces an aqueous ethanol mixture; yeast dies when ethanol reaches around 12–15%. Fractional distillation is required to separate and purify ethanol at its boiling point of 78 °C.

Misconception

Esters are acidic because they are made from carboxylic acids.

Correct Fact

Esters are neutral covalent compounds because the acidic hydrogen atom of the carboxyl group has been replaced by an alkyl group.

Summary

Key facts to remember.

  • Alkanes: Saturated, unreactive, undergo complete/incomplete combustion and photochemical substitution with halogens in the presence of UV light.
  • Alkenes: Unsaturated with C=C, undergo addition of H2, X2, HX, H2O, acidified KMnO4, and addition polymerisation. Burn with sootier flame than alkanes due to higher % C.
  • Alcohols: Contain –OH, prepared by fermentation of glucose or hydration of ethene. Undergo clean combustion, oxidation to carboxylic acids, and dehydration to alkenes.
  • Carboxylic Acids: Contain –COOH, acidic properties, undergo esterification with alcohols when catalysed by concentrated H2SO4.
  • Esters: Functional group –COO–, sweet/fruity smell, immiscible with water, named as [alkyl from alcohol] [carboxylate from acid].

Quick Activity

Complete all parts before checking your score.

A. Objective Quiz

1

Which of the following reaction conditions is required to convert ethene gas, C2H4, into ethanol, C2H5OH?

2

When propan-1-ol, C3H7OH, is heated with acidified potassium dichromate(VI) solution under reflux, what is the observation and the organic product formed?

B. Fill in the Blanks

3

In the esterification reaction between ethanoic acid and ethanol, the water molecule is formed from the –OH removed from the acid and the atom ________ removed from the alcohol.

4

During the laboratory preparation of ethanol by fermentation, the distillate collected at 78 °C from the fractionating column is separated from the reaction mixture by the process of ________ distillation.

C. Match Each Chemical Reaction with Its Conversion Type and Condition

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

Answer Choices

Ethene to Ethane Ethanol to Ethene Methane to Chloromethane Ethanoic Acid + Ethanol to Ethyl Ethanoate

Reaction Classification & Condition

1 Hydrogenation using H2 gas with Ni catalyst at 180 °C
2 Dehydration using heated porcelain chips as catalyst
3 Halogen substitution using Cl2 under UV rays
4 Esterification using concentrated H2SO4 catalyst and heat
Complete every answer before checking.
 

Keywords

Tap or hover over a term to read its meaning.

Substitution Reaction Addition Reaction Hydrogenation Hydration Dehydration Fermentation Esterification Addition Polymerisation