ChemistryICSE Class 10 · Chapter 12

Organic Chemistry ICSE Class 10: Hydrocarbons, Nomenclature, Ethanol and Acetic Acid

Updated 14 October 2026 · Fact-checked

Organic chemistry is the chemistry of carbon compounds. Carbon's tetravalency and catenation give chains, branches and rings. ICSE Class 10 covers structural formulae up to five carbons, chain and position isomerism, homologous series, IUPAC names with one functional group, and the preparation and reactions of methane, ethane, ethene, ethyne, ethanol and acetic acid.

What Organic Chemistry is about

Carbon forms more compounds than all other elements together, for two reasons: it is tetravalent (forms four covalent bonds) and it shows catenation (links to other carbon atoms in long chains, branches and rings). Compounds of carbon and hydrogen only are hydrocarbons.

The CISCE 2027 syllabus covers: (i) the unique nature of carbon, single, double and triple bonds, straight-chain, branched and cyclic compounds (only benzene's structure); (ii) structural formulae of alkanes, alkenes and alkynes up to 5 carbon atoms, and structural isomerism (chain and position); (iii) homologous series and the gradation of properties; (iv) simple nomenclature (trivial and IUPAC) for compounds with one functional group: halides, double bond, triple bond, alcohol, aldehyde, carboxylic acid, ketone, ether and ester; (v) alkanes (methane and ethane: preparation, complete and incomplete combustion, substitution with chlorine), alkenes (ethene: by dehydrohalogenation and dehydration) and alkynes (ethyne: from calcium carbide and 1,2-dibromoethane), with addition of hydrogen and halogens, and uses; (vi) ethanol: preparation by hydrolysis of an alkyl halide, physical and chemical properties, denatured, methylated and spurious alcohol, uses; (vii) acetic acid: structure, properties (including glacial acetic acid and esterification) and uses.

Weightage: CISCE publishes no chapter or unit weightage for ICSE Chemistry. Organic Chemistry is in the 9. Organic Chemistry part of the 2027 syllabus and can be asked in both Section A and Section B of the 80-mark paper.Source: CISCE ICSE Chemistry (Science Paper 2) syllabus, examination year 2027

Key concepts

Carbon: tetravalency, catenation and types of chains

Tetravalency: carbon (2, 4) shares its four valence electrons to form four covalent bonds. Every carbon in a structural formula must show four bonds.
Catenation: carbon atoms bond to one another to form chains and rings. This is why so many carbon compounds exist.
Bonds between carbons may be single (C–C), double (C=C) or triple (C≡C). Compounds with only single bonds are saturated; those with a double or triple bond are unsaturated.
Straight chain: CH₃–CH₂–CH₂–CH₃ (butane). Branched chain: CH₃–CH(CH₃)–CH₃ (2-methylpropane). Cyclic: benzene, C₆H₆, a ring of six carbons with alternate single and double bonds, each carbon carrying one H.

Worked example

Which of these is saturated: C₅H₁₂, C₅H₁₀, C₅H₈? Explain.

Try it first, then show the solutionSolution

Saturated hydrocarbons are alkanes, CₙH₂ₙ₊₂.
For n = 5: C₅H₁₂.
C₅H₁₀ (alkene) and C₅H₈ (alkyne) have multiple bonds.

Answer: C₅H₁₂ (pentane).

Homologous series

A homologous series is a group of organic compounds with the same functional group and similar chemical properties, in which successive members differ by a –CH₂– unit (14 u in molecular mass).

Characteristics:
1. Same general formula: alkanes CₙH₂ₙ₊₂, alkenes CₙH₂ₙ, alkynes CₙH₂ₙ₋₂.
2. Successive members differ by CH₂.
3. Similar chemical properties (same functional group).
4. Gradation in physical properties: as molecular mass increases, the melting and boiling points increase and the state changes from gas to liquid to solid (C₁-C₄ alkanes are gases, C₅-C₁₇ liquids, higher ones solids).
5. Similar methods of preparation.

Worked example

Which set belongs to the same homologous series: (a) CH₄, C₂H₄, C₃H₄; (b) C₂H₆, C₃H₈, C₄H₁₀; (c) C₂H₂, C₂H₄, C₂H₆?

Try it first, then show the solutionSolution

Test each against one general formula.
(b): C₂H₆, C₃H₈, C₄H₁₀ all fit CₙH₂ₙ₊₂ and differ by CH₂.

Answer: (b), the alkanes.

Structural formulae and isomerism (chain and position)

Isomers have the same molecular formula but different structural formulae.

Chain isomerism: different carbon skeletons.

C₄H₁₀: butane CH₃–CH₂–CH₂–CH₃ and 2-methylpropane CH₃–CH(CH₃)–CH₃.
C₅H₁₂: pentane, 2-methylbutane CH₃–CH(CH₃)–CH₂–CH₃, and 2,2-dimethylpropane C(CH₃)₄.

Position isomerism: the same skeleton, but the multiple bond or functional group is in a different position.

C₄H₈: but-1-ene CH₂=CH–CH₂–CH₃ and but-2-ene CH₃–CH=CH–CH₃.
C₃H₇Cl: 1-chloropropane and 2-chloropropane.

In a structural formula, draw every C and H with every bond as a line; check each carbon has four bonds. Methane, ethane, propane cannot show isomers (too few carbons).

Worked example

Write the structural formulae (condensed) and IUPAC names of the two position isomers of pentyne in which the triple bond is at the end or next to it.

Try it first, then show the solutionSolution

Pent-1-yne: CH≡C–CH₂–CH₂–CH₃.
Pent-2-yne: CH₃–C≡C–CH₂–CH₃.
Same formula C₅H₈, triple bond in a different position.

Answer: Pent-1-yne and pent-2-yne.

IUPAC and trivial names (one functional group)

Rules: (1) find the longest chain containing the functional group: word root meth- (1), eth- (2), prop- (3), but- (4), pent- (5); (2) number from the end that gives the functional group (or multiple bond) the lowest number; (3) add prefixes for branches/halogens and the suffix for the group.

Group	Suffix / prefix	Example (IUPAC)	Trivial name
halide –X	chloro-, bromo-, iodo-	chloromethane, CH₃Cl	methyl chloride
C=C	-ene	ethene, CH₂=CH₂	ethylene
C≡C	-yne	ethyne, CH≡CH	acetylene
–OH	-ol	ethanol, C₂H₅OH	ethyl alcohol
–CHO	-al	ethanal, CH₃CHO	acetaldehyde
–COOH	-oic acid	ethanoic acid, CH₃COOH	acetic acid
C=O (ketone)	-one	propanone, CH₃COCH₃	acetone
–O– (ether)	alkoxyalkane	methoxymethane, CH₃OCH₃; ethoxyethane, C₂H₅OC₂H₅	dimethyl ether; diethyl ether
–COO– (ester)	alkyl alkanoate	ethyl ethanoate, CH₃COOC₂H₅	ethyl acetate

Carbon atoms in –CHO, –COOH and ketone C=O count in the chain. Two identical substituents take di-, tri-, tetra- with a number for each (2,3-dichloropentane).

Worked example

Name: (a) CH₃–CHCl–CHCl–CH₂–CH₃, (b) CH₃–CH₂–COOH, (c) CH₃–CH(CH₃)–CH₂–CH₃.

Try it first, then show the solutionSolution

(a) Five carbons, Cl on C-2 and C-3: 2,3-dichloropentane.
(b) Three carbons including the COOH carbon: propanoic acid.
(c) Longest chain four carbons, methyl on C-2: 2-methylbutane.

Answer: (a) 2,3-dichloropentane (b) propanoic acid (c) 2-methylbutane

Alkanes: methane and ethane

Preparation:

From sodium salts by heating with soda lime (NaOH + CaO), decarboxylation:

$C H_{3} C O O N a + N a O H \to C H_{4} + N a_{2} C O_{3}$ (sodium ethanoate gives methane)
$C_{2} H_{5} C O O N a + N a O H \to C_{2} H_{6} + N a_{2} C O_{3}$ (sodium propanoate gives ethane)

From alkyl halides by reduction with nascent hydrogen (zinc-copper couple and ethanol):

$C H_{3} I + 2 [H] \to C H_{4} + H I$ ; $C_{2} H_{5} B r + 2 [H] \to C_{2} H_{6} + H B r$

Combustion: complete (plenty of air, blue flame): $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ ; $2 C_{2} H_{6} + 7 O_{2} \to 4 C O_{2} + 6 H_{2} O$ . Incomplete (limited air, poisonous CO and soot): $2 C H_{4} + 3 O_{2} \to 2 C O + 4 H_{2} O$ .

Substitution with chlorine in diffused sunlight (one H replaced at a time):
$C H_{4} + C l_{2} \to C H_{3} C l + H C l$ → CH₂Cl₂ → CHCl₃ → CCl₄
$C_{2} H_{6} + C l_{2} \to C_{2} H_{5} C l + H C l$

Methane is a greenhouse gas (released from paddy fields, cattle and landfills). Uses: methane as fuel (natural gas, CNG, biogas) and for making hydrogen and carbon black; ethane as fuel and to make ethene.

Worked example

Write the equations for the four stages of substitution of methane by chlorine and name the final product.

Try it first, then show the solutionSolution

$C H_{4} + C l_{2} \to C H_{3} C l + H C l$
$C H_{3} C l + C l_{2} \to C H_{2} C l_{2} + H C l$
$C H_{2} C l_{2} + C l_{2} \to C H C l_{3} + H C l$
$C H C l_{3} + C l_{2} \to C C l_{4} + H C l$

Answer: Final product: tetrachloromethane (carbon tetrachloride).

Alkenes and alkynes: ethene and ethyne

Ethene, C₂H₄:

Dehydrohalogenation: boil bromoethane with alcoholic KOH: $C_{2} H_{5} B r + K O H \to C_{2} H_{4} + K B r + H_{2} O$
Dehydration of ethanol with conc. H₂SO₄ at about 170 °C: $C_{2} H_{5} O H \to C_{2} H_{4} + H_{2} O$

Ethyne, C₂H₂:

From calcium carbide and water (no heat): $C a C_{2} + 2 H_{2} O \to C a (O H)_{2} + C_{2} H_{2}$
From 1,2-dibromoethane with alcoholic KOH: $C_{2} H_{4} B r_{2} + 2 K O H \to C_{2} H_{2} + 2 K B r + 2 H_{2} O$

Addition reactions (the characteristic reaction of unsaturated compounds):

Hydrogen (nickel catalyst, about 300 °C): $C_{2} H_{4} + H_{2} \to C_{2} H_{6}$ ; $C_{2} H_{2} + 2 H_{2} \to C_{2} H_{6}$
Halogens: $C_{2} H_{4} + C l_{2} \to C_{2} H_{4} C l_{2}$ (1,2-dichloroethane); $C_{2} H_{4} + B r_{2} \to C_{2} H_{4} B r_{2}$ ; $C_{2} H_{2} + 2 B r_{2} \to C_{2} H_{2} B r_{4}$ (1,1,2,2-tetrabromoethane); $C_{2} H_{2} + 2 C l_{2} \to C_{2} H_{2} C l_{4}$ ; with iodine, $C_{2} H_{4} + I_{2} \to C_{2} H_{4} I_{2}$ .
Test for unsaturation: reddish brown bromine water is decolourised by ethene or ethyne, but not by ethane.

Uses: ethene for making polythene and for artificial ripening of fruits; ethyne in the oxy-acetylene flame for welding and cutting metals, and for making PVC.

Worked example

How would you distinguish ethane from ethene using bromine water?

Try it first, then show the solutionSolution

Pass each gas through reddish brown bromine water.
Ethene adds bromine: $C_{2} H_{4} + B r_{2} \to C_{2} H_{4} B r_{2}$ ; the colour disappears.
Ethane is saturated and does not add bromine: the colour stays.

Answer: Ethene decolourises bromine water; ethane does not.

Ethanol

Preparation: hydrolysis of an alkyl halide by boiling with aqueous NaOH or KOH:
$C_{2} H_{5} B r + N a O H \to C_{2} H_{5} O H + N a B r$
(Aqueous alkali gives the alcohol; alcoholic KOH gives ethene.)

Physical: colourless liquid with a pleasant smell and burning taste; miscible with water in all proportions; less dense than water (about 0.79 g/cm³); boils at 78 °C.

Chemical:

Combustion (pale blue flame): $C_{2} H_{5} O H + 3 O_{2} \to 2 C O_{2} + 3 H_{2} O$
With sodium (hydrogen released): $2 C_{2} H_{5} O H + 2 N a \to 2 C_{2} H_{5} O N a + H_{2}$
Esterification with acetic acid (conc. H₂SO₄, warm): $C_{2} H_{5} O H + C H_{3} C O O H ⇌ C H_{3} C O O C_{2} H_{5} + H_{2} O$ , giving ethyl acetate with a fruity smell.
Dehydration with conc. H₂SO₄ at about 170 °C gives ethene.

Denatured alcohol: ethanol made unfit for drinking by adding poisonous substances (such as methanol or pyridine, often with a dye), so it can be sold cheaply for industry. Methylated spirit: ethanol mixed with some methanol. Spurious alcohol: illicit liquor contaminated with methanol, which can cause blindness and death.

Uses: solvent (medicines, perfumes, varnishes), fuel (mixed with petrol), antiseptic, in thermometers for low temperatures, and in alcoholic drinks.

Worked example

Write equations for ethanol with (a) sodium, (b) acetic acid. Name the organic product of each.

Try it first, then show the solutionSolution

(a) $2 C_{2} H_{5} O H + 2 N a \to 2 C_{2} H_{5} O N a + H_{2}$ : sodium ethoxide.
(b) $C_{2} H_{5} O H + C H_{3} C O O H ⇌ C H_{3} C O O C_{2} H_{5} + H_{2} O$ : ethyl ethanoate (ethyl acetate).

Answer: Sodium ethoxide; ethyl ethanoate.

Acetic acid (ethanoic acid)

Structure: CH₃–COOH: a methyl group attached to a carboxyl group, in which carbon is double-bonded to one O and single-bonded to an –OH.

Physical: colourless liquid with the pungent smell of vinegar (vinegar is a dilute solution of acetic acid in water). Pure acetic acid freezes at about 16.6 °C; on sufficient cooling it forms ice-like crystals, so it is called glacial acetic acid.

Chemical:

Litmus: turns blue litmus red (a weak monobasic acid).
Alkalis: $C H_{3} C O O H + N a O H \to C H_{3} C O O N a + H_{2} O$ (sodium acetate).
Alcohols (esterification): with ethanol and a little conc. H₂SO₄, a sweet, fruity-smelling ester forms: $C H_{3} C O O H + C_{2} H_{5} O H ⇌ C H_{3} C O O C_{2} H_{5} + H_{2} O$ . Concentrated sulphuric acid acts as a dehydrating agent, removing water.

Uses: vinegar for flavouring and preserving food; making esters (perfumes, flavours), dyes and plastics; as a solvent; to coagulate rubber latex.

Worked example

Why is pure acetic acid called glacial acetic acid?

Try it first, then show the solutionSolution

Pure acetic acid freezes at about 16.6 °C.
On cooling it forms colourless, ice-like crystals (like a glacier).

Answer: It forms ice-like crystals on sufficient cooling.

Chemistry in real life: Organic Chemistry

Each concept, met in an everyday situation. Read the situation, try the problem, then check the solution. The one-line hook is what to remember.

Addition test for unsaturation

Suppose a quality inspector receives two unlabelled cylinders of colourless gas, one ethane (for fuel) and one ethene (for ripening fruit), and has only a bottle of bromine water.

How can she identify the ethene? Write the equation.

Try it first, then show the solutionSolution

Bubble each gas through a little reddish brown bromine water.
Ethene is unsaturated and adds bromine: $C_{2} H_{4} + B r_{2} \to C_{2} H_{4} B r_{2}$ , so the colour disappears.
Ethane does not react, so the colour remains.

Answer: The gas that decolourises bromine water is ethene.

Memory hook: Double bonds drink bromine.

Spurious alcohol

Suppose news reports describe people losing their eyesight after drinking cheap illicit liquor, and a student wonders how this is linked to chemistry class.

Explain what spurious alcohol is and how denatured alcohol differs.

Try it first, then show the solutionSolution

Spurious alcohol is illicit liquor contaminated with methanol, which is poisonous and can cause blindness and death.
Denatured alcohol is ethanol deliberately made unfit to drink by adding poisonous substances (such as methanol or pyridine) and a dye, so it can be used in industry without being drunk.
The danger arises when denatured or methanol-containing alcohol is sold as drink.

Answer: Spurious: methanol-contaminated liquor (poisonous). Denatured: industrial ethanol deliberately made undrinkable.

Memory hook: Methanol in a drink is a poison, not a kick.

Complete and incomplete combustion

Suppose a family's LPG stove has a clogged air vent and the flame turns yellow and sooty, while a well-adjusted stove burns with a blue flame.

Using methane as the example fuel, write equations for both cases and explain the danger.

Try it first, then show the solutionSolution

Plenty of air: $C H_{4} + 2 O_{2} \to C O_{2} + 2 H_{2} O$ (blue flame).
Limited air: $2 C H_{4} + 3 O_{2} \to 2 C O + 4 H_{2} O$ , with some unburnt carbon (soot) giving a yellow flame.
Carbon monoxide is a poisonous gas.

Answer: Limited air gives CO and soot; keep the burner's air supply clear.

Memory hook: Blue flame good, yellow flame danger.

Topics in this chapter

IUPAC Nomenclature and IsomerismComing soon
Alkanes, Alkenes and AlkynesComing soon
Ethanol and Acetic AcidComing soon
Acetic (Ethanoic) Acid: Structure, Properties and Uses
Ethanol: Preparation, Properties and Uses

Organic Chemistry formula sheet

General formulae: $alkane C_{n} H_{2 n + 2}; alkene C_{n} H_{2 n}; alkyne C_{n} H_{2 n - 2}$; Successive members differ by CH₂.
Methane from sodium ethanoate: $C H_{3} C O O N a + N a O H \to C H_{4} + N a_{2} C O_{3}$; Soda lime, heat.
Ethene by dehydration: $C_{2} H_{5} O H \to C_{2} H_{4} + H_{2} O$; Conc. H₂SO₄, about 170 °C.
Ethyne from calcium carbide: $C a C_{2} + 2 H_{2} O \to C a (O H)_{2} + C_{2} H_{2}$; No heating.
Ethanol by hydrolysis: $C_{2} H_{5} B r + N a O H \to C_{2} H_{5} O H + N a B r$; Aqueous alkali, boil.
Esterification: $C H_{3} C O O H + C_{2} H_{5} O H ⇌ C H_{3} C O O C_{2} H_{5} + H_{2} O$; Conc. H₂SO₄; fruity smell.
Complete combustion of ethanol: $C_{2} H_{5} O H + 3 O_{2} \to 2 C O_{2} + 3 H_{2} O$; Pale blue flame.

Organic Chemistry: diagrams to practise

Practise each one on paper: draw it from the checklist, then tick off every label.

Laboratory preparation of methane

Draw the set-up to prepare methane from sodium ethanoate (sodium acetate).

Draw

A hard-glass test tube (or flask) containing a mixture of sodium ethanoate and soda lime, clamped slightly tilted, heated by a burner.
A delivery tube leading into a trough of water.
Methane collected in an inverted gas jar over water (downward displacement of water: methane is almost insoluble).

Label

sodium ethanoate + soda lime
heat
delivery tube
water trough
methane

Watch out: Note that soda lime is NaOH + CaO; CaO keeps NaOH dry.

Preparation of ethyne from calcium carbide

Draw the set-up to prepare ethyne.

Draw

A conical flask containing calcium carbide lumps on a layer of sand.
A dropping funnel with water through the cork.
A delivery tube to a water trough; ethyne collected over water in an inverted gas jar.

Label

calcium carbide
sand
water from dropping funnel
ethyne
over water

Watch out: The sand absorbs heat so the flask does not crack. No heating is needed.

Structure of benzene

Draw the structure of benzene.

Draw

A hexagon of six carbon atoms.
Alternate single and double bonds around the ring.
One H on each carbon, pointing outwards.

Label

C₆H₆
alternate double bonds

Watch out: Only benzene's structure is needed for cyclic compounds.

How the board asks Organic Chemistry

The question patterns to expect, and the method that works for each.

Question type	How it is asked	Method
IUPAC names and structures	Section A, Question 2 (five 1-mark items): name given structures; draw structural formulae of named compounds.	Longest chain with the group, lowest number, correct suffix; show four bonds on every carbon and C=O in -CHO and -COOH.
Homologous series and isomers	MCQ: which set is one homologous series; which is saturated; identify a pair of isomers from labelled compounds.	Test with general formulae; isomers need the same molecular formula.
Complete and balance equations	Preparation and reaction equations: soda lime, CaC₂, dibromoethane, ethanol with Na, esterification, combustion.	Learn the products; balance H and O carefully.
Name the reaction type	Ethanol to ethene (dehydration), bromoethane to ethene (dehydrohalogenation), alkene + Br₂ (addition), methane + Cl₂ (substitution), acid + alcohol (esterification).	Match the change: water lost, HX lost, atoms added, atom replaced.
Choose from a list	From methanal, ethanol, ethanoic acid, ethene, ethyne: which ripens fruit, which is glacial, which is used in welding.	Learn uses compound by compound.

Tricks to remember Organic Chemistry

Short memory aids. Say each one aloud once, then use it on a question the same day.

Meth-Eth-Prop-But-Pent
'My Enormous Pet Bear Paddles' = 1 to 5 carbons.
Use it when: Word roots.
Saturated = Substitution, Unsaturated = Addition
S-S and U-A: single bonds substitute; double/triple bonds add.
Use it when: Reaction types.
Alcoholic KOH Eliminates, Aqueous KOH substitutes
Alcoholic → Alkene (lose HBr); aqueous → alcohol (OH replaces Br).
Use it when: Ethene versus ethanol from bromoethane.
Ester = Alcohol + Acid, named alcohol-first
Ethanol + ethanoic acid → ethyl ethanoate: the alcohol part is the first word.
Use it when: Naming esters.

Organic Chemistry: board-level solved questions

Ordered from board level to HOTS. Cover the solution, write your own, then compare step by step.

Example 1: Addition reactions · 1 mark · MCQBoard level

Which of these is an addition reaction?
(a) CH₄ + Cl₂ → CH₃Cl + HCl
(b) C₂H₄ + H₂ → C₂H₆
(c) C₂H₅OH → C₂H₄ + H₂O
(d) CH₃COONa + NaOH → CH₄ + Na₂CO₃

Try it first, then show the solutionSolution

(a) replaces an H atom (substitution); (c) removes water (dehydration); (d) removes CO₂ as carbonate (decarboxylation).
In (b) the double bond of the unsaturated ethene opens and two H atoms are added, giving one product.

Answer: (b)

Example 2: Dehydration · 1 mark · Assertion-reasonHarder board level

Assertion (A): The conversion of ethanol to ethene by concentrated sulphuric acid is a dehydration.
Reason (R): A molecule of hydrogen is removed from ethanol.
(a) Both (A) and (R) are true, and (R) is the correct explanation for (A).
(b) Both (A) and (R) are true, but (R) is not the correct explanation for (A).
(c) (A) is true and (R) is false.
(d) (A) is false and (R) is true.

Try it first, then show the solutionSolution

A molecule of water (not hydrogen) is removed: $C_{2} H_{5} O H \to C_{2} H_{4} + H_{2} O$ .

Answer: (c)

Example 3: IUPAC naming · 5 marks · Very short answerHarder board level

Give the IUPAC names of: (a) CH₃–CH₂–CHO, (b) CH₃–CO–CH₂–CH₃, (c) CH₃–CH₂–O–CH₂–CH₃, (d) CH₃COOCH₃, (e) CH₃–CH=CH–CH₃.

Try it first, then show the solutionSolution

(a) Three C including CHO: propanal.
(b) Four C, C=O on C-2: butan-2-one (butanone).
(c) Ether with two ethyl groups: ethoxyethane (diethyl ether).
(d) Ester from methanol and ethanoic acid: methyl ethanoate.
(e) Four C, double bond at C-2: but-2-ene.

Answer: (a) Propanal (b) Butan-2-one (c) Ethoxyethane (d) Methyl ethanoate (e) But-2-ene

Example 4: Preparations · 3 marks · Short answerHarder board level

Write balanced equations for the preparation of (a) ethane from sodium propanoate, (b) ethyne from 1,2-dibromoethane, (c) ethanol from bromoethane.

Try it first, then show the solutionSolution

(a) $C_{2} H_{5} C O O N a + N a O H \to C_{2} H_{6} + N a_{2} C O_{3}$ (soda lime, heat)
(b) $C_{2} H_{4} B r_{2} + 2 K O H \to C_{2} H_{2} + 2 K B r + 2 H_{2} O$ (alcoholic KOH)
(c) $C_{2} H_{5} B r + N a O H \to C_{2} H_{5} O H + N a B r$ (aqueous NaOH, boil)

Answer: As above.

Example 5: Chain isomers · 3 marks · Short answerBoard level

Draw (condensed) and name the three chain isomers of C₅H₁₂.

Try it first, then show the solutionSolution

CH₃–CH₂–CH₂–CH₂–CH₃: pentane.
CH₃–CH(CH₃)–CH₂–CH₃: 2-methylbutane.
C(CH₃)₄: 2,2-dimethylpropane.

Answer: Pentane, 2-methylbutane, 2,2-dimethylpropane.

Example 6: Ethanol reactions · 4 marks · Long answerBoard level

Write equations for: (a) complete combustion of ethanol, (b) ethanol with sodium, (c) ethanol with acetic acid. Name the type of reaction in (c).

Try it first, then show the solutionSolution

(a) $C_{2} H_{5} O H + 3 O_{2} \to 2 C O_{2} + 3 H_{2} O$
(b) $2 C_{2} H_{5} O H + 2 N a \to 2 C_{2} H_{5} O N a + H_{2}$
(c) $C_{2} H_{5} O H + C H_{3} C O O H ⇌ C H_{3} C O O C_{2} H_{5} + H_{2} O$
(c) is esterification.

Answer: As above.

Example 7: Incomplete combustion · 2 marks · Short answerHarder board level

Balance the incomplete combustion of ethane giving carbon monoxide and water, and the complete combustion of ethane.

Try it first, then show the solutionSolution

Incomplete: $2 C_{2} H_{6} + 5 O_{2} \to 4 C O + 6 H_{2} O$
Complete: $2 C_{2} H_{6} + 7 O_{2} \to 4 C O_{2} + 6 H_{2} O$

Answer: As above.

Example 8: HOTS: identify the compounds · 4 marks · Long answerHOTS / competency

Compound A (C₂H₆O) reacts with sodium to give hydrogen. A heated with conc. H₂SO₄ at 170 °C gives gas B that decolourises bromine water. A warmed with C (C₂H₄O₂) and a little conc. H₂SO₄ gives D, which smells fruity. Identify A to D and write the equation for D.

Try it first, then show the solutionSolution

A reacts with Na: an alcohol, ethanol.
Dehydration gives ethene (B), which is unsaturated.
C is acetic acid; D is ethyl ethanoate.
$C_{2} H_{5} O H + C H_{3} C O O H ⇌ C H_{3} C O O C_{2} H_{5} + H_{2} O$

Answer: A ethanol, B ethene, C acetic acid, D ethyl ethanoate.

Case-study practice (Section B style)

Read the situation, then answer every part. In the board paper this is a 5-mark question split into 1 + 1 + 2 + 1 marks.

Six labelled compounds

Suppose a worksheet lists: A CH₃–CH₂–OH; B CH₃–O–CH₃; C CH₃–CH₃; D CH₂=CH–CH₂–CH₃; E CH≡CH; F CH₃–CH=CH–CH₃.

(i) · 1 mark
Which pair are position isomers?
Answer
Answer: D and F (but-1-ene and but-2-ene).
(ii) · 1 mark
Which pair have the same molecular formula but different functional groups?
Answer
Answer: A and B (ethanol and methoxymethane, both C₂H₆O).
(iii) · 2 marks
Which compound is used in welding, and how is it made from calcium carbide?
Answer
Answer: E (ethyne): $C a C_{2} + 2 H_{2} O \to C a (O H)_{2} + C_{2} H_{2}$
(iv) · 1 mark
Which compound undergoes only substitution with chlorine?
Answer
Answer: C (ethane), a saturated hydrocarbon.

Where the marks are

How a typical answer is split into step marks. Show every step: a right final answer alone rarely earns full marks.

Draw the structural formulae of (a) propanal, (b) but-2-yne. (2 marks) (2 marks)

Step	Marks
(a) CH₃–CH₂–CHO drawn in full: three carbons in a chain, the end carbon with a C=O double bond and one H, every carbon with four bonds	1
(b) CH₃–C≡C–CH₃ with the triple bond between C-2 and C-3, each end carbon with three H	1

Give the equation, condition and observation for the reaction of ethene with bromine, and name the product. (3 marks) (3 marks)

Step	Marks
$C_{2} H_{4} + B r_{2} \to C_{2} H_{4} B r_{2}$ (room temperature, bromine in CCl₄ or water)	1
Reddish brown colour of bromine disappears	1
1,2-dibromoethane	1

State four characteristics of a homologous series. (4 marks) (4 marks)

Step	Marks
Same general formula	1
Successive members differ by CH₂ (14 u)	1
Similar chemical properties (same functional group)	1
Gradual change in physical properties with molecular mass	1

Where students lose marks in Organic Chemistry

The slips examiners see most often, and what to write instead.

Mistake: Writing two separate locants instead of 'di'
CISCE examiners' comments, 2024: candidates wrote '2 chloro, 3 chloropentane' instead of 2,3-dichloropentane, wrote pentene for pentane and confused propanoic acid with propanol or propanal.
Wrong working
2 chloro, 3 chloropentane
Right working
2,3-dichloropentane
Do this: Group identical substituents with di-, tri-, tetra- and list all locants with commas.
Usually costs: 1 mark.
Mistake: Structures without tetravalency
CISCE examiners' comments, 2024 and 2025: candidates did not show the tetravalency of carbon in propyne, missed the C=O in methanal, drew condensed formulae when a structural diagram was asked, and put extra H on a double-bonded carbon.
Wrong working
Propanal drawn as CH₃CH₂COH with no bonds shown.
Right working
Every bond drawn; the aldehyde carbon with C=O and one H.
Do this: Count four bonds on every carbon before moving on.
Usually costs: 1 mark per structure.
Mistake: Confusing dehydration with dehydrogenation
CISCE examiners' comments, 2024: for ethanol to ethene, candidates chose dehydrogenation, mixing up dehydration, dehydrogenation and dehydrohalogenation.
Wrong working
Ethanol → ethene is dehydrogenation.
Right working
Dehydration (removal of H₂O).
Do this: Dehydration = −H₂O; dehydrohalogenation = −HX; dehydrogenation = −H₂.
Usually costs: 1 mark.
Mistake: Unbalanced organic equations
CISCE examiners' comments, 2024 and 2025: candidates over-balanced the decarboxylation, wrote C₂H₆ for C₂H₂ in the elimination, wrote NaHCO₃ for Na₂CO₃, and could not balance others.
Wrong working
C₂H₄Br₂ + KOH → C₂H₆ + KBr
Right working
$C_{2} H_{4} B r_{2} + 2 K O H \to C_{2} H_{2} + 2 K B r + 2 H_{2} O$
Do this: Write the product formula first from the reaction type, then balance K, Br, H, O.
Usually costs: 1 mark each.
Mistake: Wrong homologous series
CISCE examiners' comments, 2025: candidates could not use the general formulae to pick members of the same homologous series and confused C₅H₁₀ with the saturated C₅H₁₂.
Wrong working
C₅H₁₀ is saturated.
Right working
C₅H₁₂ (CₙH₂ₙ₊₂) is saturated; C₅H₁₀ is an alkene.
Do this: Check each formula against CₙH₂ₙ₊₂, CₙH₂ₙ, CₙH₂ₙ₋₂.
Usually costs: 1 mark.
Mistake: Wrong compound for a use
CISCE examiners' comments, 2024: candidates chose ethanol instead of ethene for ripening fruit, and missed 'not' in a choose-from-the-list item.
Wrong working
Fruit ripening: ethanol
Right working
Ethene
Do this: Learn one use per compound and circle 'not'.
Usually costs: 1 mark.

Practice questions on Organic Chemistry

Write each answer on paper first. Open the hint only if you are stuck, and the answer only when you have finished.

Q1 · 1 mark · MCQBoard level
Which of these is a saturated hydrocarbon?
(a) C₃H₆
(b) C₃H₄
(c) C₄H₁₀
(d) C₂H₂
Hint
CₙH₂ₙ₊₂.
Answer
(c)
Q2 · 1 mark · MCQBoard level
Ethanol is warmed with acetic acid and a few drops of conc. H₂SO₄. The sweet-smelling product is:
(a) ethanal
(b) ethyl ethanoate
(c) ethene
(d) sodium ethoxide
Hint
Alcohol + acid gives an ester, named alcohol-first.
Answer
(b)
Q3 · 1 mark · MCQHarder board level
Statement I: Ethyne decolourises bromine water.
Statement II: Ethyne adds two molecules of bromine.
(a) Both true, and II explains I
(b) Both true, but II does not explain I
(c) I true, II false
(d) I false, II true
Hint
Triple bond.
Answer
(a)
Q4 · 2 marks · Very short answerBoard level
Write the IUPAC names of: (a) CH₃CHO, (b) HCOOH, (c) CH₃CH₂CH₂Cl, (d) CH₃OCH₃.
Hint
Half mark each.
Answer
(a) Ethanal (b) Methanoic acid (c) 1-chloropropane (d) Methoxymethane
Q5 · 3 marks · Short answerHarder board level
Draw the structural formulae of (a) 2-methylpropane, (b) propanone, (c) ethyl ethanoate.
Hint
Four bonds on every carbon.
Answer
(a) CH₃–CH(CH₃)–CH₃ (b) CH₃–CO–CH₃ with C=O on the middle carbon (c) CH₃–CO–O–CH₂–CH₃ with C=O on the acid carbon (all bonds drawn).
Q6 · 3 marks · Short answerBoard level
Write equations for: (a) methane from sodium ethanoate, (b) ethyne + excess hydrogen, (c) ethene + chlorine.
Hint
Soda lime; Ni; addition.
Answer
(a) $C H_{3} C O O N a + N a O H \to C H_{4} + N a_{2} C O_{3}$ (b) $C_{2} H_{2} + 2 H_{2} \to C_{2} H_{6}$ (c) $C_{2} H_{4} + C l_{2} \to C_{2} H_{4} C l_{2}$
Q7 · 4 marks · Long answerHarder board level
Bromoethane gives two different products with KOH. Name the product and the reaction type for (a) aqueous KOH and (b) alcoholic KOH, with equations.
Hint
Substitution vs elimination.
Answer
(a) Ethanol, hydrolysis: $C_{2} H_{5} B r + K O H \to C_{2} H_{5} O H + K B r$ . (b) Ethene, dehydrohalogenation: $C_{2} H_{5} B r + K O H \to C_{2} H_{4} + K B r + H_{2} O$ .
Q8 · 2 marks · Short answerHarder board level
Give two differences between ethanol and acetic acid using simple tests.
Hint
Litmus; carbonate.
Answer
Acetic acid turns blue litmus red, ethanol does not. Acetic acid gives CO₂ with sodium carbonate (or hydrogen carbonate), ethanol does not. (Smell: vinegar vs pleasant.)
Q9 · 2 marks · Short answerHarder board level
Explain the difference between denatured alcohol and spurious alcohol.
Hint
Deliberate vs illicit.
Answer
Denatured alcohol is ethanol deliberately made unfit for drinking (with poisonous additives and dye) for industrial use. Spurious alcohol is illicit liquor contaminated with methanol, which is poisonous.
Q10 · 4 marks · Long answerHOTS / competency
An open-chain hydrocarbon X (C₃H₄) contains a triple bond and adds 2 moles of bromine. Write the structural formula and IUPAC name of X, the equation for its reaction with bromine, and the name of the product.
Hint
An alkyne with three carbons.
Answer
X: propyne, CH≡C–CH₃. $C_{3} H_{4} + 2 B r_{2} \to C_{3} H_{4} B r_{4}$ ; product 1,1,2,2-tetrabromopropane.
Q11 · 2 marks · Short answerHOTS / competency
The boiling points of the first four alkanes rise from about -162 °C to about -0.5 °C. Explain this trend and predict the state of pentane at room temperature.
Hint
Gradation with molecular mass.
Answer
In a homologous series, physical properties change gradually with molecular mass: larger molecules have stronger forces between them, so boiling points rise. Pentane (C₅) is a liquid at room temperature.

Test your recall

Answer in your head or on paper first, then open the card. Pulling an answer out of memory makes it stick far better than re-reading.

1.What is catenation?Show answer
The property of carbon atoms to link with each other to form chains and rings.
2.General formula of alkynes?Show answer
CₙH₂ₙ₋₂.
3.IUPAC name of acetone?Show answer
Propanone.
4.IUPAC name of diethyl ether?Show answer
Ethoxyethane.
5.Reagent to convert bromoethane to ethene?Show answer
Alcoholic potassium hydroxide.
6.Catalyst for adding hydrogen to ethene?Show answer
Nickel (about 300 °C).
7.Why is pure acetic acid called glacial?Show answer
It forms ice-like crystals on cooling.
8.Test for unsaturation?Show answer
Decolourises reddish brown bromine water.
9.Smell of an ester?Show answer
Sweet, fruity.

Best order to study it

Learn tetravalency, catenation, general formulae and homologous series.
Practise structural formulae to C₅ and chain/position isomers.
Learn the IUPAC rules and the table of functional groups with trivial names.
Learn methane, ethane, ethene and ethyne: preparation, reactions and uses.
Finish with ethanol and acetic acid: preparation, properties, esterification and uses.

Quick revision checklist

The night before the exam: can you do each of these without looking?

Can you name and draw any alkane, alkene or alkyne up to C₅?
Can you name compounds with each of the nine functional groups?
Can you write the soda lime, calcium carbide, alcoholic KOH and aqueous NaOH reactions?
Can you write addition reactions with H₂, Cl₂, Br₂ and I₂?
Can you write ethanol with Na, O₂, acetic acid and conc. H₂SO₄?
Can you explain denatured, methylated and spurious alcohol?

Organic Chemistry: frequently asked questions

Which functional groups are in the ICSE 2027 nomenclature syllabus?

Halides, double bond, triple bond, alcohol, aldehyde, carboxylic acid, ketone, ether and ester, for compounds with only one functional group. Halides, ketone, ether and ester were added for 2027.

How many carbon atoms do I need for structural formulae?

Up to 5 carbon atoms for alkanes, alkenes and alkynes.

Which cyclic compound must I know?

Only benzene's structure (C₆H₆, a six-carbon ring with alternate double bonds).

Is optical or functional isomerism needed?

No. Only structural isomerism of the chain and position types is in the syllabus.

Based on: CISCE ICSE 2027 Chemistry (Science Paper 2) syllabus; CISCE ICSE 2027 Chemistry (Science Paper 2) specimen question paper and marking scheme; CISCE Analysis of Pupil Performance, ICSE Chemistry (Science Paper 2), 2025; CISCE Analysis of Pupil Performance, ICSE Chemistry (Science Paper 2), 2024