Organic Chemistry

Q: Write the general formulae of alkanes, alkenes and alkynes, and give the molecular formula of the member of each with four carbon atoms.

CnH2n+2, CnH2n and CnH2n-2; butane C4H10, butene C4H8, butyne C4H6.

Q: Draw the condensed structural formulae of the two chain isomers of butane and name them.

n-butane, CH3-CH2-CH2-CH3, and 2-methylpropane (isobutane), CH3-CH(CH3)-CH3.

Q: Write balanced equations for the preparation of methane from sodium ethanoate and of ethane from bromoethane.

CH3COONa + NaOH → CH4 + Na2CO3 (with CaO, heat); C2H5Br + 2[H] → C2H6 + HBr (Zn-Cu couple in ethanol).

ICSE Class 10 · Chemistry · Chapter 12

What this chapter covers

Unique nature of the carbon atom
Single, double and triple bonds, chains and rings
Structure and isomerism
Homologous series
Simple nomenclature
Alkanes: methane and ethane
Alkenes: ethene
Alkynes: ethyne
Uses of methane, ethane, ethene and ethyne
Ethanol
Acetic acid

Key definitions

Organic chemistry is the branch of chemistry that studies the compounds of carbon, mainly the hydrocarbons and the compounds derived from them.

An organic compound is a compound in which carbon atoms are joined covalently to hydrogen and often to other atoms such as oxygen, nitrogen or the halogens.

Tetravalency is the property of the carbon atom of forming four covalent bonds, because it has four valence electrons that it shares with other atoms.

Catenation is the property of an element by which its atoms link with one another through covalent bonds to form long chains, branched chains or rings.

A single covalent bond is a bond formed by the sharing of one pair of electrons between two atoms, shown by a single line, as in C-C.

A double covalent bond is a bond formed by the sharing of two pairs of electrons between two atoms, shown by two lines, as in C=C.

A triple covalent bond is a bond formed by the sharing of three pairs of electrons between two atoms, shown by three lines, as in CequivC.

A saturated hydrocarbon is a hydrocarbon in which all the carbon–carbon bonds are single bonds, so that every carbon atom holds the largest possible number of hydrogen atoms.

An unsaturated hydrocarbon is a hydrocarbon that contains at least one carbon–carbon double or triple bond, so that it can take up more hydrogen or halogen atoms by addition.

A hydrocarbon is a compound made of carbon and hydrogen only.

A cyclic compound is a compound in which the carbon atoms are joined end to end to form a closed ring.

A structural formula is a formula that shows how the atoms in a molecule are joined to one another, with a line for each covalent bond.

Key points

Carbon forms millions of compounds because of tetravalency (four covalent bonds) and catenation (self-linking into chains and rings).
Carbon atoms join by single, double or triple bonds and form straight chains, branched chains or rings such as benzene, C₆H₆.
Isomers have the same molecular formula but different structures; chain isomers differ in the carbon skeleton, position isomers in where the multiple bond or functional group sits.
A homologous series has one general formula, members differing by CH₂, similar chemical properties and gradually changing physical properties.
Alkanes C_nH_2n+2, alkenes C_nH_2n, alkynes C_nH_2n-2.
IUPAC names use the longest chain containing the functional group and the smallest number for that group.
Methane and ethane are made by heating sodium ethanoate or sodium propanoate with soda lime, or by reducing iodomethane or bromoethane.
Alkanes burn completely to carbon dioxide and water, incompletely to carbon monoxide or carbon, and react with chlorine by substitution in diffused sunlight.
Methane is a powerful greenhouse gas from paddy fields, cattle, landfills and gas leaks.
Ethene is made by dehydrohalogenation of bromoethane or dehydration of ethanol at 170 ^\circC; ethyne from calcium carbide and water or from 1,2-dibromoethane and alcoholic potassium hydroxide.
Alkenes and alkynes undergo addition of hydrogen (nickel catalyst) and of chlorine, bromine and iodine; decolourising bromine is a test for unsaturation.
Ethanol is made by hydrolysis of bromoethane, boils at about 78 ^\circC, mixes with water, burns cleanly, reacts with sodium, forms esters and is dehydrated to ethene.
Denatured and methylated spirit are ethanol made unfit to drink; spurious alcohol contains poisonous methanol.
Acetic acid, CH₃COOH, smells of vinegar, freezes into glacial acetic acid, turns blue litmus red, is neutralised by alkalis and forms esters with alcohols.

Questions answered

Define catenation, and give two reasons why carbon shows it more than any other element.

Catenation is the property of an element by which its atoms link with one another through covalent bonds to form chains or rings. Carbon atoms are small, so the carbon–carbon bond is short and strong; and carbon is tetravalent, so each atom in a chain still has valencies free to bond with other atoms.

Why does carbon form covalent bonds and not ionic ones?

Carbon has four valence electrons; losing or gaining four electrons would need too much energy, so it completes its octet by sharing electrons.

Write the general formulae of alkanes, alkenes and alkynes, and give the molecular formula of the member of each with four carbon atoms.

C_nH_2n+2, C_nH_2n and C_nH_2n-2; butane C₄H₁₀, butene C₄H₈, butyne C₄H₆.

State three characteristics of a homologous series.

Members share one general formula; successive members differ by CH₂ (14 units of mass); they have similar chemical properties and physical properties that change gradually with molecular mass.

Draw the condensed structural formulae of the two chain isomers of butane and name them.

n-butane, CH₃-CH₂-CH₂-CH₃, and 2-methylpropane (isobutane), CH₃-CH(CH₃)-CH₃.

Name the type of isomerism shown by but-1-ene and but-2-ene, and give a reason.

Position isomerism, because both have the same four-carbon straight chain and differ only in the position of the double bond.

Give the IUPAC names of: (a) acetaldehyde, (b) acetone, (c) methyl chloride, (d) ethyl acetate, (e) dimethyl ether.

(a) ethanal, (b) propanone, (c) chloromethane, (d) ethyl ethanoate, (e) methoxymethane.

Write balanced equations for the preparation of methane from sodium ethanoate and of ethane from bromoethane.

CH₃COONa + NaOH → CH₄ + Na₂CO₃ (with CaO, heat); C₂H₅Br + 2[H] → C₂H₆ + HBr (Zn-Cu couple in ethanol).

Past board exam questions

ICSE 2027 sample paper · 1 mark

Identify the organic compound(s) with the molecular formula C₃H₈O. 1. propanol 2. propanal 3. propanoic acid

Only 1
Only 2
Both 2 and 3
Both 1 and 2

Show answer

Only 1

ICSE 2027 sample paper · 5 marks

(a) Give the IUPAC name of the following organic compounds: 1. HO–C(=O)–CH₂–CH₂–CH₃ (the carbon bonded to OH carries a double-bonded O) 2. H–C(H)(H)–C(H)(CH₃)–C(H)(H)–C(H)(CH₃)–H (a four-carbon chain drawn left to right: the 1st carbon carries three H; the 2nd carries one H above and a CH₃ below; the 3rd carries two H; the 4th carries one H above, a CH₃ below and an H on the right) (b) Draw the structural diagram for the following compounds: 1. diethyl ether 2. isopentane 3. propanone

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(a) 1. Butanoic acid (butan-1-oic acid) 2. 2-methylpentane (longest chain CH₃–CH(CH₃)–CH₂–CH₂–CH₃ has five carbons, methyl at C-2) (b) 1. Diethyl ether: CH₃–CH₂–O–CH₂–CH₃, i.e. H₃C–CH₂–O–CH₂–CH₃ with every H shown on the carbons. 2. Isopentane (2-methylbutane): CH₃–CH(CH₃)–CH₂–CH₃ 3. Propanone: CH₃–C(=O)–CH₃ (C=O on the middle carbon)

ICSE 2027 sample paper · 2 marks

What is the ratio of methane to the acidic gaseous product obtained when methane undergoes complete combustion? Give a balanced equation for the above reaction.

Show answer

CH₄ + 2O₂ → CO₂ + 2H₂O The acidic gas is CO₂; ratio of methane : carbon dioxide = 1 : 1

ICSE 2027 sample paper · 3 marks

Give balanced equations for each of the following: (a) Action of warm water on calcium carbide. (b) Oxidation of sulphur with conc. nitric acid. (c) Laboratory preparation of ethane by using sodium propionate and soda lime.

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(a) CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ (b) S + 6HNO₃ (conc.) → H₂SO₄ + 6NO₂ + 2H₂O (c) C₂H₅COONa + NaOH —(CaO, heat)→ C₂H₆ + Na₂CO₃

ICSE 2027 sample paper · 4 marks

The structures of five organic compounds, C, D, E, F and G are shown. - C: CH₃–CH=CH₂ - D: CH₃–CH₂–CH₂–OH - E: CH₃–CH₂–CH₂–CH₃ - F: CH₂Br–CH₂Br - G: CH₂=CH₂ (Each was printed as a full structural formula with every bond shown.) Answer the following questions about these compounds. (Each compound may be used once, more than once or not at all.) State which compound, C, D, E, F or G: (a) on halogenation will form F. (b) is an alcohol. (c) is an isomer of 2-methyl propane. (d) is in the same homologous series as ethene.

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(a) G (CH₂=CH₂ + Br₂ → CH₂Br–CH₂Br) (b) D (propan-1-ol) (c) E (n-butane, C₄H₁₀) (d) C (propene, an alkene)

ICSE 2026 sample paper · 1 mark

The structures of four hydrocarbons are shown below: 1. CH₃–CH(CH₃)–CH₃ (a central C bonded to H, to CH₃ on the left, and to CH₃ above and below) 2. (CH₃)₂C=CH₂ (a central C with CH₃ above, CH₃ below, and =CH₂ to the right) 3. CH₃–CH₂–CH=CH₂ 4. CH₃–C(CH₃)=CH₂ (CH₃ on the left, CH₃ above, =CH₂ to the right) How many isomers of butene are shown in the above structures?

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ICSE 2026 board paper · 1 mark

Identify the equation that shows the reaction of ethane with chlorine in the presence of ultraviolet light.

C₂H₆ + Cl₂ ⟶ C₂H₆Cl₂
C₂H₆ + Cl₂ ⟶ C₂H₄Cl₂ + H₂
C₂H₆ + Cl₂ ⟶ C₂H₅Cl + HCl
C₂H₆ + Cl₂ ⟶ 2CH₃Cl

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C₂H₆ + Cl₂ ⟶ C₂H₅Cl + HCl

ICSE 2026 board paper · 1 mark

A hydrocarbon X undergoes the reaction under suitable conditions as shown below: X (H₃C–CH₂–CH₂–CH₃) ⟶ Y (H₂C=CH–CH₃) + Z (CH₄) (X: four carbon atoms in a chain joined by single bonds, each carbon carrying hydrogens; Y: H₂C=CH–CH₃, a carbon–carbon double bond between the first two carbons; Z: one carbon with four hydrogens.) Which compound(s) is/are unsaturated?

X only
Y only
X and Z
Y and Z

Show answer

Y only