10th Class Chemistry Notes 2026 — Chapter 21

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Class 10 Chemistry — Chapter 21: Organic Chemistry

Quran Education Point
Board: Punjab Board

A. Multiple Choice Questions (MCQs)

21.1 Choose the correct IUPAC name for the compound CH₂=CH−CH₂−CH₃.

(a) But-3-ene
(b) But-1-ene
(c) But-2-ene
(d) Butene

Correct Answer: (b) But-1-ene

21.2 Select a branched-chain compound.

(a) CH₂=CH−CH₂−CH₃
(b) CH₂=C(CH₃)−CH₂−CH₃
(c) CH₃−CH=CH−CH₃
(d) CH₃−CH=CH−CH₂−CH₃

Correct Answer: (b) CH₂=C(CH₃)−CH₂−CH₃

21.3 Which is the displayed formula of ethanol?

(a) C₂H₅OH
(b) CH₃−CH₂−OH
(c) H−C(H)₂−C(H)₂−OH
(d) H₃C−CH₂−O−H, with all individual bonds displayed

Correct Answer: (d) H₃C−CH₂−O−H, with all individual bonds displayed

21.4 Identify the aldehydic functional group.

(a) −C(=O)−OH
(b) −OH
(c) −C(=O)−
(d) −C(=O)−H

Correct Answer: (d) −CHO

21.5 Select the general formula of the alkyne family.

(a) CₙH₂ₙ₊₁
(b) CₙH₂ₙ
(c) CₙH₂ₙ₋₂
(d) CₙH₂ₙ₋₁

Correct Answer: (c) CₙH₂ₙ₋₂

21.6 How many structural isomers are possible for a saturated hydrocarbon containing five carbon atoms?

(a) 2
(b) 3
(c) 4
(d) 5

Correct Answer: (b) 3

The three isomers are n-pentane, 2-methylbutane (isopentane), and 2,2-dimethylpropane (neopentane).

21.7 Which of the following esters is ethyl propanoate?

(a) CH₃−C(=O)−OCH₃
(b) CH₃−CH₂−C(=O)−OCH₃
(c) CH₃−C(=O)−OC₂H₅
(d) CH₃−CH₂−C(=O)−OC₂H₅

Correct Answer: (d) CH₃−CH₂−C(=O)−OC₂H₅

21.8 Which functional group is present in alkenes?

(a) No functional group
(b) Carbon–carbon triple bond
(c) Carbon–carbon double bond
(d) Carbon–carbon single bond

Correct Answer: (c) Carbon–carbon double bond

B. Short Answer Questions

21.1 What is the difference between saturated and unsaturated hydrocarbons?

Saturated hydrocarbons: These contain only single covalent bonds between carbon atoms. They are called alkanes. Examples include methane and ethane.

Unsaturated hydrocarbons: These contain at least one carbon–carbon double bond or triple bond. Alkenes contain a double bond, while alkynes contain a triple bond. Examples include ethene and ethyne.

21.2 Write down the structural formula of butan-2-ol.

Condensed structural formula:

CH₃−CH(OH)−CH₂−CH₃

21.3 Why is ethylamine, CH₃−CH₂−NH₂, called an organic compound?

Ethylamine is an organic compound because it contains a carbon-based chain with covalent bonds and an amino functional group (−NH₂).

21.4 Write down the formula of an ester called ethyl methanoate.

Condensed structural formula:

H−C(=O)−O−CH₂−CH₃

Molecular formula: HCOOCH₂CH₃

21.5 Define structural isomerism.

Structural isomerism is the phenomenon in which compounds have the same molecular formula but different structural arrangements of atoms.

21.6 Give the IUPAC name of CH₃−CH₂−CH₂−CH₃.

The IUPAC name is butane.

21.7 What is common in the carboxyl, amide, and ester functional groups?

All three contain a carbonyl group, \(\mathrm{C=O}\). However, the atoms attached to the carbonyl carbon differ in each functional group.

C. Constructed Response Questions

21.1 Why do the members of a homologous series have similar chemical properties?

All members of a homologous series possess the same functional group. Since the functional group determines the characteristic chemical reactions of organic compounds, members of the same homologous series show similar chemical properties.

21.2 How do the properties of 2-methylpropane differ from those of n-butane, although both contain the same number of carbon atoms?

Both compounds have the molecular formula C₄H₁₀, but their structures differ.

  • n-Butane: Has a straight carbon chain.

  • 2-Methylpropane: Has a branched carbon chain.

2-Methylpropane has a lower boiling point than n-butane because its branched structure provides a smaller effective surface area for intermolecular attractions. Therefore, weaker London dispersion forces act between its molecules.

21.3 Write down the structural formulae of the isomers of a saturated hydrocarbon containing five carbon atoms.

The molecular formula is C₅H₁₂. It has three structural isomers.

1. n-Pentane

CH₃−CH₂−CH₂−CH₂−CH₃

2. 2-Methylbutane (Isopentane)

CH₃−CH(CH₃)−CH₂−CH₃

3. 2,2-Dimethylpropane (Neopentane)

C(CH₃)₄

21.4 Write down the structural formulae of the isomers shown by butene.

Butene has the molecular formula C₄H₈. Its structural isomers include:

1. But-1-ene

CH₂=CH−CH₂−CH₃

2. But-2-ene

CH₃−CH=CH−CH₃

3. 2-Methylprop-1-ene

CH₂=C(CH₃)₂

21.5 Explain the difference among the functional groups present in aldehydes, ketones, and carboxylic acids.

Aldehyde (−CHO): The carbonyl carbon is bonded to at least one hydrogen atom. Example: propanal, CH₃−CH₂−CHO.

Ketone (−CO−): The carbonyl carbon is bonded to two carbon atoms. Example: propanone, CH₃−CO−CH₃.

Carboxylic acid (−COOH): The carbonyl carbon is directly bonded to a hydroxyl group (−OH). Example: ethanoic acid, CH₃−COOH.

D. Descriptive Questions

21.1 What is an organic compound? How are organic compounds classified based on the type of carbon chain present in them?

Organic compounds: Organic compounds are mainly carbon-based compounds, usually containing carbon–hydrogen bonds, and their derivatives. Some carbon-containing compounds, such as carbonates and carbon oxides, are generally classified as inorganic.

Organic compounds can be classified according to their carbon chains as follows:

1. Open-chain (acyclic or aliphatic) compounds

In these compounds, carbon atoms are joined in straight or branched chains.

Examples: alkanes, alkenes, and alkynes.

2. Closed-chain (cyclic) compounds

In these compounds, carbon atoms or carbon and other atoms form rings.

They are divided into:

  • Homocyclic or carbocyclic compounds: Rings consist entirely of carbon atoms. These include alicyclic compounds and aromatic compounds such as benzene.

  • Heterocyclic compounds: Rings contain carbon atoms along with one or more other elements, such as nitrogen, oxygen, or sulphur.

21.2 Explain structural formula, giving examples.

A structural formula shows how atoms are connected to one another in a molecule and how the bonds between them are arranged.

Example 1: Methane (CH₄)

Methane contains one carbon atom bonded to four hydrogen atoms by single covalent bonds.

H

|

H − C − H

|

H

Example 2: Propane (C₃H₈)

Condensed structural formula:

CH₃−CH₂−CH₃

21.3 Write down five characteristics of a homologous series.

  1. All members can be represented by the same general formula.

  2. Consecutive members differ by a −CH₂− group, corresponding to a difference of 14 atomic mass units in molecular mass.

  3. All members contain the same functional group and show similar chemical properties.

  4. Physical properties, such as boiling point, melting point, and density, change gradually with increasing molecular mass.

  5. Members of the same homologous series can often be prepared by similar general methods.

21.4 Give the IUPAC rules for naming alcohols.

  1. Select the longest carbon chain containing the hydroxyl group (−OH) as the parent chain.

  2. Use the name of the corresponding alkane and replace the final “e” with “ol.”

  3. Number the carbon chain from the end nearest the hydroxyl group so that the −OH group receives the lowest possible number.

  4. Indicate the position of the hydroxyl group in the name, such as propan-2-ol.

  5. Identify and number any alkyl substituents, writing them as prefixes in alphabetical order.

Examples:

  • CH₃−CH₂−CH₂OH — Propan-1-ol

  • CH₃−CH(OH)−CH₃ — Propan-2-ol

21.5 Name the following compounds.

(a) CH₃−CH₂−CH₂OH

IUPAC name: Propan-1-ol

(b) CH₃−CH₂−CHO

IUPAC name: Propanal

(c) CH₃−CH₂−CH₂−COOH

IUPAC name: Butanoic acid

(d) CH₃−CH₂−CO−O−CH₃

IUPAC name: Methyl propanoate

 

Notes by Sir Dilawar

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