Class 10 ICSE – Organic Chemistry: ALKYNES The Metal Mayur Gupta

Описание: Class 10 ICSE – Organic Chemistry: ALKYNES
(Metal Mayur Gupta style: topic-wise, exam-oriented description)

ALKYNES – Topic per Description
1. Definition
Alkynes are unsaturated hydrocarbons which contain at least one triple bond (–C≡C–) between two carbon atoms.

👉 General formula: CₙH₂ₙ₋₂

Example:

Ethyne (Acetylene): C₂H₂

Propyne: C₃H₄

2. Homologous Series
Alkynes form a homologous series where:

Successive members differ by –CH₂–

They have similar chemical properties

Physical properties show a gradual change

3. Nomenclature (ICSE level)
Longest carbon chain containing triple bond is selected

Suffix “-yne” is used

Numbering starts from the end nearest to the triple bond

Example:
CH≡C–CH₃ → Propyne

4. Structure of Ethyne
Each carbon atom is sp hybridised

Bond angle = 180° (linear molecule)

Contains:

1 σ bond

2 π bonds (in triple bond)

5. Preparation of Ethyne (Very Important)
From Calcium Carbide

Reaction:
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ ↑

📌 Lab method & industrial method – same reaction

6. Physical Properties
Lower members are colourless gases

Insoluble in water

Soluble in organic solvents

Highly inflammable

7. Chemical Properties of Alkynes
(a) Combustion
Ethyne burns with sooty flame due to high carbon content.

2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O

(b) Addition Reactions (Key Property)
Because of triple bond, alkynes undergo addition reactions.

1. Hydrogenation
C₂H₂ + H₂ → C₂H₄ → C₂H₆
(Nickel catalyst)

2. Halogenation
C₂H₂ + Br₂ → C₂H₂Br₂

🔹 Bromine water is decolourised

3. Hydrohalogenation
C₂H₂ + HCl → Vinyl chloride

8. Acidic Nature of Ethyne
Ethyne is weakly acidic due to presence of terminal hydrogen.

With Ammoniacal AgNO₃:

Forms white precipitate of silver acetylide

📌 This test is used to distinguish ethyne from ethene & ethane

9. Uses of Ethyne
Oxy-acetylene welding

Artificial ripening of fruits

Manufacture of plastics & synthetic rubber

10. Difference Based Questions (ICSE Favourite)
Property Alkane Alkene Alkyne
Saturation Saturated Unsaturated Unsaturated
Bond Single Double Triple
Formula CₙH₂ₙ₊₂ CₙH₂ₙ CₙH₂ₙ₋₂
Bromine water No effect Decolourises Decolour
ALKYNES – Topic per Description
1. Definition
Alkynes are unsaturated hydrocarbons which contain at least one triple bond (–C≡C–) between two carbon atoms.

👉 General formula: CₙH₂ₙ₋₂

Example:

Ethyne (Acetylene): C₂H₂

Propyne: C₃H₄

2. Homologous Series
Alkynes form a homologous series where:

Successive members differ by –CH₂–

They have similar chemical properties

Physical properties show a gradual change

3. Nomenclature (ICSE level)
Longest carbon chain containing triple bond is selected

Suffix “-yne” is used

Numbering starts from the end nearest to the triple bond

Example:
CH≡C–CH₃ → Propyne

4. Structure of Ethyne
Each carbon atom is sp hybridised

Bond angle = 180° (linear molecule)

Contains:

1 σ bond

2 π bonds (in triple bond)

5. Preparation of Ethyne (Very Important)
From Calcium Carbide

Reaction:
CaC₂ + 2H₂O → Ca(OH)₂ + C₂H₂ ↑

📌 Lab method & industrial method – same reaction

6. Physical Properties
Lower members are colourless gases

Insoluble in water

Soluble in organic solvents

Highly inflammable

7. Chemical Properties of Alkynes
(a) Combustion
Ethyne burns with sooty flame due to high carbon content.

2C₂H₂ + 5O₂ → 4CO₂ + 2H₂O

(b) Addition Reactions (Key Property)
Because of triple bond, alkynes undergo addition reactions.

1. Hydrogenation
C₂H₂ + H₂ → C₂H₄ → C₂H₆
(Nickel catalyst)

2. Halogenation
C₂H₂ + Br₂ → C₂H₂Br₂

🔹 Bromine water is decolourised

3. Hydrohalogenation
C₂H₂ + HCl → Vinyl chloride

8. Acidic Nature of Ethyne
Ethyne is weakly acidic due to presence of terminal hydrogen.

With Ammoniacal AgNO₃:

Forms white precipitate of silver acetylide

📌 This test is used to distinguish ethyne from ethene & ethane

9. Uses of Ethyne
Oxy-acetylene welding

Artificial ripening of fruits

Manufacture of plastics & synthetic rubber

10. Difference Based Questions (ICSE Favourite)
Property Alkane Alkene Alkyne
Saturation Saturated Unsaturated Unsaturated
Bond Single Double Triple
Formula CₙH₂ₙ₊₂ CₙH₂ₙ CₙH₂ₙ₋₂
Bromine water No effect Decolourises Decolour