4 Most SAQ’s of Organic Chemistry Some Basic Principles and Techniques Chapter in Inter 1st Year Chemistry (TS/AP)

4 Marks

SAQ-1 : Explain (a) Position isomerism (b) Functional group isomerism with one exam.

Introduction

Isomerism is a chemical concept wherein compounds with the same molecular formula exhibit different structural formulas. This means the atoms are connected in different ways or sequences. Two prevalent types of isomerism are position isomerism and functional group isomerism.

Position Isomerism

1. Definition: Position Isomerism occurs in compounds that have the same molecular formula but differ in the arrangement of functional groups, substituents, or double/triple bonds.
2. Example: The molecule C₄​H₈​ can exist in different forms based on the position of its double bond:
   • 1-butene: $$CH_2=CH-CH_2-CH_3$$
   • 2-butene: $$CH_3-CH=CH-CH_3$$

Functional Group Isomerism

1. Definition: Functional Group Isomerism is when compounds have the same molecular formula but different functional groups.
2. Example: The molecular formula C₂​H₆​O can represent two different compounds:
   • Ethyl Alcohol (Ethanol): $$C_2H_5OH$$
   • Dimethyl Ether: $$CH_3OCH_3$$

Summary

Isomerism showcases the diversity of chemical compounds. With the same set of atoms, molecules can have distinct structures and, consequently, different properties. Position isomerism focuses on different arrangements of functional groups within a molecule, while functional group isomerism centers on the variety of functional groups a molecule can possess. Recognizing these types of isomerism is crucial in understanding the structural versatility of organic compounds.

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SAQ-2 : How does ethylene react with following: a) Cl₂ b) HBr c) H₂SO₄ d) O₃

Introduction

Ethylene (H₂C = CH₂) is a simple hydrocarbon known for its reactivity with various chemicals. We’ll explore its reactions with chlorine, hydrogen bromide, sulfuric acid, and ozone.

1. Reaction with Cl₂ (Chlorine)
   • Process: Ethylene reacts with chlorine in the presence of CCl₄ to produce 1,2-dichloroethane.
   • Equation: H₂​C = CH₂​ + Cl₂​ (in CCl₄) → CH₂​Cl − CH₂​Cl
2. Reaction with HBr (Hydrogen Bromide)
   • Process: Ethylene reacts with hydrogen bromide to form ethyl bromide.
   • Equation: H₂​C = CH₂​ + HBr → CH₃​ − CH₂​ − Br
3. Reaction with H₂SO₄ (Sulfuric Acid)
   • Process: Ethylene interacts with cold concentrated sulfuric acid to produce ethyl hydrogen sulfate.
   • Equation: H₂​C = CH₂​ + H₂​SO₄​ → CH₃​ − CH₂​ − HSO₄​
4. Reaction with O₃ (Ozone)
   • Process: Ethylene reacts with ozone to produce ethylene ozonide. Upon hydrolysis in the presence of Zn and water, this compound transforms into formaldehyde.
   • Equation: H₂​C = CH₂​ + O₃​ → Ethylene Ozonide → (with Zn/H₂O) 2HCHO + H₂​O₂​

Summary

Ethylene exhibits diverse chemical behavior, as seen in its reactions with chlorine, hydrogen bromide, sulfuric acid, and ozone. These reactions produce a variety of products, ranging from halogenated compounds and esters to aldehydes, highlighting the versatile nature of ethylene in organic chemistry.

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SAQ-3 : What do you understand about geometrical isomerism? Explain the geometrical isomers of 2-butene. Draw the cis and trans isomers of (i) CHCl = CHCL (ii) C₂H₅ CCH₃ = CCH₃ C₂ H₅

Introduction

Geometrical isomerism, also known as cis-trans isomerism, is a form of stereoisomerism. This type of isomerism occurs due to restricted rotation around a double bond, leading to different spatial arrangements of groups or atoms around the double bond.

Understanding Geometrical Isomerism

1. It arises because of the restricted rotation around a carbon-carbon double bond (C=C).
2. The presence of different atoms or groups on either side of the C=C bond gives rise to distinct spatial arrangements.

2-Butene as an Example

1. 2-butene exhibits geometrical isomerism.
2. Cis-2-butene: Similar atoms or groups (methyl groups) are on the same side of the C=C bond.
3. Trans-2-butene: A methyl group and a hydrogen atom are on opposite sides of the double bond.

Drawing Geometrical Isomers

1. For CHCl=CHCl:
   • Cis-isomer: Both chlorine atoms are on the same side of the double bond.
   • Trans-isomer: The chlorine atoms are on opposite sides of the double bond.
2. For C₂H₅C(CH₃)=C(CH₃)C₂H₅:
   • Cis-isomer: Both ethyl (C₂H₅) groups are on the same side of the double bond.
   • Trans-isomer: The ethyl groups are on opposite sides of the double bond.

Summary

Geometrical isomerism provides insight into how different spatial arrangements of atoms or groups around a double bond can result in compounds with unique properties. 2-butene serves as a classic example of this phenomenon, showcasing both the cis and trans arrangements. This type of isomerism highlights the intricacies and nuances of molecular structures in organic chemistry.

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SAQ-4 : How is acetylene prepared from the following compounds:
a) Calcium carbide b) 1,2-dibromoethane

Introduction

Acetylene (C₂H₂), a simple alkyne, can be prepared using various methods. Two common methods involve starting from calcium carbide and 1,2-dibromoethane.

1. From Calcium Carbide
   • Process: The reaction of calcium carbide with water produces acetylene.
   • Reaction:
     • Equation: $$CaC_2 + 2H_2O \rightarrow C_2H_2 + Ca(OH)_2​$$
     • Description: Calcium carbide (CaC₂) reacts with water to produce acetylene and calcium hydroxide (Ca(OH)₂).
2. From 1,2-Dibromoethane
   • Process: Acetylene can also be prepared by dehalogenation of 1,2-dibromoethane using a strong base like potassium hydroxide.
   • Reaction:
     • Equation: $$C_2H_4Br_2 + 2KOH \rightarrow C_2H_2 + 2KBr + 2H_2O$$
     • Description: 1,2-dibromoethane reacts with potassium hydroxide (KOH) to yield acetylene, potassium bromide (KBr), and water.

Summary

The preparation of acetylene can be achieved through different methods, with calcium carbide and 1,2-dibromoethane being common starting materials. In both methods, acetylene is produced through a reaction involving water, highlighting the versatility of these compounds in synthetic organic chemistry.