Chapter Summary
Chemical reactions involving carbon compounds are called organic reactions. These reactions are fundamental to creating many substances we use daily, including drugs, polymers, fuels, plastics, paints, and clothing materials. The major types of organic reactions include substitution reactions, addition reactions, combustion, thermal cracking, and polymerization.
Substitution reactions involve replacing one atom or group with another in a compound. Addition reactions occur when unsaturated compounds with double or triple bonds combine with other molecules to form saturated compounds. Polymerization is the process where simple molecules (monomers) join together to form large complex molecules (polymers). Thermal cracking breaks down large hydrocarbon molecules into smaller ones when heated without air. Combustion reactions occur when hydrocarbons burn in oxygen to produce carbon dioxide and water with heat and light.
Important organic compounds discussed include methanol, ethanol, ethanoic acid, and esters. These compounds have significant industrial applications and are used in manufacturing various products. Understanding these reactions and compounds is essential for comprehending modern chemistry and its applications in daily life.
Questions and Answers
1. What are organic reactions? Give examples of their applications in daily life.
Answer: Organic reactions are chemical reactions involving carbon compounds. These reactions are responsible for creating many substances we use in our daily lives. Examples include drugs for medical treatment, polymers for making plastics, fuels for energy, paints for coloring surfaces, and synthetic fibers for clothing. The contribution of organic chemistry through these reactions has revolutionized modern life by providing essential materials and products.
2. Define substitution reactions with an example.
Answer: Substitution reactions are chemical reactions in which an atom or group of atoms is replaced with another atom or group of atoms in a compound. A classic example is the reaction of methane with chlorine in the presence of sunlight: CH₄ + Cl₂ → CH₃Cl + HCl. In this reaction, one hydrogen atom in methane is replaced by a chlorine atom, forming chloromethane (methyl chloride) and hydrogen chloride.
3. What happens when methane undergoes successive substitution with chlorine?
Answer: When methane undergoes successive substitution with chlorine in the presence of sunlight, multiple products are formed:
- Step 1: CH₄ + Cl₂ → CH₃Cl (chloromethane) + HCl
- Step 2: CH₃Cl + Cl₂ → CH₂Cl₂ (dichloromethane) + HCl
- Step 3: CH₂Cl₂ + Cl₂ → CHCl₃ (trichloromethane/chloroform) + HCl
- Step 4: CHCl₃ + Cl₂ → CCl₄ (tetrachloromethane/carbon tetrachloride) + HCl
Each step replaces one more hydrogen atom with chlorine until all hydrogens are substituted.
4. Explain addition reactions with suitable examples.
Answer: Addition reactions are reactions in which unsaturated organic compounds with double or triple bonds combine with certain molecules to form saturated compounds. For example:
- Ethene + Hydrogen: CH₂=CH₂ + H₂ → CH₃-CH₃ (in the presence of nickel catalyst at high temperature)
- Ethene + Chlorine: CH₂=CH₂ + Cl₂ → CH₂Cl-CH₂Cl
- Ethyne + Hydrogen: HC≡CH + H₂ → CH₂=CH₂
In these reactions, the double or triple bonds are broken and new atoms are added to form saturated compounds.
5. What is polymerization? Distinguish between addition and condensation polymerization.
Answer: Polymerization is the process by which simple molecules (monomers) join together to form large complex molecules (polymers).
Addition Polymerization: This involves repeated addition reactions of monomers without the removal of any small molecules. Example: nCH₂=CH₂ → [-CH₂-CH₂-]ₙ (ethene to polyethene). No small molecules are eliminated during this process.
Condensation Polymerization: This involves different monomers combining together to form larger compounds with the removal of small molecules like water. Example: Nylon 66 is formed from adipic acid and hexamethylenediamine with the removal of water molecules.
6. Give examples of addition polymers and their uses.
Answer: Common addition polymers include:
- Polyethene (Polythene): From ethene monomer, used for manufacturing tarpaulin sheets and carry bags
- Polyvinyl Chloride (PVC): From vinyl chloride monomer, used for manufacturing pipes, plastic furniture, and coating electric conductors
- Teflon: From tetrafluoroethene monomer, used for coating the inner surface of non-stick cookware
- Natural Rubber: From isoprene monomer, used for manufacturing tires
- Orlon: From acrylonitrile monomer, used for manufacturing synthetic fibers
7. What is thermal cracking? Give examples.
Answer: Thermal cracking is a process where hydrocarbons with high molecular weight decompose into hydrocarbons with lower molecular weight when heated in the absence of air. Examples include:
- Propane cracking: CH₃-CH₂-CH₃ → CH₄ + CH₂=CH₂
- Butane cracking: CH₃-CH₂-CH₂-CH₃ → CH₃-CH=CH₂ + CH₄
- Pentane cracking: CH₃-CH₂-CH₂-CH₂-CH₃ → CH₃-CH₂-CH₃ + CH₂=CH₂
This process produces both saturated and unsaturated hydrocarbons and is useful for controlling pollution by breaking down plastic wastes.
8. Describe the combustion of hydrocarbons with examples.
Answer: Combustion is the process where hydrocarbons burn in oxygen to form carbon dioxide and water along with heat and light. All hydrocarbons give the same products when they burn completely. Examples:
- Methane combustion: CH₄ + 2O₂ → CO₂ + 2H₂O + Heat
- Butane combustion: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O + Heat
The combustion of hydrocarbons is an important source of energy and is widely used in heating, cooking, and power generation.
9. How is methanol prepared industrially? What are its uses?
Answer: Methanol is industrially produced by treating carbon monoxide with hydrogen in the presence of catalysts at 573 K temperature: CO + 2H₂ → CH₃-OH (catalyst, 573 K)
Uses of Methanol:
- Manufacturing varnish and paint
- Manufacturing formic acid and formaldehyde
- Production of various organic compounds
- As a solvent in industrial processes
Note: Methanol is a poisonous substance and must be handled carefully.
10. Explain the industrial preparation of ethanol from molasses.
Answer: Ethanol is manufactured by the fermentation of molasses using yeast. The process involves two enzymatic steps:
Step 1: The enzyme invertase present in yeast converts sugar solution to glucose and fructose: C₁₂H₂₂O₁₁ + H₂O → C₆H₁₂O₆ + C₆H₁₂O₆ (invertase)
Step 2: The enzyme zymase converts glucose and fructose into ethanol: C₆H₁₂O₆ → 2C₂H₅OH + 2CO₂ (zymase)
The 8-10% ethanol obtained is called 'wash'. When subjected to fractional distillation, 95.6% ethanol (rectified spirit) is obtained. 100% ethanol is called absolute alcohol.
11. What is denatured spirit and methylated spirit?
Answer: Denatured Spirit: When toxic substances like methanol, pyridine, or rubber distillate are added to ethanol to prevent its misuse as a beverage, the resulting ethanol is called denatured spirit. This makes ethanol unfit for drinking while maintaining its industrial utility.
Methylated Spirit: When denaturing is done specifically with methanol, the resulting ethanol is called methylated spirit. This is a common form of denatured alcohol used for industrial and household purposes.
12. List the uses of ethanol.
Answer: Ethanol has numerous industrial and commercial uses:
- Production of power alcohol (20% absolute alcohol + 80% petrol) used as vehicle fuel
- As a solvent for medicines and pharmaceutical preparations
- Manufacturing paints and varnishes
- As preservatives in various products
- Production of other organic compounds
- In the manufacture of perfumes and cosmetics
13. How is ethanoic acid prepared industrially? What are its uses?
Answer: Industrial Preparation: Ethanoic acid is prepared industrially by treating methanol with carbon monoxide in the presence of a catalyst: CH₃-OH + CO → CH₃-COOH (catalyst)
Alternative Method: Fermentation of ethanol with acetobacter bacteria in the presence of air yields 5-8% ethanoic acid, known as vinegar.
Uses of Ethanoic Acid:
- Manufacture of vinegar for food purposes
- Production of acetic anhydride and acetate esters
- Manufacturing synthetic fibers
- As a solvent for polymers and resins
- Manufacture of disinfectants and medicines
14. What are esters? How are they formed?
Answer: Esters are organic compounds formed when alcohols react with carboxylic acids. This reaction is called esterification. The general formula of esters is R-COO-R₁, where R and R₁ are alkyl groups.
Formation Example: CH₃-COOH + HO-CH₂-CH₃ → CH₃-COO-CH₂-CH₃ + H₂O (Ethanoic acid + Ethanol → Ethyl ethanoate + Water)
This reaction occurs in the presence of concentrated sulfuric acid as a catalyst. Esters are characterized by their pleasant fragrances resembling flowers and fruits, making them useful in artificial perfumes and food flavoring.
15. Give examples of esters and their fragrances.
Answer: Different esters produce characteristic fragrances:
- Isoamyl acetate: Formed from ethanoic acid and isoamyl alcohol, gives banana fragrance
- Benzyl ethanoate: Formed from ethanoic acid and benzyl alcohol, gives jasmine fragrance
- Octyl ethanoate: Formed from ethanoic acid and octyl alcohol, gives orange fragrance
- Ethyl butanoate: Formed from butanoic acid and ethyl alcohol, gives pineapple fragrance
These esters are widely used in the food and cosmetic industries to create artificial flavors and fragrances.
16. What are the different categories of medicines mentioned? Give their functions and examples.
Answer: The main categories of medicines include:
Analgesics: These medicines relieve pain. Example: Aspirin, which is commonly used for headaches and body pain.
Antipyretics: These medicines reduce body temperature during fever. Example: Paracetamol, which is effective in bringing down high fever.
Antiseptics: These medicines control microorganisms on living tissues. Example: Dettol, used for cleaning wounds and preventing infection.
Antibiotics: These medicines destroy infectious microorganisms and prevent their growth. Example: Penicillin, used to treat bacterial infections.
17. Describe the properties and uses of paracetamol and aspirin.
Answer: Paracetamol (N-acetyl-p-amino phenol):
- Functions both as an antipyretic (reduces fever) and analgesic (relieves pain)
- Has comparatively fewer side effects than other pain relievers
- Higher consumption levels can adversely affect the liver
- Included in the World Health Organization's list of essential medicines
Aspirin (Acetyl salicylic acid):
- Primarily used as an analgesic for pain relief
- Has anti-blood coagulant properties, making it useful for preventing heart attacks
- Helps in reducing the risk of blood clot formation
- Commonly used for treating headaches and reducing inflammation
18. What is methyl salicylate? What are its properties and uses?
Answer: Methyl salicylate is a methyl ester of the aromatic carboxylic acid, salicylic acid. It is also known as oil of wintergreen because it is extracted from certain species of evergreen plants that grow in winter.
Properties and Uses:
- Relieves pain in joints and muscles, making it useful for treating arthritis and muscle soreness
- Has a characteristic wintergreen fragrance
- Used as a flavoring agent in foods and medicines
- Can be prepared both by extraction from natural sources and through synthetic methods
- Applied topically for pain relief in various pharmaceutical preparations
19. Complete the combustion equation: 2C₄H₁₀ + 13O₂ → ? + ? + Heat
Answer: 2C₄H₁₀ + 13O₂ → 8CO₂ + 10H₂O + Heat
In this combustion reaction of butane, the hydrocarbon combines with oxygen to produce carbon dioxide and water along with the release of heat and light energy. This follows the general pattern of hydrocarbon combustion where the products are always CO₂ and H₂O.
20. What happens when ethyne undergoes addition reaction with hydrogen?
Answer: When ethyne (acetylene) undergoes addition reaction with hydrogen in the presence of a nickel catalyst, it can undergo partial or complete addition:
Partial Addition: HC≡CH + H₂ → CH₂=CH₂ (ethyne to ethene) Complete Addition: HC≡CH + 2H₂ → CH₃-CH₃ (ethyne to ethane)
The triple bond in ethyne can add one molecule of hydrogen to form ethene (with a double bond) or two molecules of hydrogen to form ethane (completely saturated). The reaction conditions and catalyst determine whether partial or complete addition occurs.