Master the core concepts of organic chemistry with this detailed guide to solved exercises from the ‘Fundamental Principles of Organic Chemistry’ chapter. This resource covers key topics such as bonding, hybridization, isomerism, functional groups, and reaction mechanisms. Aligned with the latest syllabus for Lahore Board, Federal Board, and other academic boards, it includes step-by-step solutions, solved MCQs, short questions, and conceptual problems to reinforce learning. Ideal for students aiming to excel in organic chemistry, this guide simplifies complex principles and enhances exam preparation.
Q4. How organic compounds are classified? Give a suitable example of each type.
Organic compounds are classified based on their structure, functional groups, and bonding into the following major categories:
- Acyclic or Open-Chain Compounds: These are compounds with straight or branched chains.
- Example: Butane (C₄H₁₀)
- Cyclic Compounds: These compounds have atoms arranged in a ring structure.
- Example: Cyclohexane (C₆H₁₂)
- Aromatic Compounds: Compounds containing one or more benzene rings (arenes).
- Example: Benzene (C₆H₆)
- Heterocyclic Compounds: Cyclic compounds where one or more atoms in the ring are not carbon.
- Example: Pyridine (C₅H₅N)
Q5. What are homocyclic and heterocyclic compounds? Give one example of each.
- Homocyclic Compounds: Compounds whose rings are made up entirely of carbon atoms.
- Example: Benzene (C₆H₆)
- Heterocyclic Compounds: Compounds that contain at least one atom other than carbon in the ring structure.
- Example: Pyridine (C₅H₅N) (contains nitrogen in the ring)
Q6. Write the structural formulas of the two possible isomers of C₄H₁₀.
The two isomers of C₄H₁₀ are:
- n-Butane (Straight-chain isomer):
Structure: CH₃-CH₂-CH₂-CH₃ - Iso-Butane (Branched-chain isomer):
Structure: (CH₃)₃CH
Q7. Why is ethene an important industrial chemical?
Ethene (ethylene) is crucial in the chemical industry because:
- It is used as a raw material for producing polymers such as polyethylene, the most widely used plastic.
- It is involved in the production of other chemicals such as ethanol, ethylene oxide, and ethylene glycol, which are used in manufacturing antifreeze, detergents, and solvents.
- Ethene is also used as a plant hormone to stimulate fruit ripening.
Q8. What is meant by a functional group? Name typical functional groups containing oxygen.
A functional group is a specific group of atoms within a molecule responsible for the characteristic chemical reactions of that molecule. Typical oxygen-containing functional groups include:
- Hydroxyl group (-OH): Found in alcohols (e.g., ethanol)
- Carbonyl group (C=O): Found in aldehydes and ketones (e.g., formaldehyde)
- Carboxyl group (-COOH): Found in carboxylic acids (e.g., acetic acid)
- Ether group (R-O-R’): Found in ethers (e.g., diethyl ether)
Q9. What is an organic compound? Explain the importance of Wöhler’s work in the development of organic chemistry.
An organic compound is a chemical compound containing carbon atoms, usually bonded to hydrogen, oxygen, and/or other elements. Organic compounds are the basis of life and include molecules such as carbohydrates, proteins, and fats.
Wöhler’s work was groundbreaking because he synthesized urea (an organic compound) from ammonium cyanate (an inorganic compound) in 1828. This demonstrated for the first time that organic compounds could be synthesized from inorganic substances, disproving the belief that organic compounds could only be produced by living organisms, leading to the rise of modern organic chemistry.
Q10. Write a short note on cracking of hydrocarbons.
Cracking is a process in which large hydrocarbon molecules (usually alkanes) are broken down into smaller, more useful molecules, often by applying heat and pressure. This process is crucial in the petroleum industry to convert long-chain hydrocarbons into gasoline, diesel, and other products. There are two main types of cracking:
- Thermal Cracking: High temperature and pressure are used to break the bonds.
- Catalytic Cracking: A catalyst is used to lower the temperature and pressure needed for the process.
Q11. Explain reforming of petroleum with the help of a suitable example.
Reforming is a chemical process used to convert low-octane hydrocarbons into high-octane gasoline components. This process improves the quality of gasoline by rearranging the molecular structure of hydrocarbons.
- Example: In naphtha reforming, straight-chain alkanes are converted into branched-chain alkanes, cycloalkanes, and aromatic hydrocarbons. For instance, heptane (C₇H₁₆) can be converted into methylcyclohexane or toluene, which have higher octane ratings, improving fuel efficiency.
Q12. Describe important sources of organic compounds.
Important sources of organic compounds include:
- Petroleum: The largest source, used for producing fuels, plastics, and chemicals.
- Natural Gas: Contains methane and is used as a source for organic synthesis.
- Coal: A source of hydrocarbons, aromatic compounds, and various other organics.
- Plants and Animals: Provide carbohydrates, proteins, fats, and other biochemicals used in medicine, food, and textiles.
Q13. What is orbital hybridization? Explain sp³, sp², and sp modes of hybridization of carbon.
Orbital hybridization is the mixing of atomic orbitals in an atom to form new hybrid orbitals that influence molecular geometry and bonding properties.
- sp³ Hybridization: Involves the mixing of one s and three p orbitals. The geometry is tetrahedral with bond angles of 109.5°.
- Example: Methane (CH₄)
- sp² Hybridization: Involves the mixing of one s and two p orbitals. The geometry is trigonal planar with bond angles of 120°.
- Example: Ethene (C₂H₄)
- sp Hybridization: Involves the mixing of one s and one p orbital. The geometry is linear with bond angles of 180°.
- Example: Ethyne (C₂H₂)
Q14. Explain the type of bonds and shapes of the following molecules using hybridization approach.
- CH₃-CH₂-CH₂-CH₃ (Butane):
- Hybridization: sp³ for each carbon atom
- Shape: Tetrahedral around each carbon
- CH=CH₂ (Ethene):
- Hybridization: sp² for each carbon
- Shape: Trigonal planar
- CHCl (Chloromethane):
- Hybridization: sp³ for the carbon
- Shape: Tetrahedral around the carbon
- HCHO (Formaldehyde):
- Hybridization: sp² for carbon
- Shape: Trigonal planar
Q15. Why is there no free rotation around a double bond and free rotation around a single bond? Discuss cis-trans isomerism.
In a double bond, one of the bonds is a pi bond (π) that restricts rotation because breaking this bond requires a significant amount of energy. This is unlike a single bond, which is a sigma bond (σ) that allows free rotation because of the symmetric overlap of orbitals along the bond axis.
Cis-trans isomerism occurs due to the restricted rotation around double bonds, resulting in different spatial arrangements of groups attached to the carbon atoms involved in the double bond. In cis-isomers, similar groups are on the same side of the double bond, while in trans-isomers, they are on opposite sides.
