Chapter 7: Fundamental Principles of Organic Chemistry – Chemistry 2nd Year Notes

This chapter covers the Fundamental Principles of Organic Chemistry from the 2nd Year (FSc Part-II) Chemistry syllabus of the Punjab Curriculum and Textbook Board (PTB/PCTB). Organic chemistry is the chemistry of carbon compounds. Carbon is unique because it can bond to itself in long chains and rings (catenation) and form single, double and triple bonds, which is why there are millions of organic compounds.

This chapter lays the foundation for the organic section: where organic compounds come from, how they are classified, functional groups and homologous series, hybridization of carbon, isomerism, and the main types of organic reaction.

Learning Objectives

• Define organic chemistry and explain the special nature of carbon.
• State the main sources of organic compounds.
• Classify organic compounds into aliphatic, alicyclic and aromatic types.
• Define functional group and homologous series with examples.
• Explain sp3, sp2 and sp hybridization of carbon.
• Explain structural isomerism and identify the main types of organic reactions.

Key Concepts

Why Carbon Is Special

Carbon has four valence electrons, so it forms four covalent bonds. It shows catenation (linking with other carbon atoms into chains and rings) to a greater extent than any other element, and it can form multiple bonds. Because carbon to carbon bonds are strong, the resulting molecules are stable, allowing an almost unlimited variety of compounds.

Sources and Classification

The main natural sources are the fossil fuels: coal, petroleum and natural gas, together with plants and animals. Organic compounds are broadly divided into open-chain (aliphatic) and closed-chain (cyclic) compounds. Cyclic compounds are further divided into alicyclic and aromatic (those containing the benzene ring). Aliphatic compounds may be saturated (only single bonds, the alkanes) or unsaturated (containing double or triple bonds, the alkenes and alkynes).

Functional Groups and Homologous Series

A functional group is the atom or group of atoms that gives a molecule its characteristic chemical properties, for example the hydroxyl group makes a compound an alcohol. A homologous series is a family of compounds with the same functional group and general formula, in which each member differs from the next by a CH2 unit; members have similar chemical properties and a gradual change in physical properties. The general formulas are alkane CnH2n+2, alkene CnH2n and alkyne CnH2n-2.

Hybridization of Carbon

In sp3 hybridization (as in methane) carbon forms four single bonds arranged tetrahedrally at 109.5 degrees. In sp2 hybridization (as in ethene) carbon forms three sigma bonds in a plane at 120 degrees plus one pi bond (a double bond). In sp hybridization (as in ethyne) carbon forms two sigma bonds at 180 degrees plus two pi bonds (a triple bond).

Isomerism and Types of Reactions

Isomers have the same molecular formula but different structures; in structural isomerism the atoms are joined in a different order, for example n-butane and isobutane both have the formula C4H10. The three main reaction types are addition (atoms add across a multiple bond, typical of unsaturated compounds), substitution (one atom or group replaces another, typical of saturated compounds) and elimination (a small molecule is removed to create a double bond).

Important Definitions

Formulas & Rules

Item	Fact
General formulas	alkane CnH2n+2, alkene CnH2n, alkyne CnH2n-2
sp3 / sp2 / sp	109.5 single / 120 double / 180 triple
Reaction types	addition (unsaturated), substitution (saturated), elimination (forms double bond)
Homologues	differ by one CH2 unit and share chemical properties

Diagrams & Illustrations

Classification of organic compounds: a tree diagram dividing organic compounds into open-chain (aliphatic, saturated and unsaturated) and closed-chain (alicyclic and aromatic) types.

Hybridization of carbon: three shapes showing sp3 (tetrahedral, 109.5 degrees, methane), sp2 (planar, 120 degrees, ethene) and sp (linear, 180 degrees, ethyne).

Common functional groups: a chart listing alkene, alkyne, alcohol, aldehyde, ketone, carboxylic acid, amine, ester and halide with their group symbols.

Solved Examples & Numericals

Example 1: Hybridization

State the hybridization of carbon in ethyne. Each carbon in ethyne is sp hybridized, forming two sigma bonds (linear, 180 degrees) and a triple bond that includes two pi bonds.

Example 2: Isomerism

Describe the structural isomers of C4H10. There are two: n-butane (a straight chain) and isobutane or 2-methylpropane (a branched chain). Same formula, different structure.

Example 3: Reaction type

Classify the reaction of methane with chlorine giving chloromethane and hydrogen chloride. It is a substitution reaction, because a hydrogen atom of methane is replaced by a chlorine atom.

Short Questions & Answers

Long Questions & Answers

Q1: Explain the classification of organic compounds.

Organic compounds are first divided, according to the shape of their carbon skeleton, into open-chain (aliphatic) and closed-chain (cyclic) compounds. Open-chain compounds have carbon atoms joined in straight or branched chains; they may be saturated with only single bonds (the alkanes) or unsaturated with double or triple bonds (the alkenes and alkynes). Closed-chain compounds have their carbon atoms in a ring and are further divided into alicyclic compounds, whose rings behave like aliphatic chains, and aromatic compounds, which contain the specially stable benzene ring. This classification, with the idea of functional groups, allows the millions of organic compounds to be studied in an orderly way.

Q2: Describe the hybridization of carbon with examples.

Hybridization is the mixing of a carbon atom one 2s and some 2p orbitals to give equivalent hybrid orbitals. In sp3 hybridization the 2s mixes with all three 2p orbitals to give four sp3 orbitals pointing to the corners of a tetrahedron at 109.5 degrees; carbon then forms four single bonds, as in methane. In sp2 hybridization the 2s mixes with two 2p orbitals to give three sp2 orbitals in a plane at 120 degrees, while the leftover p-orbital forms a pi bond, giving a double bond as in ethene. In sp hybridization the 2s mixes with one 2p orbital to give two sp orbitals at 180 degrees, while two leftover p-orbitals form two pi bonds, giving a triple bond as in ethyne. Thus the type of hybridization decides the shape of the molecule and the number of multiple bonds.

Q3: Distinguish between addition, substitution and elimination reactions.

In an addition reaction, atoms or groups are added across a carbon to carbon double or triple bond, so an unsaturated compound becomes more saturated; for example ethene reacts with hydrogen to give ethane. In a substitution reaction, an atom or group in a molecule is replaced by another; saturated compounds such as alkanes undergo this, as when methane reacts with chlorine to give chloromethane and hydrogen chloride. In an elimination reaction, a small molecule such as water or a hydrogen halide is removed to create a double bond, for example the dehydration of ethanol to ethene. Recognising the class helps predict the products.

MCQs with Answers

Quick Revision Summary

• Carbon: catenation plus multiple bonds give millions of compounds.
• Sources: coal, petroleum, natural gas.
• Classification: aliphatic (open) versus cyclic (alicyclic, aromatic); saturated versus unsaturated.
• Functional group decides properties; homologous series differ by CH2.
• Hybridization: sp3 (109.5, single), sp2 (120, double), sp (180, triple).
• Reactions: addition (unsaturated), substitution (saturated), elimination (forms double bond).

Exam Tips

• Learn the three hybridizations with example, shape and angle; a guaranteed question.
• Memorise general formulas: alkane CnH2n+2, alkene CnH2n, alkyne CnH2n-2.
• Match reaction type to compound: unsaturated to addition, saturated to substitution.
• Give a clear example for functional group and homologous series definitions.
• Practise drawing structural isomers of C4H10 and C5H12.