Organic Chemistry in Chemistry Class 11 Notes: Overview and Important Reactions

Organic Chemistry is the study of organic compounds found in animals, plants, and living organisms. Essentially, it is the study of hydrocarbons that can be found in fuels, DNA, drugs, and other substances.

Why Study Organic Chemistry?

Organic chemistry helps us understand biological processes and develop new materials and medicines. Organic compounds are essential for life on Earth. 

Examples of Organic Chemistry:

• DNA and proteins in our bodies
• Materials like clothing, fuels, and polymers
• Medicines and dyes we use daily
• Food components that sustain us
• Methane, ethanol, glucose, etc.

Carbon can form four covalent bonds with other atoms; this property is called tetravalence, which is explained by carbon’s electronic configuration and orbital hybridization. 

Hybridization affects both physical and chemical properties of molecules.

The Shapes of Carbon Compounds

Carbon forms different types of bonds and creates different molecular shapes, such as:

1. sp³ Hybridization (Tetrahedral)

Example-Methane (CH₄), Bond angles: 109.5°, four single bonds.

2. sp² Hybridization (Trigonal Planar)

Example: Ethene (C₂H₄), Bond angles: 120°, Three sigma bonds + one pi bond

3. sp Hybridization (Linear)

Example: Ethyne (C₂H₂), Bond angles: 180°, Two sigma bonds + two pi bonds

Also read: Tetravalence of Carbon

Characteristics of π (Pi) Bonds

Pi bonds have the following features:

• Formed by the sideways overlap of p orbitals
• All atoms must be in the same plane
• Restrict rotation around the bond
• Create reactive centers in molecules
• Electrons are easily available for reactions

Example: In ethene (H₂C=CH₂), rotation around the C=C bond is restricted due to the pi bond.

Organic compounds can be represented in several ways. The structural representation of organic compounds describes how each of them has different purposes. 

Types of Structural Formulas

1. Complete Structural Formulas

Show all atoms and bonds explicitly.

Example:

H   H

|   |

H-C-C-H  (Ethane)

|   |

H   H

2. Condensed Structural Formulas

A condensed structural formula requires you to remove some bonds and group similar atoms together.

Examples, Ethane (CH₃CH₃), Ethanol (CH₃CH₂OH), Octane (CH₃(CH₂)₆CH₃)

3. Bond-line Structural Formulas

In the bond-line structural formulas, we use lines to represent carbon-carbon bonds in a zig-zag pattern. Carbon and hydrogen atoms are not shown but line junctions represent carbon atoms and terminals represent methyl groups. Only heteroatoms (O, N, S, halogens) are written.

Example: 3-Methyloctane

Based on the structure and functional groups, organic compounds have been classified into two categories: Acyclic compounds and Cyclic compounds.

1. Acyclic Compounds (Open-Chain)

Acyclic compounds, also known as aliphatic compounds, have straight or branched chains.

Examples: Methane CH₄, Butane CH₃CH₂CH₂CH₃, Isobutane (CH₃)₃CH

2. Cyclic Compounds (Closed Chain)

Cyclic compounds contain atoms arranged in ring structures. They are of two types: Alicyclic compounds and Aromatic compounds.

(a) Alicyclic Compounds- Compounds that contain only carbon atoms in rings show properties similar to aliphatic compounds

Examples:

Cyclopropane: C₃H₆ (3-membered ring)

Cyclohexane: C₆H₁₂ (6-membered ring)

(b) Aromatic Compounds- These are special ring compounds with unique stability, including benzene and related compounds.

Examples:

Benzene: C₆H₆

Naphthalene: C₁₀H₈

Read more: Classification of Organic Compounds

Functional Groups

An atom or a group of atoms that determines the chemical properties of organic compounds is called a functional group.

Common functional groups:

• Hydroxyl: -OH (alcohols)
• Carbonyl: >C=O (ketones, aldehydes)
• Carboxyl: -COOH (carboxylic acids)
• Amino: -NH₂ (amines)

Homologous Series

A homologous series is a group of compounds with the same functional group and the same general formula. Successive members differing by a CH₂ unit have similar chemical properties.

For example- Methane: CH₄, Ethane: C₂H₆, Propane: C₃H₈ ,Butane: C₄H₁₀

Naming compounds systematically helps identify them easily. The IUPAC (International Union of Pure and Applied Chemistry) system provides standardized rules.

Common names are traditional names based on origin or properties:

For example- Formic acid (from ants - formica in Latin), Acetic acid (from vinegar - acetum in Latin)

IUPAC names are systematic and structure-based, allowing anyone to determine the structure from the name.

Also read: Nomenclature: Common names vs IUPAC names

A phenomenon where compounds have the same molecular formula but different structures, which leads to different properties, is called Isomerism. There are different types of Isomerism:

Structural Isomerism

Compounds with the same molecular formula but different structural arrangements are structural isomerism.

1. Chain Isomerism- Various carbon skeleton structures. Example: C₅H₁₂

Pentane: CH₃CH₂CH₂CH₂CH₃, Isopentane: (CH₃)₂CHCH₂CH₃

2. Position Isomerism- Alternative positions of functional groups or substituents. Example: C₃H₈O

Propan-1-ol: CH₃CH₂CH₂OH, Propan-2-ol: CH₃CH(OH)CH₃

3. Functional Group Isomerism- Alternative functional groups with the same molecular formula. Example: C₃H₆O

Propanal: CH₃CH₂CHO (aldehyde), Propanone: CH₃COCH₃ (ketone)

4. Metamerism- Alternative alkyl groups surrounding a functional group. Example: C₄H₁₀O

Methoxypropane: CH₃OC₃H₇, Ethoxyethane: C₂H₅OC₂H₅

Stereoisomerism

Compounds with the same structure but different three-dimensional arrangements. There are two types:

1. Geometrical Isomerism- Various structures around double bonds or rings.

2. Optical Isomerism- Various structures around asymmetric carbon atoms.

These are the fundamental concepts of how organic reactions help predict products and design synthetic pathways.

Fission of Covalent Bonds

Covalent bonds can break in two ways: Heterolytic cleavage and Homolytic cleavage.

1. Heterolytic Cleavage

The bond breaks such that both electrons go to one fragment, which results in a Carbocation and a Carbanion.

Carbocation (C⁺): Carbon with a positive charge and six electrons, Carbanion (C⁻): Carbon with a negative charge and eight electrons.

2. Homolytic Cleavage

Each fragment gets one electron from the bond, which results in free radicals, neutral species with unpaired electrons. Very reactive and short-lived.

Nucleophiles and Electrophiles

Nucleophiles (Nu:)

Nucleophiles are “Nucleus-loving” species that are electron-rich, donate electron pairs, and attack electron-deficient centers.

Examples: OH⁻, CN⁻, NH₃, H₂O

Electrophiles (E⁺)

Electrophiles are “Electron-loving” species that are electron-deficient, accept electron pairs, and attack electron-rich centers.

Examples: H⁺, BF₃, carbocations, carbonyl carbon

Electronic Effects

1. Inductive Effect- due to the inductive effect, electrons get permanently displaced along sigma bonds due to electronegativity differences.
2. Resonance Effect- Electron displacement through pi bonds or lone pairs.
3. Hyperconjugation- Delocalization of sigma electrons with an empty orbital.
4. Electromeric Effect- Temporary electron displacement in multiple bonds when reagents approach.

Types of Organic Reactions

There are four types of organic reaction:

• Substitution Reaction: One group replaces another
• Addition Reaction: New groups add across multiple bonds
• Elimination Reaction: Groups are removed to form multiple bonds
• Rearrangement Reaction: Atoms reorganize within the molecule