Introduction to Organic Chemistry

Welcome to this comprehensive guide on Organic Chemistry for CXC Chemistry examination 2024-2025. This lesson covers fundamental concepts, structural representations, functional groups, and key organic reactions.

1. What is Organic Chemistry?

Organic chemistry is the study of carbon-containing compounds and their properties. Carbon's unique ability to form four covalent bonds, combined with its capacity to form stable bonds with itself (catenation) and other elements, has led to over 10 million known organic compounds.

Definition: Organic chemistry is the branch of chemistry that studies the structure, properties, composition, reactions, and preparation of carbon-containing compounds.

1.1 Historical Development

1.2 Why Carbon is Special

Carbon's unique properties make it central to organic chemistry:

H H | | H—C—C—H Example of Carbon-Carbon bonding | | (Ethane molecule) H H

2. Hydrocarbons - The Simplest Organic Compounds

Hydrocarbons are organic compounds composed only of carbon and hydrogen atoms. They form the basis of organic chemistry and are classified based on their structures.

2.1 Types of Hydrocarbons

Type Characteristics Example
Alkanes Single bonds only (C-C), saturated hydrocarbons, CnH2n+2 Methane (CH4), Ethane (C2H6)
Alkenes Contains at least one C=C double bond, unsaturated, CnH2n Ethene (C2H4), Propene (C3H6)
Alkynes Contains at least one C≡C triple bond, unsaturated, CnH2n-2 Ethyne (C2H2), Propyne (C3H4)
Cycloalkanes Closed ring of carbon atoms with single bonds, CnH2n Cyclohexane (C6H12)
Aromatic Contains benzene rings or similar structures Benzene (C6H6), Toluene (C7H8)

2.2 Alkanes - The Simplest Hydrocarbons

Alkanes are the simplest hydrocarbons, containing only single bonds between carbon atoms.

H | H—C—H Methane (CH4) | H
H H | | H—C—C—H Ethane (C2H6) | | H H

2.2.1 Homologous Series of Alkanes

Name Formula Condensed Formula
Methane CH4 CH4
Ethane C2H6 CH3CH3
Propane C3H8 CH3CH2CH3
Butane C4H10 CH3(CH2)2CH3
Pentane C5H12 CH3(CH2)3CH3
Hexane C6H14 CH3(CH2)4CH3
Important: For CXC examinations, you should be familiar with the names, formulae, and properties of the first 10 alkanes.

2.3 Alkenes - Unsaturated Hydrocarbons

Alkenes contain at least one carbon-carbon double bond (C=C).

H H | | H-C=C-H Ethene (C2H4)

2.4 Alkynes - Triple Bond Hydrocarbons

Alkynes contain at least one carbon-carbon triple bond (C≡C).

H-C≡C-H Ethyne (C2H2)

3. Representations of Organic Compounds

Organic chemists use various methods to represent molecular structures:

3.1 Molecular Formula

Indicates the actual number of each type of atom in a molecule.

Example: C2H6O (ethanol)

3.2 Structural Formula

Shows how atoms are bonded to each other.

H H | | H—C—C—O—H Ethanol (C2H6O) | | H H

3.3 Condensed Structural Formula

A simplified version of the structural formula.

Example: CH3CH2OH (ethanol)

3.4 Displayed Formula

Shows all atoms and bonds in a molecule.

3.5 Skeletal Formula

Carbon atoms are not shown explicitly but are understood to be at the vertices and ends of lines. Hydrogen atoms attached to carbon are not shown.

O | \/\ Cyclohexanol (skeletal representation) | OH

4. Functional Groups in Organic Chemistry

Functional groups are specific atoms or groups of atoms that are responsible for the characteristic chemical reactions of organic compounds.

Functional Group Structure Example Class of Compound
Hydroxyl -OH CH3CH2OH (Ethanol) Alcohols
Carbonyl C=O CH3CHO (Ethanal) Aldehydes/Ketones
Carboxyl -COOH CH3COOH (Ethanoic acid) Carboxylic acids
Ester -COO- CH3COOCH3 (Methyl ethanoate) Esters
Amino -NH2 CH3NH2 (Methylamine) Amines
Amide -CONH2 CH3CONH2 (Ethanamide) Amides
Halo -X (F, Cl, Br, I) CH3Cl (Chloromethane) Halogenoalkanes

4.1 Alcohols (-OH)

Alcohols contain the hydroxyl (-OH) functional group attached to a carbon atom.

4.2 Aldehydes and Ketones (C=O)

Both contain the carbonyl group (C=O), but in aldehydes, the carbonyl is at the end of the carbon chain, while in ketones, it's within the chain.

4.3 Carboxylic Acids (-COOH)

Contain the carboxyl group (-COOH).

5. Isomerism in Organic Chemistry

Isomers are compounds with the same molecular formula but different structures or arrangements of atoms.

5.1 Structural Isomerism

Occurs when compounds have the same molecular formula but different structural formulas.

H H H | | | H—C—C—C—H Propane | | | H H H
H H H | | | H—C—C—C—H vs. H—C—C—H Chain isomerism of C4H10 | | | | | (Butane and 2-methylpropane) H H H H—C—H | H

5.2 Stereoisomerism

Occurs when compounds have the same molecular and structural formulas but different spatial arrangements of atoms.

6. Common Organic Reactions

Organic reactions are chemical reactions involving organic compounds.

6.1 Combustion

Organic compounds react with oxygen to produce carbon dioxide and water.

Example: CH4 + 2O2 → CO2 + 2H2O

6.2 Substitution Reactions

One atom or group is replaced by another.

Example: CH4 + Cl2 → CH3Cl + HCl

6.3 Addition Reactions

Molecules add across a double or triple bond.

Example: C2H4 + H2 → C2H6

6.4 Elimination Reactions

Two atoms or groups are removed to form a multiple bond.

Example: CH3CH2OH → C2H4 + H2O

6.5 Oxidation and Reduction

Oxidation involves the loss of hydrogen or gain of oxygen, while reduction involves the gain of hydrogen or loss of oxygen.

Example: CH3CH2OH + [O] → CH3CHO + H2O (Oxidation of ethanol to ethanal)

7. Naming Organic Compounds (IUPAC Nomenclature)

IUPAC (International Union of Pure and Applied Chemistry) nomenclature provides systematic rules for naming organic compounds.

7.1 Basic Rules

  1. Identify the longest continuous carbon chain (parent chain)
  2. Name the parent chain according to the number of carbon atoms
  3. Number the carbon atoms in the parent chain starting from the end closest to the first substituent
  4. Name and number the substituents
  5. Combine the prefix, parent name, and suffix in the proper order

7.2 Common IUPAC Prefixes and Suffixes

Class Prefix/Suffix Example
Alkanes -ane ethane (C2H6)
Alkenes -ene ethene (C2H4)
Alkynes -yne ethyne (C2H2)
Alcohols -ol ethanol (C2H5OH)
Aldehydes -al ethanal (CH3CHO)
Ketones -one propanone (CH3COCH3)
Carboxylic acids -oic acid ethanoic acid (CH3COOH)

8. Polymers - Macromolecules of Organic Chemistry

Polymers are large molecules composed of repeating structural units (monomers) connected by covalent bonds.

8.1 Types of Polymers

8.2 Examples of Common Polymers

Polymer Monomer Uses
Polyethene (Polyethylene) Ethene (C2H4) Plastic bags, bottles
Polypropene (Polypropylene) Propene (C3H6) Ropes, carpets
PVC (Polyvinyl chloride) Vinyl chloride (C2H3Cl) Pipes, window frames
Nylon Diamine and dicarboxylic acid Clothing, fishing lines
Polyester Diols and dicarboxylic acids Fabrics, bottles
Environmental Impact: While polymers have revolutionized modern life, many synthetic polymers are non-biodegradable and contribute to environmental pollution. Biodegradable and recyclable alternatives are being developed.

9. Organic Chemistry in Everyday Life

Organic chemistry plays a crucial role in various aspects of our daily lives:

9.1 Food and Nutrition

9.2 Medicines and Pharmaceuticals

9.3 Fuels and Energy

9.4 Materials

Glossary of Terms

Organic Chemistry: The study of carbon-containing compounds and their properties.

Catenation: The ability of carbon atoms to form long chains or rings by bonding with other carbon atoms.

Hydrocarbon: A compound composed only of carbon and hydrogen atoms.

Alkanes: Saturated hydrocarbons with single bonds only, general formula CnH2n+2.

Alkenes: Unsaturated hydrocarbons containing at least one carbon-carbon double bond, general formula CnH2n.

Alkynes: Unsaturated hydrocarbons containing at least one carbon-carbon triple bond, general formula CnH2n-2.

Functional Group: A specific atom or group of atoms that determines the characteristic chemical reactions of a compound.

Homologous Series: A series of compounds with the same functional group but differing in the number of -CH2- units.

Isomers: Compounds with the same molecular formula but different structural arrangements.

Structural Isomerism: Isomerism where compounds have the same molecular formula but different connectivity of atoms.

Geometric Isomerism: Isomerism due to different spatial arrangements around a double bond.

IUPAC Nomenclature: Systematic method for naming organic compounds developed by the International Union of Pure and Applied Chemistry.

Addition Reaction: A reaction where atoms are added to a molecule, typically across a multiple bond.

Substitution Reaction: A reaction where one atom or group is replaced by another.

Polymer: A large molecule made up of repeating structural units called monomers.

Monomer: The basic repeating unit in a polymer.

Saturated Compound: A compound with only single bonds between carbon atoms.

Unsaturated Compound: A compound with at least one double or triple bond between carbon atoms.

Aliphatic Compounds: Organic compounds that do not contain a benzene ring.

Aromatic Compounds: Organic compounds containing a benzene ring or similar structure.

Self-Assessment Questions

Multiple Choice Questions

  1. Which of the following is NOT a characteristic of carbon that makes it central to organic chemistry?

    A) It forms 4 covalent bonds

    B) It has a strong electronegativity

    C) It can form bonds with itself (catenation)

    D) It forms stable bonds with hydrogen

    B) It has a strong electronegativity. Carbon's electronegativity is moderate, not particularly strong.

  2. What is the general formula for alkanes?

    A) CnH2n

    B) CnH2n+2

    C) CnH2n-2

    D) CnHn

    B) CnH2n+2

  3. Which of the following compounds is an aldehyde?

    A) CH3CH2OH

    B) CH3COCH3

    C) CH3CHO

    D) CH3COOH

    C) CH3CHO (ethanal/acetaldehyde)

  4. What type of isomerism involves different spatial arrangements around a carbon-carbon double bond?

    A) Chain isomerism

    B) Position isomerism

    C) Functional group isomerism

    D) Geometric (cis-trans) isomerism

    D) Geometric (cis-trans) isomerism

  5. Which functional group is characteristic of alcohols?

    A) -COOH

    B) -CHO

    C) -OH

    D) -C=O