Alkanes - Homologous Series, Substitution Reaction & Isomerism | Chemistry for JAMB PDF Download

Introduction

Aliphatic hydrocarbons are organic compounds that consist of carbon and hydrogen atoms arranged in open chains or branched structures. Alkanes are a type of aliphatic hydrocarbon characterized by single bonds between carbon atoms, also known as saturated hydrocarbons.

1. Properties:

• Physical State: Alkanes are generally colorless, odorless, and exist as gases, liquids, or solids at room temperature, depending on their molecular size.
• Boiling and Melting Points: As the molecular size increases, so does the boiling and melting points of alkanes due to increased London dispersion forces between molecules.
• Solubility: Alkanes are nonpolar molecules and are therefore insoluble in water. They are soluble in nonpolar solvents such as benzene and hexane.

2. Isomerism:
Alkanes exhibit structural isomerism, where compounds with the same molecular formula have different structural arrangements. Isomers can be classified into chain isomers (different carbon chain arrangements) and positional isomers (different positions of functional groups on the carbon chain).

3. Uses:

• Fuel: Alkanes, such as methane (CH₄), ethane (C₂H₆), and propane (C₃H₈), are important constituents of natural gas and liquefied petroleum gas (LPG), commonly used as fuel for heating, cooking, and transportation.
• Lubricants: Higher molecular weight alkanes, like hexane (C₆H₁₄) and octane (C₈H₁₈), are used as lubricants to reduce friction and wear in machinery.
• Chemical Feedstock: Alkanes serve as starting materials for the production of various chemicals, including plastics, polymers, and pharmaceuticals.

Note on Homologous Series in Relation to Physical Properties

A homologous series is a group of organic compounds that have the same functional group and exhibit a similar chemical behavior. Members of a homologous series possess a gradual increase in molecular size and a systematic variation in physical properties as the number of carbon atoms increases.

1. Physical Properties:

• Molecular Size and Boiling Point: As the number of carbon atoms increases, the molecular size and London dispersion forces also increase. Consequently, the boiling point of homologous series compounds gradually increases.
• Solubility: The solubility of homologous series compounds in water decreases with an increase in the number of carbon atoms due to their nonpolar nature. However, their solubility in organic solvents generally increases.

2. Isomerism:
Homologous series compounds can exhibit structural isomerism (different carbon chain arrangements), functional group isomerism (different functional groups), and positional isomerism (different positions of functional groups on the carbon chain).

3. Uses:

• Alcohols: The homologous series of alcohols, such as methanol, ethanol, and propanol, find applications as solvents, disinfectants, and fuel additives.
• Aldehydes and Ketones: Compounds like formaldehyde and acetone are widely used as solvents, intermediates in chemical synthesis, and preservatives.
• Carboxylic Acids: Homologous series of carboxylic acids, such as formic acid and acetic acid, have applications as food preservatives, flavorings, and organic synthesis intermediates.

Note on Substitution Reactions and Examples of Halogenated Products

Substitution reactions occur when an atom or a functional group in a molecule is replaced by another atom or group. Halogenated products refer to organic compounds that contain halogen atoms (fluorine, chlorine, bromine, iodine) as substituents.

Substitution Reactions:

• Halogenation: In this reaction, a halogen atom replaces a hydrogen atom in an organic compound. It occurs in the presence of a halogen (e.g., chlorine or bromine) and is commonly initiated by heat or light.
• Nitration: Nitration involves the substitution of a hydrogen atom with a nitro group (-NO₂). It is often carried out using a mixture of concentrated nitric acid and sulfuric acid.
• Alkylation: Alkylation is the substitution of a hydrogen atom with an alkyl group (-R). It can be achieved by reacting an organic compound with an alkyl halide in the presence of a base.

Examples and Uses of Halogenated Products:

• Chloroform (CHCl₃): Used as a solvent, anesthetic, and in the production of refrigerants.
• Dichloromethane (CH₂Cl₂): Commonly used as a solvent in various industries, including pharmaceuticals, paints, and adhesives.
• Tetrachloroethylene (C₂Cl₄): Primarily used as a dry-cleaning solvent and in the production of textiles, metal degreasing, and as a refrigerant.