Introduction

Alkanes are fundamental organic compounds that play a crucial role in chemistry and industrial processes. As saturated hydrocarbons, they exhibit unique reactivity patterns that are essential to understand for both theoretical knowledge and practical applications.

Molecular Structure and Reactivity

Alkanes are classified as saturated hydrocarbons because their carbon atoms form single bonds exclusively, with each carbon atom bonded to a maximum of four other atoms. This molecular structure directly influences their chemical behavior and possible reaction pathways.

Types of Reactions

1. Substitution Reactions

Substitution reactions represent one of the most important transformations of alkanes. In these reactions, hydrogen atoms are replaced by halogen atoms (such as chlorine or bromine) through a process called halogenation.

Key characteristics of substitution reactions include:

For example, the stepwise chlorination of methane proceeds as follows:

$CH_4 + Cl_2 \xrightarrow{UV} CH_3Cl + HCl$ $CH_3Cl + Cl_2 \xrightarrow{UV} CH_2Cl_2 + HCl$ $CH_2Cl_2 + Cl_2 \xrightarrow{UV} CHCl_3 + HCl$ $CHCl_3 + Cl_2 \xrightarrow{UV} CCl_4 + HCl$

Substitution Selectivity

The substitution process follows a specific order of preference:

2. Catalytic Cracking

Catalytic cracking is an industrial process where long-chain alkanes are broken down into smaller hydrocarbons. This process:

3. Combustion Reactions

Alkanes undergo two types of combustion reactions:

Complete Combustion

In the presence of excess oxygen, alkanes undergo complete combustion:

$C_nH_{2n+2} + \frac{3n+1}{2}O_2 \rightarrow nCO_2 + (n+1)H_2O$

For example, octane combustion: $2C_8H_{18} + 25O_2 \rightarrow 16CO_2 + 18H_2O$

Incomplete Combustion

With limited oxygen supply: $2C_8H_{18} + 17O_2 \rightarrow 8CO_2 + 8C + 18H_2O$

Testing for Alkanes

The bromine water test can differentiate between saturated and unsaturated hydrocarbons: