Welcome to Organic Chemistry: The World of Alkanes!

Hello there! You've just stepped into the exciting field of Organic Chemistry, which is essentially the chemistry of carbon compounds. Don't worry if the long names seem intimidating—we're starting with the simplest group: the Alkanes.

Alkanes are the foundation of many things we use daily, most importantly, fuels. Understanding them is key to understanding everything else in organic chemistry. We'll look at their structure, how they are obtained from crude oil, and their most important (and sometimes destructive) reactions.

Quick Review: Key Organic Chemistry Concepts

Before diving into alkanes, let's refresh some essential vocabulary:

  • Hydrocarbon: A compound containing only hydrogen and carbon atoms.
  • Homologous Series: A family of compounds with the same general formula and similar chemical properties (e.g., Alkanes).
  • General Formula: A formula that represents any member of a homologous series. For alkanes, this is \(C_nH_{2n+2}\).

3.3.2 Alkanes: Structure and Properties

What makes an Alkane an Alkane?

Alkanes are described as saturated hydrocarbons. This means two important things:

  1. They only contain carbon and hydrogen.
  2. They contain only single covalent bonds (C–C and C–H). This is what "saturated" means—the carbon atoms are bonded to the maximum possible number of hydrogen atoms.

The general formula for non-cyclic alkanes is: $$C_nH_{2n+2}$$

Example: Methane (\(n=1\)) is \(C_1H_{(2\times 1)+2} = CH_4\). Ethane (\(n=2\)) is \(C_2H_{(2\times 2)+2} = C_2H_6\).

Why are Alkanes relatively unreactive?

Alkanes are often called paraffins (from the Latin for 'little affinity'). This lack of reactivity comes down to two factors:

  • Non-polar bonds: The C–C and C–H bonds are non-polar (or very weakly polar) because carbon and hydrogen have similar electronegativities.
  • High bond enthalpy: The C–C and C–H bonds are strong and require a large amount of energy to break.
This means they are not easily attacked by typical reagents like acids, bases, or oxidising agents. They mainly react under harsh conditions (like high heat or UV light).

Key Takeaway: Alkanes are simple, saturated molecules with only C–C and C–H single bonds. Their non-polar, strong bonds make them largely unreactive at room temperature.

3.3.2.1 Fractional Distillation of Crude Oil

Alkanes are the main components of crude oil (petroleum), which is a thick, black, smelly liquid found underground. It is a complex mixture of thousands of hydrocarbons. To use them, we first need to separate them.

Separation Technique: Fractional Distillation

Crude oil is separated based on the differences in the boiling points of the components.

  1. Crude oil is vaporised at about 350 °C and passed into the bottom of a fractionating column (a tall tower).
  2. As the hot vapour rises, it cools down.
  3. The components condense at different heights, depending on their boiling point.
  4. Longer chain hydrocarbons (high boiling point) condense lower down the column.
  5. Shorter chain hydrocarbons (low boiling point/more volatile) rise higher up the column before condensing, or leave as gases at the very top.

Analogy: Think of a ladder. The heavy, long chains can only climb a few steps (condense quickly at the bottom). The light, short chains can easily climb all the way to the top before condensing.

3.3.2.2 Modification of Alkanes by Cracking

After fractional distillation, we often find that the supply of long-chain alkanes (like heavy fuel oil) is much greater than the demand, while the demand for shorter-chain alkanes (like petrol/gasoline) is much higher.

Cracking is the process used to break down large, less useful hydrocarbon molecules into smaller, more useful ones.

The Economic Reasons for Cracking

The primary reason for cracking is economic.

  • It helps match the supply of fractions (from distillation) with the industrial demand.
  • Cracking produces small alkanes (good for petrol) AND alkenes (used to make plastics and polymers, which are high-value products).

Two Types of Cracking

Cracking involves breaking the strong C–C bonds in the alkane backbone.

1. Thermal Cracking:

  • Conditions: Very high pressure (\(7000\) kPa) and high temperature (\(400-900\) °C).
  • Product: Produces a high percentage of alkenes (which have C=C double bonds).
  • Note: The mechanism for cracking is not required in this syllabus section, but you must know the conditions and major product type.

2. Catalytic Cracking:

  • Conditions: Slight pressure, high temperature (\(450\) °C), and a zeolite catalyst.
  • Product: Mainly used to produce hydrocarbons suitable for motor fuels and aromatic hydrocarbons.
  • Advantage: This is cheaper than thermal cracking due to the lower pressure required, and the catalyst allows the reaction to be more selective, yielding higher quality gasoline.

Key Takeaway: Cracking is vital for increasing the yield of valuable products (like petrol and alkenes) from crude oil, making it economically viable.

3.3.2.3 Combustion of Alkanes

Alkanes are primarily used as fuels because they burn readily in oxygen, releasing large amounts of heat (they are exothermic reactions).

Complete vs. Incomplete Combustion

The products depend on the amount of oxygen available:

1. Complete Combustion (Plenty of Oxygen):

When sufficient oxygen is available, the alkane burns cleanly to produce only carbon dioxide and water. Example (Methane):
$$CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O$$

2. Incomplete Combustion (Limited Oxygen):

If there isn't enough oxygen, the reaction produces dangerous pollutants like carbon monoxide (CO) and/or carbon (soot). Example (Incomplete Methane):
$$2CH_4 + 3O_2 \rightarrow 2CO + 4H_2O$$ (Carbon Monoxide production)
$$CH_4 + O_2 \rightarrow C + 2H_2O$$ (Soot production)

WARNING: Carbon monoxide is a highly toxic, odourless gas that prevents red blood cells from carrying oxygen. Always ensure good ventilation when using hydrocarbon fuels!

Air Pollution from Internal Combustion Engines

Car engines operate under very high temperatures and pressures, leading to additional pollution issues:

  • Nitrogen Oxides (NOx): At high temperatures, atmospheric nitrogen and oxygen react: \(N_2 + O_2 \rightarrow 2NO\). NOx contributes to acid rain and photochemical smog.
  • Unburned Hydrocarbons: Some fuel may pass through the engine unburnt, contributing to smog and acting as greenhouse gases.
  • CO and C: Result from incomplete combustion, as noted above.

The Solution: Catalytic Converters

Catalytic converters are fitted to car exhausts to convert these pollutants into less harmful substances. They use expensive metal catalysts (like Platinum, Rhodium, or Palladium) on a ceramic honeycomb structure to provide a high surface area.

  • Conversion: \(2CO + 2NO \rightarrow N_2 + 2CO_2\)

Sulfur Removal (Flue Gas Desulfurisation)

Crude oil sometimes contains impurities, especially sulfur compounds. When these burn, they produce sulfur dioxide (\(SO_2\)), a major cause of acid rain.

Removal of \(SO_2\):

In power stations (flue gas desulfurisation), \(SO_2\) is removed by reacting it with bases such as calcium oxide (CaO) or calcium carbonate (\(CaCO_3\)).

Equation using Calcium Carbonate:
$$\text{CaO(s)} + \text{SO}_2\text{(g)} \rightarrow \text{CaSO}_3\text{(s)}$$
Equation using Calcium Carbonate:
$$\text{CaCO}_3\text{(s)} + \text{SO}_2\text{(g)} \rightarrow \text{CaSO}_3\text{(s)} + \text{CO}_2\text{(g)}$$

Key Takeaway: Alkanes are vital fuels, but burning them causes pollution (CO, C, NOx, SO₂). Catalytic converters and flue gas desulfurisation help clean up the exhaust.

3.3.2.4 Chlorination of Alkanes: Free-Radical Substitution

Because alkanes are generally unreactive, they need specific high-energy conditions to undergo a reaction. They react with halogens (like chlorine or bromine) in the presence of UV light or high temperature.

This reaction is called a free-radical substitution, where a hydrogen atom is replaced (substituted) by a halogen atom.

Example: The chlorination of methane:

$$CH_4 + Cl_2 \xrightarrow{\text{UV light}} CH_3Cl + HCl$$

Step-by-Step Mechanism

This reaction is a chain reaction and happens in three distinct stages.

Prerequisite concept: A free radical is a species with an unpaired electron. In organic chemistry, the unpaired electron is shown by a dot (\(\cdot\)).

Stage 1: Initiation

This step starts the chain reaction by creating the free radicals. The UV light provides enough energy to break the bond in the chlorine molecule (homolytic fission), producing two chlorine free radicals.

$$\text{Cl}_2 \xrightarrow{\text{UV light}} 2\text{Cl}\cdot$$

Stage 2: Propagation

This is the "chain" part of the reaction, where a radical reacts to form a new radical, keeping the reaction going. This happens in two main steps:

Step 2a: The chlorine radical attacks the methane molecule, abstracting a hydrogen atom and forming a methyl radical ($\text{CH}_3\cdot$) and a stable HCl molecule. $$\text{Cl}\cdot + \text{CH}_4 \rightarrow \text{HCl} + \text{CH}_3\cdot$$
Step 2b: The methyl radical then attacks a fresh chlorine molecule, forming the desired product (chloromethane, $\text{CH}_3\text{Cl}$) and regenerating a new chlorine radical ($\text{Cl}\cdot$). $$\text{CH}_3\cdot + \text{Cl}_2 \rightarrow \text{CH}_3\text{Cl} + \text{Cl}\cdot$$
(The Cl$\cdot$ radical produced in Step 2b then cycles back to Step 2a, allowing the chain to repeat many times.)

Stage 3: Termination

The chain stops when two radicals collide, forming a stable molecule. This removes the free radicals from the system.

Possible termination steps include:

  • Two chlorine radicals combine: $$\text{Cl}\cdot + \text{Cl}\cdot \rightarrow \text{Cl}_2$$
  • Two methyl radicals combine (forming ethane): $$\text{CH}_3\cdot + \text{CH}_3\cdot \rightarrow \text{C}_2\text{H}_6$$
  • A methyl radical and a chlorine radical combine: $$\text{CH}_3\cdot + \text{Cl}\cdot \rightarrow \text{CH}_3\text{Cl}$$

Don't worry if this seems tricky at first! The key is to remember that radicals start reactions (Initiation), are used and re-made during the middle (Propagation), and are finally used up (Termination).

✅ Quick Review: The Free-Radical Mechanism
  • Initiation: Breaks \(Cl_2\) into $2Cl\cdot$ using UV light.
  • Propagation: The two-step chain reaction that uses a radical to make a product AND a new radical.
  • Termination: Two radicals combine to stop the chain.
Common Mistake to Avoid: Since the reaction is uncontrolled, substitution can happen multiple times, leading to a mixture of products like $\text{CH}_2\text{Cl}_2$, $\text{CHCl}_3$, and $\text{CCl}_4$!

Key Takeaway: The reaction of alkanes with halogens is free-radical substitution, requiring UV light and proceeding through three stages (I-P-T) involving highly reactive species with unpaired electrons.