Welcome to the World of Alkanes!

Hello! This chapter is your foundation for studying Organic Chemistry—the chemistry of carbon compounds. Alkanes are the simplest type of hydrocarbon, and understanding them is crucial because they teach us fundamental concepts like bonding, molecular shape, and reaction mechanisms.

Don't worry if organic chemistry seems like a lot of diagrams at first! We'll break down the structure and reactions of alkanes, showing you exactly why these molecules are so stable, and how we can make them react when needed.

1. Structure and General Properties of Alkanes (14.1.2)

1.1 Definitions and Formulas

Alkanes belong to the family of organic compounds called hydrocarbons.

  • Hydrocarbon: A compound made up of carbon (C) and hydrogen (H) atoms only.
  • Alkane: A saturated hydrocarbon containing only single covalent bonds. They have no functional group (13.1.2, 13.1.3).
General Formula

The general formula for non-cyclic alkanes is: \(\text{C}_n\text{H}_{2n+2}\)

Example: Methane (\(n=1\)) is \(\text{CH}_4\). Ethane (\(n=2\)) is \(\text{C}_2\text{H}_6\).

Homologous Series

Alkanes form a homologous series (13.2.1a). This means they:

  • Can be represented by the same general formula (\(\text{C}_n\text{H}_{2n+2}\)).
  • Differ from the next member by a \(\text{CH}_2\) unit.
  • Show a gradual change in physical properties (e.g., boiling point increases as molecules get larger).
  • Have similar chemical properties.

1.2 Bonding and Shape

  • Bonding: All bonds in alkanes are single, strong sigma (\(\sigma\)) covalent bonds (3.4.2a).
  • Hybridisation: Every carbon atom in an alkane is \(\text{sp}^3\) hybridised (13.3.2).
  • Shape: This leads to a tetrahedral shape around each carbon atom, with bond angles of \(109.5^{\circ}\) (13.3.2).
  • Saturated: Alkanes are called saturated because they contain the maximum number of hydrogen atoms possible—they have no double or triple bonds (13.2.1b).
Quick Review: Key Terms

Saturated: Only single C-C bonds.
Hydrocarbon: Contains only C and H.
\(\text{C}_n\text{H}_{2n+2}\): The magic formula for alkanes.

2. The General Unreactivity of Alkanes (14.1.5)

Alkanes are often called paraffins, meaning "little affinity." They are chemically very stable and do not easily react with many common chemical reagents like acids, bases, or oxidising agents (polar reagents).

2.1 Explanation of Unreactivity

The low reactivity of alkanes is due to two main factors:

1. Bond Strength:

  • The \(\text{C-C}\) and \(\text{C-H}\) bonds are both very strong (3.4.3a).
  • A lot of energy (high activation energy) is required to break these bonds, meaning they rarely react spontaneously (5.1.2).

2. Relative Lack of Polarity:

  • Carbon and hydrogen have very similar electronegativity values (C is 2.5, H is 2.1).
  • This results in the \(\text{C-H}\) bonds being essentially non-polar (3.1.4).
  • Since the molecules are non-polar, they are not attacked by typical polar reagents, such as electrophiles (positive seeking) or nucleophiles (nucleus seeking) (13.2.1e).

Analogy: Think of alkanes as billiard balls—they are smooth, stable, and don't have any 'sticky' charged patches for other molecules to latch onto.

Key Takeaway: Alkanes only react under extreme conditions (high heat, UV light) because you need a lot of energy to break their strong, non-polar bonds.

3. Sources and Preparation of Alkanes (14.1.1, 14.1.4)

Alkanes are primarily obtained from crude oil (petroleum). Since crude oil is a mixture of hydrocarbons, processes are needed to get the useful products we need.

3.1 Cracking (14.1.1b, 14.1.4)

Crude oil contains a high percentage of large, long-chain hydrocarbons (heavy fractions) which are less useful. Cracking is the process of breaking these large, less useful alkanes into smaller, more valuable molecules.

  • What happens? Large alkanes are broken down into smaller alkanes and alkenes (which are much more reactive and useful in the petrochemical industry).
  • Conditions: High heat and a catalyst, such as aluminium oxide (\(\text{Al}_2\text{O}_3\)).
  • Reaction Type: Thermal decomposition.

Example of Cracking:

\(\text{C}_{10}\text{H}_{22} \rightarrow \text{C}_8\text{H}_{18} + \text{C}_2\text{H}_4\)
Decane (large alkane) \(\rightarrow\) Octane (smaller alkane) + Ethene (alkene)

3.2 Hydrogenation of Alkenes (14.1.1a)

This is a method used to *produce* alkanes from their unsaturated counterparts, alkenes.

  • Reagents: Hydrogen gas (\(\text{H}_2(g)\)).
  • Conditions: Platinum (Pt) or Nickel (Ni) catalyst and heat.
  • Reaction Type: Addition reaction (since the C=C double bond is broken and hydrogen atoms are added).

Example: Producing Ethane

\(\text{C}_2\text{H}_4(g) + \text{H}_2(g) \rightarrow \text{C}_2\text{H}_6(g)\)
Ethene + Hydrogen \(\rightarrow\) Ethane

Key Takeaway: Cracking makes small hydrocarbons from big ones (produces alkenes too); Hydrogenation makes saturated alkanes from unsaturated alkenes.

4. Reactions of Alkanes

4.1 Combustion (Burning) (14.1.2a)

Alkanes are highly important as fuels because they burn readily in oxygen, releasing large amounts of heat (exothermic reaction).

Did you know? Methane is the main component of natural gas used for heating homes!

Complete Combustion

Occurs when there is an excess of oxygen. The products are carbon dioxide and water.

\(\text{Alkane} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}\)

Example (Methane):
\(\text{CH}_4(g) + 2\text{O}_2(g) \rightarrow \text{CO}_2(g) + 2\text{H}_2\text{O}(l)\)

Incomplete Combustion

Occurs when the supply of oxygen is limited. This produces poisonous carbon monoxide ($\text{CO}$) or solid carbon (soot, $\text{C}$), in addition to water.

Example (Incomplete Methane Combustion):
\(\text{CH}_4(g) + 1.5\text{O}_2(g) \rightarrow \text{CO}(g) + 2\text{H}_2\text{O}(l)\)

4.2 Free-Radical Substitution (Halogenation) (14.1.2b, 14.1.3)

Since alkanes are unreactive towards polar reagents, the only major reaction they undergo (other than combustion) is substitution with halogens (like chlorine or bromine).

  • Reagents: \(\text{Cl}_2\) or \(\text{Br}_2\).
  • Conditions: Ultraviolet (UV) light or high temperature (to provide enough energy to break bonds).
  • Reaction Type: Substitution—a hydrogen atom in the alkane is replaced by a halogen atom.

Example (Halogenation of Ethane):
\(\text{C}_2\text{H}_6 + \text{Cl}_2 \xrightarrow{\text{UV light}} \text{C}_2\text{H}_5\text{Cl} + \text{HCl}\)
Ethane + Chlorine \(\rightarrow\) Chloroethane + Hydrogen Chloride

This reaction is called Free-Radical Substitution (FRS) because it proceeds via highly reactive species called free radicals.

5. The Free-Radical Substitution Mechanism (14.1.3)

The mechanism describes the step-by-step pathway a reaction follows. FRS involves three main stages: Initiation, Propagation, and Termination.

Don't worry if this looks complicated! You just need to remember which species are formed and where the radicals come from.

What is a Free Radical?
A free radical (1.3.9, 13.2.1d) is a species with one or more unpaired electrons. They are extremely reactive. Free radicals are formed by homolytic fission (13.2.1c), where a covalent bond breaks equally, leaving one electron on each atom.

Step 1: Initiation

The process starts when the weakest bond in the reaction mixture, usually the halogen bond ($\text{Cl-Cl}$ or $\text{Br-Br}$), breaks using energy from UV light (sunlight). This is homolytic fission.

\(\text{Cl}_2 \xrightarrow{\text{UV light}} 2\text{Cl}^{\cdot}\)
(A chlorine molecule breaks into two highly reactive chlorine free radicals)

Step 2: Propagation (The Chain Reaction)

This is the main part of the reaction, where the radical is used up, but another radical is created, allowing the chain reaction to continue.

Propagation Step 1: A chlorine radical steals a hydrogen atom from the alkane molecule.

\(\text{C}_2\text{H}_6 + \text{Cl}^{\cdot} \rightarrow \text{C}_2\text{H}_5^{\cdot} + \text{HCl}\)
(An alkyl free radical is formed)

Propagation Step 2: The new alkyl radical reacts with an unreacted halogen molecule to form the product, and regenerates the chlorine radical.

\(\text{C}_2\text{H}_5^{\cdot} + \text{Cl}_2 \rightarrow \text{C}_2\text{H}_5\text{Cl} + \text{Cl}^{\cdot}\)
(The catalyst radical is reformed to continue the chain)

Memory Aid: In Propagation, you start with a radical and end with a radical.

Step 3: Termination

The reaction stops when two radicals combine to form a stable molecule. This removes radicals from the system.

Termination reactions are typically inefficient because the concentration of radicals is very low.

Possible termination reactions include:
1. \(\text{Cl}^{\cdot} + \text{Cl}^{\cdot} \rightarrow \text{Cl}_2\)
2. \(\text{C}_2\text{H}_5^{\cdot} + \text{Cl}^{\cdot} \rightarrow \text{C}_2\text{H}_5\text{Cl}\) (Desired product)
3. \(\text{C}_2\text{H}_5^{\cdot} + \text{C}_2\text{H}_5^{\cdot} \rightarrow \text{C}_4\text{H}_{10}\) (Undesired side product, butane)

Common Mistake Alert!

Since termination reactions can form longer carbon chains (like in Reaction 3 above), free-radical substitution usually results in a mixture of products. If you use excess chlorine, you can even substitute all the hydrogen atoms (e.g., \(\text{CH}_4 \rightarrow \text{CCl}_4\)).

Key Takeaway: FRS requires UV light to initiate the reaction by breaking the halogen bond homolytically to form highly reactive free radicals, which then sustain the chain reaction through propagation steps.

6. Environmental Consequences of Alkanes (14.1.6)

While alkanes are excellent fuels, their use in internal combustion engines (like car engines) poses significant environmental challenges, primarily due to incomplete combustion and high operating temperatures.

6.1 Harmful Pollutants

The internal combustion engine produces three main classes of harmful pollutants:

  1. Carbon Monoxide (\(\text{CO}\)): Formed from incomplete combustion (lack of oxygen). It is highly poisonous because it binds irreversibly to haemoglobin in the blood, preventing oxygen transport.
  2. Oxides of Nitrogen (\(\text{NO}_x\)): Formed when the high temperatures and pressures inside the engine cause atmospheric nitrogen and oxygen to react (5.1.4).
    \(\text{N}_2(g) + \text{O}_2(g) \xrightarrow{\text{High Heat}} 2\text{NO}(g)\)
    $\text{NO}_x$ causes acid rain and photochemical smog.
  3. Unburnt Hydrocarbons: Alkanes that pass straight through the engine without burning fully. These contribute to smog and respiratory problems.

6.2 Catalytic Removal

To combat these pollutants, modern cars use a catalytic converter. This device uses transition metals (like Platinum, Palladium, and Rhodium) as heterogeneous catalysts (8.3.1) to convert the harmful gases into harmless ones.

The catalyst works by providing a surface for the pollutant molecules to adsorb onto (26.2.2).

  • Conversion of \(\text{CO}\) and \(\text{NO}\): The catalyst helps reduce $\text{NO}$ and oxidise $\text{CO}$.
  • Target Reactions in the Converter:

    • \(\text{2CO}(g) + \text{O}_2(g) \xrightarrow{\text{Catalyst}} 2\text{CO}_2(g)\)
    \(\text{2NO}(g) + 2\text{CO}(g) \xrightarrow{\text{Catalyst}} \text{N}_2(g) + 2\text{CO}_2(g)\)

Key Takeaway: Catalytic converters save the day by converting toxic combustion by-products (\(\text{CO}\) and \(\text{NO}_x\)) into non-toxic gases (\(\text{CO}_2\) and \(\text{N}_2\)) using expensive metal catalysts.

Summary Review of Alkanes

  • Structure: Saturated, only single C-C and C-H bonds, tetrahedral shape, \(\text{C}_n\text{H}_{2n+2}\).
  • Unreactivity: Due to strong bonds and lack of polarity.
  • Preparation: Cracking (large \(\rightarrow\) small alkanes/alkenes) and Hydrogenation (alkenes \(\rightarrow\) alkanes).
  • Reaction 1: Combustion (Complete \(\rightarrow \text{CO}_2\); Incomplete \(\rightarrow \text{CO}/\text{C}\)).
  • Reaction 2: Free-Radical Substitution (Halogenation using \(\text{Cl}_2/\text{Br}_2\) + UV light).
  • Mechanism Steps: Initiation (makes radicals), Propagation (uses radical, makes radical), Termination (stops radicals).

Keep practising those equations and mechanism steps—you've got this!