Welcome to the World of Alkanes!
Hello future chemist! This chapter is your introduction to Organic Chemistry, the fascinating study of compounds that contain carbon. Alkanes are the absolute starting point—they are the simplest family in this vast area of chemistry.
Don't worry if the formulae look intimidating at first. By breaking down the structure and learning a few simple rules, you will master this topic quickly. Let's get started!
11.4 Alkanes: The Saturated Hydrocarbons (Core Content)
1. Defining Alkanes and the Homologous Series
Alkanes belong to the first family we study in organic chemistry: the Homologous Series.
What makes Alkanes special?
- They are hydrocarbons: compounds containing only hydrogen (\(\text{H}\)) and carbon (\(\text{C}\)).
- They are saturated: This is a crucial term! It means their molecules contain only single covalent bonds between the carbon atoms. They are "full" of hydrogen atoms, meaning no more can be added across a double bond.
The General Formula
Because they are a homologous series, alkanes all follow a simple mathematical rule for their molecular formula. This is the General Formula:
$$ \text{C}_n\text{H}_{2n+2} $$
Where 'n' is the number of carbon atoms.
Example: If n=2 (Ethane), the formula is \(\text{C}_2\text{H}_{(2\times 2)+2} = \text{C}_2\text{H}_6\).
Quick Review Box: The First Four Alkanes
It is essential to remember the names and formulae of the first four alkanes:
- n=1: Methane (\(\text{CH}_4\))
- n=2: Ethane (\(\text{C}_2\text{H}_6\))
- n=3: Propane (\(\text{C}_3\text{H}_8\))
- n=4: Butane (\(\text{C}_4\text{H}_{10}\))
Memory Aid: A common mnemonic to remember the number of carbons (1, 2, 3, 4) is:
M - E - P - B (Mice Eat Peanut Butter)
Key Takeaway: Alkanes are saturated hydrocarbons with single bonds, fitting the formula \(\text{C}_n\text{H}_{2n+2}\).
2. Structure and Bonding (Core Content)
In all alkanes, carbon atoms form four bonds and hydrogen atoms form one bond. Since alkanes are saturated, every bond is a single covalent bond.
Drawing Displayed Formulae
The displayed formula shows every atom and every bond in the molecule. For alkanes, this means drawing a chain of carbon atoms, and then attaching enough hydrogen atoms so that each carbon atom has four bonds in total.
Example: Methane (\(\text{CH}_4\))
$$\text{H} \atop |$$
$$\text{H}—\text{C}—\text{H}$$
$$\text{H} \atop |$$
Example: Ethane (\(\text{C}_2\text{H}_6\))
$$\text{H}\quad\text{H} \atop |\quad|$$
$$\text{H}—\text{C}—\text{C}—\text{H}$$
$$\text{H}\quad\text{H} \atop |\quad|$$
Accessibility Tip: Always count the bonds around each carbon atom to check your drawing. If a carbon has 5 bonds, something is wrong! (It must have exactly four.)
3. General Properties of Alkanes (Core Content)
Alkanes are Generally Unreactive
Alkanes are often described as being generally unreactive. This might sound boring, but it's actually very important!
Why are they so unreactive?
The bonds in alkanes (\(\text{C}—\text{C}\) and \(\text{C}—\text{H}\)) are:
- Strong: A lot of energy is needed to break them.
- Non-polar: The electrons are shared almost equally, so they don't have charged ends to attract other molecules easily.
They mainly only react in two specific ways, which are often triggered by high energy:
- Combustion (Burning)
- Substitution (with halogens like chlorine)
Did You Know? Methane, the simplest alkane, is the main component of natural gas, which we use globally for heating and cooking. Its stability is why it is safe to transport and use (until we give it enough energy to burn!).
Key Takeaway: Due to strong, non-polar single bonds, alkanes are chemically stable and generally unreactive, requiring high energy to initiate reactions.
4. Reactions of Alkanes
A. Combustion (Core Content)
Combustion is simply the process of burning. Alkanes are extremely important fuels because they release a large amount of energy when they burn (they undergo an exothermic reaction).
Complete Combustion
When there is a plentiful supply of oxygen, the alkane burns completely, producing only carbon dioxide and water.
$$ \text{Alkane} + \text{Oxygen} \rightarrow \text{Carbon Dioxide} + \text{Water} $$
Example: Combustion of Methane (balanced symbol equation is expected at IGCSE level, though 11.4 Core 2 only asks to describe it):
$$
\text{CH}_4(\text{g}) + 2\text{O}_2(\text{g}) \rightarrow \text{CO}_2(\text{g}) + 2\text{H}_2\text{O}(\text{l})
$$
Incomplete Combustion
If the oxygen supply is limited (not enough O\(_2\)), the burning is incomplete. This produces dangerous by-products: carbon monoxide (\(\text{CO}\)) and/or soot (carbon particulates).
$$ \text{CH}_4 + 1.5\text{O}_2 \rightarrow \text{CO} + 2\text{H}_2\text{O} $$ (Note: Carbon monoxide is a toxic gas, which is why proper ventilation is vital when burning fuels.)
B. Substitution by Chlorine (Core & Supplement Content)
Alkanes react with halogens (like chlorine, \(\text{Cl}_2\)) in a specific way called a substitution reaction.
What is a Substitution Reaction?
Definition (Supplement 3): A substitution reaction is a reaction where one atom or group of atoms is replaced by another atom or group of atoms.
Think of it like swapping a piece in a board game. An alkane molecule swaps one of its hydrogen atoms for a chlorine atom.
The Conditions Required (Photochemical Reaction)
This reaction does not happen simply by mixing the alkane and chlorine. It requires energy to break the strong \(\text{Cl}—\text{Cl}\) bond and start the process. This energy is provided by ultraviolet light (UV light).
- This type of reaction is called a photochemical reaction (Supplement 4).
- The UV light provides the necessary activation energy (\(E_a\)) to start the reaction.
The Reaction Explained (Monosubstitution)
The IGCSE syllabus focuses on monosubstitution—where only one hydrogen atom is replaced.
If we react methane (\(\text{CH}_4\)) with chlorine (\(\text{Cl}_2\)) in the presence of UV light:
$$ \text{CH}_4 + \text{Cl}_2 \xrightarrow{\text{UV Light}} \text{CH}_3\text{Cl} + \text{HCl} $$
Products formed are chloromethane (\(\text{CH}_3\text{Cl}\)) and hydrogen chloride (\(\text{HCl}\)).
Drawing the Products (Supplement 4)
You must be able to draw the displayed formulae of the products.
Example: Substitution of Methane
The original methane structure swaps one H for one Cl:
Methane:
$$\text{H} \atop |$$
$$\text{H}—\text{C}—\text{H}$$
$$\text{H} \atop |$$
Chloromethane:
$$\text{H} \atop |$$
$$\text{H}—\text{C}—\text{Cl}$$
$$\text{H} \atop |$$
Common Mistake to Avoid: If excess chlorine is present, the reaction can continue, replacing all the hydrogens (e.g., forming dichloromethane, trichloromethane, etc.). However, for the IGCSE syllabus, you only need to describe and draw the product of monosubstitution ($\text{CH}_3\text{Cl}$).
Key Takeaway: Alkanes undergo substitution with chlorine using UV light (a photochemical reaction), replacing an \(\text{H}\) atom with a \(\text{Cl}\) atom.
Chapter Summary: Alkanes
- Definition: Alkanes are saturated hydrocarbons (\(\text{C}\) and \(\text{H}\) only) containing only single covalent bonds.
- General Formula: \(\text{C}_n\text{H}_{2n+2}\).
- Reactivity: Generally unreactive due to strong C-C and C-H bonds.
- Reaction 1: Combustion: Burns completely in excess oxygen to produce \(\text{CO}_2\) and \(\text{H}_2\text{O}\). In limited oxygen, produces dangerous \(\text{CO}\) and soot.
- Reaction 2: Substitution: Reacts with halogens (like chlorine) in a photochemical reaction initiated by UV light, replacing an \(\text{H}\) with a halogen atom.