Welcome to the World of Alkanes!
Hi there! Today, we are diving into Alkanes. You can think of alkanes as the "starting point" of organic chemistry. They are the simplest members of the hydrocarbon family. Even if you find chemistry a bit intimidating, don't worry! Alkanes are actually quite straightforward once you see how they work. We use them every single day—from the natural gas that cooks our food to the petrol that powers our cars.
In these notes, we’ll look at what they are, why they are a bit "lazy" when it comes to reacting, and the specific ways they do react when they finally get going.
1. What exactly is an Alkane?
Before we move forward, let’s quickly recap a prerequisite concept: A hydrocarbon is a compound made only of carbon (C) and hydrogen (H) atoms.
Alkanes are known as saturated hydrocarbons.
Analogy: Imagine a sponge that has soaked up as much water as it possibly can. It is "saturated." Similarly, in an alkane, every carbon atom is bonded to as many hydrogen atoms as possible using single bonds only. There are no double bonds (\(C=C\)) here!
Key Properties:
- General Formula: \(C_nH_{2n+2}\)
- Bonding: They only contain single covalent bonds (\(\sigma\) bonds).
- Shape: Each carbon atom in an alkane is \(sp^3\) hybridised, giving it a tetrahedral shape with bond angles of 109.5°.
Quick Review: If an alkane has 3 carbons (\(n=3\)), how many hydrogens does it have?
Using the formula \(2(3) + 2\), the answer is 8. So, Propane is \(C_3H_8\)!
2. Why are Alkanes so "Unreactive"?
If you try to react an alkane with an acid, a base, or an oxidising agent at room temperature, usually... nothing happens. This is why they were historically called paraffins (from the Latin parum affinis, meaning "little affinity").
There are two main reasons for this "laziness":
- Bond Strength: The \(C-C\) and \(C-H\) bonds are very strong. It takes a lot of energy to break them!
- Lack of Polarity: Carbon and Hydrogen have very similar electronegativities. This means the electrons in the bonds are shared almost equally. Because there are no significant partial charges (\(\delta+\) or \(\delta-\)), polar reagents (like ions) have no "hook" to grab onto to start a reaction.
Key Takeaway: Alkanes are unreactive because their bonds are strong and non-polar.
3. How do we make Alkanes? (Synthesis)
The syllabus requires you to know two main ways to produce alkanes:
A. Hydrogenation of Alkenes
We can turn an "unsaturated" alkene (with a double bond) into a "saturated" alkane by adding hydrogen gas (\(H_2\)).
Reagents and Conditions:
1. Hydrogen gas (\(H_2\))
2. Pt (Platinum) or Ni (Nickel) catalyst
3. Heat
Equation Example: \(C_2H_4 + H_2 \rightarrow C_2H_6\)
B. Cracking of Crude Oil
Large alkane molecules found in crude oil aren't very useful. We "crack" them into smaller, more useful alkanes (like petrol) and alkenes (for making plastics).
Reagents and Conditions:
1. High heat
2. \(Al_2O_3\) (Aluminium oxide) catalyst
Did you know? Cracking is like taking a long, clunky gold chain and breaking it into smaller rings and bracelets that more people want to buy!
4. Reaction 1: Combustion (Burning)
While alkanes are generally unreactive, they are excellent fuels. When they react with oxygen, they release a massive amount of energy.
Complete Combustion
This happens when there is plenty of oxygen. The only products are carbon dioxide (\(CO_2\)) and water (\(H_2O\)).
Example (Ethane): \(2C_2H_6 + 7O_2 \rightarrow 4CO_2 + 6H_2O\)
Incomplete Combustion
This happens when oxygen is limited. Instead of all the carbon turning into \(CO_2\), we get Carbon Monoxide (\(CO\)) or even just Soot (Carbon, \(C\)).
Example (Ethane): \(2C_2H_6 + 5O_2 \rightarrow 4CO + 6H_2O\)
Environmental Consequences:
- Carbon Monoxide (\(CO\)): A toxic, odourless gas that prevents your blood from carrying oxygen.
- Oxides of Nitrogen (\(NO_x\)): Formed in car engines due to high heat; these cause acid rain and smog.
- Unburnt Hydrocarbons: Contribute to smog and can be carcinogenic.
The Solution: Modern cars use a Catalytic Converter (containing Platinum and Rhodium) to turn these nasty gases into harmless \(CO_2\), \(N_2\), and \(H_2O\).
5. Reaction 2: Free-Radical Substitution
This is the "star" mechanism of this chapter. Alkanes can react with Halogens (like \(Cl_2\) or \(Br_2\)), but only if you provide Ultraviolet (UV) Light. The UV light provides the "kick-start" energy needed to break the bonds.
Memory Aid: Think of this as a "Steal and Swap" reaction. A halogen atom steals a spot from a hydrogen atom.
The Three Stages (Example: Ethane + Chlorine)
Stage 1: Initiation
The UV light breaks the \(Cl-Cl\) bond. This is called homolytic fission because the bond breaks perfectly in half, and each chlorine atom takes one electron. This creates two Free Radicals (\(Cl \cdot\)).
Equation: \(Cl_2 \xrightarrow{UV} 2Cl \cdot\)
Stage 2: Propagation (The Chain Reaction)
This is a two-step cycle where the radical keeps "living" on.
1. A chlorine radical attacks the ethane: \(C_2H_6 + Cl \cdot \rightarrow \cdot C_2H_5 + HCl\)
2. The new ethyl radical attacks a fresh \(Cl_2\) molecule: \(\cdot C_2H_5 + Cl_2 \rightarrow C_2H_5Cl + Cl \cdot\)
Notice: We created a \(Cl \cdot\) at the end, which can go back and repeat step 1! This is why it's a "chain" reaction.
Stage 3: Termination
The reaction ends when two radicals bump into each other and form a stable molecule. They "cancel" each other out.
Examples:
\(Cl \cdot + Cl \cdot \rightarrow Cl_2\)
\(\cdot C_2H_5 + \cdot C_2H_5 \rightarrow C_4H_{10}\) (Butane)
\(\cdot C_2H_5 + Cl \cdot \rightarrow C_2H_5Cl\)
Common Mistake to Avoid: Don't forget the dot (\(\cdot\))! It represents the unpaired electron that makes a free radical so reactive. No dot = no marks!
Quick Review Summary
- Alkanes: Saturated hydrocarbons, \(C_nH_{2n+2}\), tetrahedral shape.
- Reactivity: Very low due to strong, non-polar \(C-C\) and \(C-H\) bonds.
- Synthesis: Hydrogenation of alkenes (Ni/Pt catalyst) or Cracking (\(Al_2O_3\) catalyst).
- Combustion: Complete (\(CO_2 + H_2O\)) vs Incomplete (\(CO/C + H_2O\)).
- Free-Radical Substitution: Needs UV light. Follows Initiation, Propagation, and Termination.
Don't worry if the mechanism seems tricky at first—just remember it as Breaking, Swapping, and Stopping. You've got this!