Alkanes (exemplified by ethane)

Introduction to Alkanes: The "Steady" Hydrocarbons

Welcome to the study of Alkanes! If you think of Organic Chemistry as a large city, Alkanes are like the sturdy, reliable foundation of the buildings. They are the simplest type of hydrocarbons—compounds made entirely of carbon and hydrogen. In this chapter, we will focus specifically on ethane \( (C_2H_6) \) to understand how these molecules are built and how they behave. Don't worry if Organic Chemistry feels like a new language; we'll take it one "word" at a time!

1. Structure and Bonding in Ethane

Before we look at how alkanes react, we need to see how they are put together. Alkanes are saturated hydrocarbons, meaning they contain only single bonds. There are no double or triple bonds here!

Hybridisation and the \(\sigma\) Bond

In ethane \( (CH_3CH_3) \), each carbon atom is \(sp^3\) hybridised.
Imagine the carbon atom is "mixing" one s-orbital and three p-orbitals to create four identical "hybrid" orbitals. This allows the carbon to form four strong bonds.

The bonds in ethane are \(\sigma\) (sigma) bonds. These are formed by the "head-on" overlap of orbitals.
• The \(C-C\) bond is formed by \(sp^3-sp^3\) overlap.
• The \(C-H\) bonds are formed by \(sp^3-s\) overlap.

Shape and Bond Angles

Because of the four \(sp^3\) orbitals pushing away from each other (remember VSEPR theory?), the shape around each carbon atom is tetrahedral.
• The bond angle is approximately \(109.5^\circ\).
• Analogy: Think of each carbon atom as the center of a camera tripod, with the three legs being hydrogens and the top being the other carbon atom.

Quick Review:
• Hybridisation: \(sp^3\)
• Bond Type: \(\sigma\) bonds only
• Shape: Tetrahedral
• Angle: \(109.5^\circ\)

2. Why are Alkanes so Unreactive?

Alkanes are often called "paraffins," which comes from the Latin parum affinis, meaning "little affinity." In plain English: they aren't very interested in reacting with other chemicals!

There are two main reasons for this "laziness":
1. Bond Strength: The \(C-C\) and \(C-H\) \(\sigma\) bonds are very strong and require a lot of energy to break.
2. Lack of Polarity: Carbon and hydrogen have very similar electronegativities. This means the bonds are non-polar. There are no "electron-rich" or "electron-poor" spots for acids, bases, or polar reagents to attack.

Did you know? Because they are so stable and non-polar, alkanes make excellent waxes for surfboards and candles!

Key Takeaway: Alkanes are generally inert (unreactive) towards polar reagents like acids, bases, and oxidising agents.

3. Chemical Reaction: Combustion

Even though they are generally unreactive, alkanes are great fuels. When you give them enough a little "spark" (activation energy) and plenty of oxygen, they burn!

Complete Combustion

With plenty of oxygen, ethane burns to produce carbon dioxide and water:
\( C_2H_6(g) + 3.5O_2(g) \rightarrow 2CO_2(g) + 3H_3O(l) \)
(Note: You can multiply the whole equation by 2 to get rid of the 0.5 if you prefer!)

Incomplete Combustion

If oxygen is limited, you get "dirtier" products like carbon monoxide (CO) or even soot (C).
• Environmental Note: Carbon monoxide is a toxic gas that prevents blood from carrying oxygen. This is why car engines and gas heaters must be well-ventilated!

4. The Big Mechanism: Free-Radical Substitution (FRS)

This is the most important reaction for alkanes in your syllabus. While alkanes ignore polar reagents, they will react with Halogens (like \(Cl_2\) or \(Br_2\)) if you provide Ultraviolet (UV) light.

The Overall Reaction (using ethane and chlorine):
\( C_2H_6 + Cl_2 \xrightarrow{UV} C_2H_5Cl + HCl \)
One hydrogen is "substituted" (replaced) by a chlorine atom.

The Step-by-Step Mechanism

Don't worry if this seems tricky! It follows a 3-step "story" called a chain reaction.

Step 1: Initiation

UV light provides the energy to break the \(Cl-Cl\) bond. Each chlorine atom takes one electron from the shared pair. This is called homolytic fission.
\( Cl-Cl \xrightarrow{UV} 2Cl \cdot \)
The dots \( (\cdot) \) represent Free Radicals—highly reactive atoms with an unpaired electron.

Step 2: Propagation (The "Cycle")

The radicals are like "zombies"—they attack a stable molecule and turn it into a radical too!
i) A chlorine radical attacks ethane: \( CH_3CH_3 + Cl \cdot \rightarrow \cdot CH_2CH_3 + HCl \)
ii) The new ethyl radical attacks a chlorine molecule: \( \cdot CH_2CH_3 + Cl_2 \rightarrow CH_3CH_2Cl + Cl \cdot \)
Notice that a \(Cl \cdot\) is regenerated at the end! This allows the reaction to keep going like a loop.

Step 3: Termination (The "End")

The reaction stops when two radicals find each other and "cancel out" to form a stable molecule.
• \( Cl \cdot + Cl \cdot \rightarrow Cl_2 \)
• \( \cdot CH_2CH_3 + Cl \cdot \rightarrow CH_3CH_2Cl \)
• \( \cdot CH_2CH_3 + \cdot CH_2CH_3 \rightarrow CH_3CH_2CH_2CH_3 \) (Wait, ethane turned into butane! This is proof of the mechanism.)

Mnemonic for FRS Steps: I eat Potato Tots (Initiation, Propagation, Termination).

Common Mistake to Avoid: Students often forget to write the UV light condition. Without UV light, the reaction simply won't start at room temperature!

5. Alkanes and the Environment

As H2 Chemistry students, you need to be aware of the "real-world" impact of these molecules.

• Hydrocarbons as Fuels: Most of our energy comes from burning alkanes (petrol, natural gas).
• The Greenhouse Effect: Carbon dioxide \( (CO_2) \) produced from combustion is a greenhouse gas. It traps heat in the Earth's atmosphere, leading to global warming.
• Unburnt Hydrocarbons: Sometimes alkanes don't burn at all in car engines. These "unburnt hydrocarbons" contribute to photochemical smog, which is harmful to our lungs.

Quick Review Box:
• Reagents for FRS: \(Cl_2\) or \(Br_2\)
• Conditions for FRS: UV light / Sunlight
• Type of Fission: Homolytic
• Environmental Impact: \(CO_2\) (Greenhouse effect), \(CO\) (Toxicity), Unburnt hydrocarbons (Smog).

Final Summary Takeaway

Alkanes like ethane are the "strong, silent type" of Organic Chemistry. They are held together by strong, non-polar \(sp^3\) \(\sigma\) bonds, making them very stable. Their main chemical personality traits are burning as fuels and undergoing Free-Radical Substitution when exposed to UV light. Master the three steps of the FRS mechanism (Initiation, Propagation, Termination), and you've conquered the core of this chapter!