Chemistry (9202) | Hydrocarbons

🔥 Hydrocarbons: The Fuel of Our World (Chemistry 9202 Study Notes)

Hello future Chemists! Welcome to the fascinating world of Organic Chemistry. This chapter focuses on Hydrocarbons, which are the basic building blocks of fuels, plastics, and countless other materials we use every day. Don't worry if complex structures seem daunting; we will break everything down step-by-step. By the end of these notes, you’ll understand what makes these compounds so important and how they react!

Introduction: What Are Hydrocarbons?

The name says it all! Hydrocarbons are organic compounds made up of just two elements: Hydrogen (H) and Carbon (C).

• Carbon's Special Power: Carbon atoms are unique because they can link together in long chains, rings, and complex structures. This allows for millions of different organic compounds to exist.
• Covalent Bonding: In hydrocarbons, carbon and hydrogen atoms share electrons to form strong covalent bonds.

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1. The Homologous Series

Hydrocarbons are grouped into "families" called Homologous Series. Imagine a family where every member has similar traits, but each successive member is slightly bigger than the last.

Characteristics of a Homologous Series:

1. They share the same general formula.
2. Each member differs from the next by a fixed unit, usually \(CH_2\).
3. They have similar chemical properties (they react in similar ways).
4. They show a gradual trend (or gradation) in physical properties (like melting point and boiling point) as the chain length increases.

Quick Review: The longer the carbon chain, the higher the boiling point. Shorter chains are gases (like methane), and very long chains are solids (like waxes).

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2. Alkanes: Saturated Hydrocarbons

Alkanes are the simplest family of hydrocarbons. They are sometimes called "paraffins."

2.1 Structure and General Formula

Alkanes are saturated hydrocarbons. This means that every carbon atom is bonded to its maximum possible number of atoms using only single covalent bonds. There are no double or triple bonds.

Memory Aid Analogy: Think of saturation like a sponge that is completely full of water—it cannot hold any more atoms!

The General Formula for the alkane series is:

\(C_n H_{2n+2}\)

Where n is the number of carbon atoms.

Naming the First Four Alkanes (The Basics)

All alkanes end with the suffix -ane.

n (Carbons)	Name	Formula	State (Room Temp)
1	Methane	\(CH_4\)	Gas
2	Ethane	\(C_2 H_6\)	Gas
3	Propane	\(C_3 H_8\)	Gas
4	Butane	\(C_4 H_{10}\)	Gas

Mnemonic: Monkeys Eat Peeled Bananas (Methane, Ethane, Propane, Butane).

2.2 Isomerism (The Concept)

When the carbon chain gets longer (starting from butane, C4), it's possible to arrange the atoms in different structures.

Isomers are compounds that have the same molecular formula but different structural formulae. They have different physical properties (like boiling points).

Example: Butane (\(C_4 H_{10}\)) can exist as a straight chain (butane) or a branched chain (methylpropane). This concept shows how simple changes in structure can create a completely new chemical.

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3. Alkenes: Unsaturated Hydrocarbons

Alkenes are the next homologous series, and they are much more reactive than alkanes.

3.1 Structure and General Formula

Alkenes are unsaturated hydrocarbons because they contain at least one Carbon-Carbon double covalent bond (\(C=C\)).

Analogy: An unsaturated hydrocarbon is like a sponge that isn't full—it has space (the double bond) and is ready to soak up more atoms!

The General Formula for the alkene series is:

\(C_n H_{2n}\)

Notice they have two fewer hydrogen atoms than the corresponding alkane because of the double bond.

Note: The smallest alkene has 2 carbons (n must be 2 or more), as you need at least two carbons to form a double bond.

Naming the First Four Alkenes

All alkenes end with the suffix -ene.

n (Carbons)	Name	Formula
2	Ethene	\(C_2 H_4\)
3	Propene	\(C_3 H_6\)
4	Butene	\(C_4 H_8\)

Don't worry if this seems tricky at first! The crucial difference is the name ending (-ane vs. -ene) and the presence of the double bond.

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4. Chemical Reactions of Hydrocarbons

4.1 Combustion (Burning)

Both alkanes and alkenes burn easily because they contain stored energy. Combustion is simply burning the hydrocarbon in oxygen.

1. Complete Combustion (Plenty of Oxygen)

If there is plenty of oxygen, the hydrocarbon burns cleanly, producing only carbon dioxide and water.

Hydrocarbon + Oxygen \(\rightarrow\) Carbon Dioxide + Water

Example (Methane):
\(CH_4 + 2O_2 \rightarrow CO_2 + 2H_2O\)

2. Incomplete Combustion (Limited Oxygen)

If the supply of oxygen is limited, the burning is "dirty." This produces harmful products:

• Carbon Monoxide (CO): A highly toxic, odourless gas. It prevents blood from carrying oxygen effectively.
• Carbon (C): Soot or fine black particles, which can cause respiratory problems.

Common Mistake to Avoid: Always remember incomplete combustion is dangerous due to the production of Carbon Monoxide.

4.2 Addition Reactions (Specific to Alkenes)

The double bond (\(C=C\)) in alkenes makes them highly reactive. In an addition reaction, the double bond breaks open, allowing other atoms to "add on" to the carbon chain, forming a single-bonded, saturated molecule.

1. Halogenation (Adding Halogens, e.g., Bromine)

This reaction is the most important chemical test used to distinguish between saturated and unsaturated hydrocarbons.

• If you add bromine water (which is orange-brown) to an alkane, nothing happens. The solution remains orange-brown.
• If you add bromine water to an alkene, the addition reaction occurs quickly, the double bond breaks, and the bromine atoms attach. The orange-brown colour disappears (it is decolourised).

Alkene + Bromine \(\rightarrow\) Dibromoalkane (Colourless)

Did you know? This simple colour change test is crucial in industry and laboratories to identify unsaturated fats or oils!

2. Hydrogenation (Adding Hydrogen)

Hydrogen gas can be added across the double bond to turn an alkene into an alkane. This requires a Nickel catalyst and heating.

Alkene + Hydrogen \(\rightarrow\) Alkane

Real World Example: This process is used in the food industry to turn unsaturated liquid vegetable oils (containing C=C bonds) into saturated semi-solid fats, like margarine.

3. Hydration (Adding Steam/Water)

Steam (water) can be added across the double bond to produce an alcohol (a hydrocarbon derivative). This requires high temperature, high pressure, and a catalyst (often phosphoric acid).

Alkene + Steam \(\rightarrow\) Alcohol

Example: Ethene reacts with steam to form ethanol (\(C_2 H_5 OH\)), which is used in drinks and as a solvent.

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5. Crude Oil, Distillation, and Cracking

The source of most of the hydrocarbons we use is Crude Oil.

5.1 Crude Oil and Fractional Distillation

Crude oil is a complex, thick, black liquid found underground. It is a non-renewable resource and is a mixture of many different hydrocarbons (mostly alkanes) of various chain lengths.

Before we can use crude oil, we must separate this mixture into useful components, known as fractions. This is done using Fractional Distillation.

Step-by-Step: Fractional Distillation

1. Heating: Crude oil is heated to a high temperature (around \(350^\circ C\)) until most of it vaporises (turns into gas).
2. Column Entry: The hot vapour is pumped into the bottom of a tall tower called a fractionating column. The column is hotter at the bottom and cooler at the top.
3. Separation: As the vapours rise, they cool down. Each hydrocarbon fraction condenses back into a liquid when it reaches the level where the temperature is equal to its boiling point.
4. Collection:
   • Shorter chain molecules have low boiling points, so they continue to rise and are collected near the top (e.g., gases, petrol).
   • Longer chain molecules have high boiling points, so they condense quickly and are collected near the bottom (e.g., diesel, lubricating oil, bitumen).

Analogy: Imagine a building with many floors. Only light people (short chains) can run all the way to the top floor. Heavy people (long chains) condense (stop) on the lower, hotter floors.

5.2 Cracking

Fractional distillation produces large amounts of long-chain hydrocarbons (like heavy fuel oils), but the market demands more short-chain molecules, such as petrol (gasoline).

Cracking is the process of breaking down long-chain alkane molecules into smaller, more useful molecules.

Cracking produces two main things:

1. Shorter, more useful alkanes (e.g., petrol).
2. A short-chain alkene (which are highly valuable for making plastics/polymers).

Long Chain Alkane \(\rightarrow\) Shorter Alkane + Alkene

Cracking typically requires high temperature and a catalyst (catalytic cracking) or very high temperatures and pressure (thermal cracking).

Keep practising those formulae and reactions—you've got this!