Chemistry (0620) | Alkenes

Understanding Alkenes: The Family with a Double Bond!

Hello future Chemists! In the world of Organic Chemistry, we just finished learning about the Alkanes (the 'safe' family with only single bonds). Now, get ready to meet the Alkenes, a much more *reactive* family!

This chapter is all about compounds that contain a special feature: the **carbon-carbon double bond**. This double bond is the secret behind why alkenes are so useful for making important materials like plastics (polymers).

Let's dive in and explore their structure, how they are made, and their unique chemical reactions!

1. Structure and Terminology (The Basics)

1.1 What Makes an Alkene an Alkene? (Core 11.5.1)

Alkenes are a type of **hydrocarbon** (meaning they contain only hydrogen and carbon atoms). They belong to their own homologous series.

• The key feature of an alkene molecule is the presence of at least one **carbon-carbon double covalent bond** (\(C=C\)).
• The simplest alkene is **Ethene** (\(C_2H_4\)).

1.2 Saturated vs. Unsaturated

This is a really important distinction you must remember:

• Saturated Hydrocarbons (Alkanes): These have only single bonds (\(C-C\)). They are 'full' of hydrogen atoms—they cannot hold any more. (Think of a saturated sponge that can't absorb more water.)
• Unsaturated Hydrocarbons (Alkenes): These have at least one **double bond** (\(C=C\)). The double bond can "break open" to accept more atoms. They are not 'full' of hydrogen atoms yet. (Think of an unsaturated sponge ready to absorb water.)

Quick Trick: Alkane has an 'A' for All Single bonds. Alkene has an 'E' for the double bond.

1.3 General Formula

Since alkenes have a double bond, they have fewer hydrogen atoms than their corresponding alkanes.

The general formula for non-cyclic alkenes is:
\(C_{n}H_{2n}\)

Example:

• If \(n=2\), the molecule is ethene: \(C_2H_{(2 \times 2)} = C_2H_4\).
• If \(n=3\), the molecule is propene: \(C_3H_6\).

2. Manufacture of Alkenes: Cracking (Core 11.5.2 & 11.5.3)

2.1 What is Cracking?

Petroleum (crude oil) mainly contains very large alkane molecules. These large molecules are not very useful as fuels and are not useful for making plastics. Cracking is the process used to break down these large, less useful alkanes into smaller, more valuable molecules.

The cracking process produces two main things:

1. Smaller, more useful **alkanes** (like petrol/gasoline).
2. **Alkenes** (which are vital as starting materials for polymers/plastics).

2.2 Why Cracking is Necessary (Reasons for Cracking)

We need cracking for two main reasons (Core 11.5.3):

1. To meet the high demand for smaller, more efficient fuels like gasoline/petrol.
2. To produce **alkenes** (like ethene and propene) which are essential chemical feedstocks for the plastic industry.

2.3 Conditions for Cracking (Core 11.5.2)

Cracking requires harsh conditions to break the strong C-C bonds:

• High temperature
• The presence of a **catalyst** (usually aluminum oxide or silicon dioxide).

Formula Example (A general representation):
A long alkane \(\rightarrow\) A shorter alkane + An alkene

\(C_{10}H_{22} \rightarrow C_{8}H_{18} + C_{2}H_{4}\)
(Decane, a large alkane, cracks into Octane, a smaller alkane fuel, and Ethene, an alkene for plastics)

3. Chemical Properties of Alkenes: Addition Reactions

Alkenes are much more chemically reactive than alkanes because of that vulnerable C=C double bond. The double bond easily breaks, allowing two new atoms or groups of atoms to attach to the carbon chain.

3.1 Defining Addition Reactions (Supplement 11.5.5)

The characteristic reaction of alkenes is an **addition reaction**.

• In an addition reaction, the double bond opens up.
• Atoms are added across the carbons that were double-bonded.
• Crucially, **only one product is formed**.

Analogy: Think of the double bond as a zipper that is half-open. You can easily pull the zipper open completely and add two new pieces (atoms) onto the chain without anything being thrown out (unlike substitution reactions in alkanes).

3.2 Addition Reactions (Supplement 11.5.6)

(a) Reaction with Hydrogen (Hydrogenation)

When hydrogen gas (\(H_2\)) is added to an alkene, it converts the alkene into a saturated **alkane**.

• Reactants: Alkene + Hydrogen gas
• Conditions: High temperature, **Nickel catalyst**
• Product: Alkane (only single bonds)

Example (Ethene to Ethane):

\(C_2H_4 + H_2 \rightarrow C_2H_6\)

Did you know? This reaction is used industrially to make margarine! Liquid vegetable oils (which contain unsaturated fats, similar to alkenes) are hardened by adding hydrogen to convert the double bonds into single bonds, making the fat solid at room temperature.

(b) Reaction with Steam (Hydration)

Steam (\(H_2O(g)\)) is added to an alkene to produce an **alcohol**. This is the main industrial method for making ethanol.

• Reactants: Alkene + Steam
• Conditions: High temperature (around 300°C), high pressure, and an **acid catalyst** (like phosphoric acid).
• Product: Alcohol (The \(-OH\) functional group is added.)

Example (Ethene to Ethanol):

\(C_2H_4 + H_2O(g) \rightarrow C_2H_5OH\)

The displayed formula of the product, Ethanol (\(CH_3CH_2OH\)), has a two-carbon chain. One carbon is bonded to three hydrogens and the other is bonded to two hydrogens and an -OH group.

(c) Reaction with Halogens (Halogenation)

Alkenes react quickly with halogens like chlorine or bromine. This reaction is the crucial test for unsaturation (see Section 4).

• Reactants: Alkene + Halogen (e.g., Bromine, \(Br_2\))
• Conditions: Room temperature, no catalyst needed.
• Product: Dihaloalkane (an alkane with two halogen atoms attached).

Example (Ethene + Bromine):

\(C_2H_4 + Br_2 \rightarrow C_2H_4Br_2\)

The product is 1,2-dibromoethane. The two bromine atoms attach themselves to the two carbons that were originally double-bonded.

4. The Test for Unsaturation (Core 11.5.4)

Since the double bond is so reactive, we can use a simple chemical test to determine if a hydrocarbon is saturated (an alkane) or unsaturated (an alkene).

4.1 The Aqueous Bromine Test

The test uses **aqueous bromine** (bromine dissolved in water), which is usually an orange-brown colour.

Step-by-step procedure:

1. Add a few drops of **orange-brown aqueous bromine** to the unknown hydrocarbon liquid or gas.
2. Observe the colour change.

Results:

• If the hydrocarbon is saturated (Alkane):
  There is **no reaction** in the dark. The bromine water remains **orange-brown**.
• If the hydrocarbon is unsaturated (Alkene):
  The bromine adds across the double bond. The colour of the bromine water is instantly **decolorized** (turns colourless). This happens very quickly, even without light.

Why does this happen?

The bromine molecule is added across the double bond. The bromine atoms are used up in the addition reaction, so the characteristic orange colour disappears.

\(Alkene + Aqueous \: Bromine \rightarrow Dihaloalkane \: (colourless)\)