👋 Welcome to the Exciting World of Alkenes!
Hello future chemists! You have already mastered Alkanes, the saturated members of the organic family. Now, we are moving on to their more reactive cousins: the Alkenes.
Alkenes are incredibly important! They are the starting materials for creating many essential products, from plastics (like those used in water bottles) to industrial chemicals (like ethanol). Don't worry if the reactions look tricky—we will break them down step-by-step. Let's dive in!
1. Defining Alkenes: The Double Bond Difference
1.1 What Makes an Alkene?
Alkenes belong to the family of Hydrocarbons, meaning they contain only Hydrogen and Carbon atoms. What sets them apart from Alkanes is a special feature: a carbon-carbon double covalent bond (C=C).
Imagine Carbon atoms usually shaking hands once (a single bond). In an Alkene, two Carbon atoms hold hands twice! This extra bond makes the molecule much more reactive.
Key Terminology Check
- Hydrocarbon: A compound containing only carbon and hydrogen.
- Unsaturated: A term meaning the molecule contains at least one double or triple bond. Alkenes are unsaturated.
- Saturated: A term meaning the molecule contains only single bonds (like Alkanes).
🔥 Simple Trick: Alkenes are unsaturated because they could potentially hold more hydrogen atoms if the double bond were broken.
1.2 The General Formula
Alkenes form a homologous series, meaning they have a general formula and similar chemical properties.
The general formula for Alkenes is:
$$C_nH_{2n}$$
Where n is the number of carbon atoms. Notice how they have fewer hydrogens than Alkanes ($C_nH_{2n+2}$)? This is because the double bond takes up two bonding spots!
Alkanes: Single bonds (saturated), Formula \(C_nH_{2n+2}\)
Alkenes: Double bond (unsaturated), Formula \(C_nH_{2n}\)
2. Naming the Alkenes (Nomenclature)
Naming Alkenes follows the same system as Alkanes, but we use the ending -ene to show the presence of the C=C double bond.
| Carbons (n) | Prefix | Alkane Name | Alkene Name | Formula (\(C_nH_{2n}\)) |
|---|---|---|---|---|
| 1 | Meth- | Methane | (Doesn't exist as an Alkene) | N/A |
| 2 | Eth- | Ethane | Ethene | \(C_2H_4\) |
| 3 | Prop- | Propane | Propene | \(C_3H_6\) |
| 4 | But- | But- | Butene | \(C_4H_8\) |
Did you know? Ethene ($C_2H_4$) is the simplest and most common alkene. It is a natural plant hormone that helps ripen fruits!
3. The Key Reactions of Alkenes: Addition!
Alkenes are much more reactive than Alkanes. This is all down to the double bond. Since one of the bonds in the C=C pair is relatively weak, it breaks easily, allowing new atoms to join the molecule.
3.1 General Reaction Type: Addition
In an Addition Reaction, atoms are added across the double bond, breaking the C=C bond and turning the unsaturated Alkene into a saturated molecule (like an Alkane or an equivalent compound).
It's like breaking a double handshake (C=C) and forming two new, single handshakes with new partners!
3.2 Specific Addition Reactions
3.2.1 Addition of Hydrogen (Hydrogenation)
When an Alkene reacts with hydrogen gas ($H_2$), the double bond breaks, and a hydrogen atom adds to each carbon atom. This requires heat and a catalyst (usually nickel, Ni).
Alkene + Hydrogen \(\rightarrow\) Alkane
Example: Ethene reacts with hydrogen to form Ethane.
$$C_2H_4 (g) + H_2 (g) \xrightarrow{Ni, Heat} C_2H_6 (g)$$
Real World Use: Hydrogenation is used to turn unsaturated oils (liquid) into saturated fats (solid), such as in the manufacturing of margarine.
3.2.2 Addition of Halogens (Halogenation)
Halogens (like Chlorine or Bromine) add across the double bond easily, even at room temperature, and without a catalyst.
Alkene + Halogen \(\rightarrow\) Dihaloalkane
Example: Ethene reacts with Bromine ($Br_2$).
$$C_2H_4 (g) + Br_2 (aq) \rightarrow C_2H_4Br_2 (l)$$
3.2.3 Addition of Steam (Hydration)
Alkenes react with steam ($H_2O$) under specific industrial conditions (high temperature, high pressure, and a catalyst, usually phosphoric acid). This adds an -OH group and an -H group across the double bond.
Alkene + Steam \(\rightarrow\) Alcohol
Example: Ethene reacts with steam to produce Ethanol (an important alcohol).
$$C_2H_4 (g) + H_2O (g) \xrightarrow{Acid Catalyst} C_2H_5OH (l)$$
This is a major industrial route for making ethanol.
3.3 Combustion of Alkenes
Like all hydrocarbons, Alkenes burn (combust) in oxygen, releasing carbon dioxide and water. However, because Alkenes have a higher carbon content (relative to hydrogen) compared to Alkanes, they tend to undergo incomplete combustion more easily.
Important observation: When Alkenes burn, they usually produce a smoky flame (due to unburnt carbon soot), unlike the clean blue flame of Methane (an Alkane).
Alkenes are defined by addition reactions where the C=C bond breaks, allowing two atoms/groups to attach. This turns the molecule into a saturated compound.
4. The Diagnostic Test: Testing for Unsaturation
How can we prove that a hydrocarbon sample is an Alkene (unsaturated) and not an Alkane (saturated)? We use the addition reaction with Bromine water!
4.1 The Bromine Water Test
This is the most important chemical test you need to know for Alkenes.
Reagent: Aqueous Bromine (Bromine water).
Initial Colour: Orange-brown (or yellowish-brown).
The Process (What happens?):
- When Bromine water is added to an Alkene, the Bromine adds across the double bond.
- The Bromine ($Br_2$) is used up in the reaction.
- Since the Bromine is consumed, the characteristic orange-brown colour disappears (the solution becomes colourless).
If the sample is an Alkane: No reaction occurs, and the orange-brown colour of the Bromine water remains.
Summary of the Test
- Alkene present: Orange-brown colour decolourises (goes colourless) immediately.
- Alkane present: Orange-brown colour remains.
Students often forget that Bromine water is inherently coloured. The positive result for an alkene is the loss of colour, not the formation of a new colour.
5. Polymerisation: Making Giant Molecules
One of the most valuable reactions of Alkenes is their ability to join together repeatedly to form long chains called Polymers. This process is called polymerisation.
5.1 Monomers and Polymers
- Monomer: The small, reactive alkene molecule (like Ethene).
- Polymer: The very long chain molecule formed when many monomers link up.
Think of monomers as individual train carriages. Polymerisation is the process of linking thousands of these carriages together to form one giant train (the polymer).
5.2 How Polymerisation Works
Under high pressure, heat, and sometimes with a catalyst, the double bonds (C=C) of the alkene monomers break open. These carbons then form single bonds with neighbouring monomers, creating a long, saturated chain.
Example: Ethene (monomer) turns into Poly(ethene) (polymer).
$$n C_2H_4 \xrightarrow{Heat, Pressure, Catalyst} (C_2H_4)_n$$
The 'n' shows that many (thousands) of monomers join together.
5.3 Uses of Poly(alkenes)
The polymers formed from alkenes (often called plastics) are incredibly useful because they are generally unreactive, flexible, and durable.
- Poly(ethene): Used to make plastic bags, bottles, and sheeting.
- Poly(propene): Used for ropes, crates, and car parts.
You have now grasped the concept of unsaturation! The most important ideas here are the general formula ($C_nH_{2n}$), the addition mechanism, and the distinctive Bromine water test. Keep reviewing the reaction conditions, and you will ace this chapter! You've got this!