Welcome to the World of Alkenes!
In your journey through Organic Chemistry, you’ve already met the Alkanes—the "quiet" hydrocarbons. Now, it's time to meet their more exciting cousins: Alkenes. While alkanes are held together by single bonds, alkenes have a double bond that changes everything. This double bond makes them reactive, useful, and the starting point for many things we use every day, like plastic bottles and clothing. Don't worry if Organic Chemistry feels like a new language; we'll take it one "word" at a time!
1. Structure, Bonding, and Reactivity
Alkenes are unsaturated hydrocarbons. But what does "unsaturated" actually mean? Think of it like a sponge. A "saturated" sponge can't hold any more water. A "saturated" hydrocarbon (alkane) has as many Hydrogen atoms as possible. An unsaturated hydrocarbon (alkene) has a Carbon-Carbon double bond (\( C=C \)), which means it has room to "soak up" more atoms in a reaction.
The Nature of the Double Bond
The \( C=C \) double bond isn't just "two single bonds." It is a center of high electron density. Because there are four electrons shared between the two Carbon atoms instead of two, that area of the molecule is very "negative."
Analogy: Imagine the double bond is a giant magnet for anything positive. Because it has so many electrons, it attracts particles called electrophiles (which literally means "electron-lovers").
Quick Review:
• Alkenes: Unsaturated hydrocarbons with at least one \( C=C \) bond.
• General Formula: \( \text{C}_n\text{H}_{2n} \) (for alkenes with one double bond).
• Reactivity: Much higher than alkanes because of the electron-rich double bond.
Key Takeaway: The double bond makes alkenes reactive because it is a "honey pot" of electrons that attracts electrophiles.
2. Stereoisomerism in Alkenes: \( E\text{–}Z \) Isomerism
Because of the \( C=C \) double bond, alkenes can exhibit a special type of stereoisomerism called \( E\text{–}Z \) isomerism.
Why Does \( E\text{–}Z \) Isomerism Occur?
Two conditions are required for \( E\text{–}Z \) isomerism:
1. Restricted rotation: There is no free rotation around the rigid \( C=C \) double bond.
2. Two different groups: Each carbon atom of the \( C=C \) double bond must be attached to two different atoms or groups of atoms.
Cahn-Ingold-Prelog (CIP) Priority Rules
To name \( E\text{–}Z \) isomers, we use the CIP priority rules:
1. Look at the two atoms directly bonded to each carbon of the double bond.
2. Assign priority based on atomic number (the higher the atomic number, the higher the priority).
3. If the highest-priority groups are on the same side of the double bond, it is the \( Z \) isomer ("Zame Zide" / zusammen).
4. If the highest-priority groups are on opposite sides across the double bond, it is the \( E \) isomer ("Enemy / Opposite" / entgegen).
Key Takeaway: Restricted rotation around \( C=C \) leads to \( E\text{–}Z \) stereoisomers, identified by comparing the atomic numbers of attached groups using CIP rules.
3. Addition Reactions: Adding to the Family
Most reactions involving alkenes are Electrophilic Addition reactions. In these reactions, the \( C=C \) double bond opens up, and new atoms are "added" to the Carbon atoms.
Testing for Alkenes (The Bromine Water Test)
This is a classic lab test you need to know. When you add orange bromine water (\( \text{Br}_2(aq) \)) to an alkene:
1. The bromine adds across the double bond.
2. The solution turns from orange to colourless.
Note: If you add it to an alkane, it stays orange!
Common Addition Reactions
Alkenes react with several key substances. You need to know the mechanisms for these:
• Hydrogen Bromide (\( \text{HBr} \)): Produces a halogenoalkane.
• Bromine (\( \text{Br}_2 \)): Produces a dihalogenoalkane.
• Sulfuric Acid (\( \text{H}_2\text{SO}_4 \)): Concentrated sulfuric acid adds across the double bond; adding water hydrolyses the intermediate to form an alcohol.
• Industrial Hydration (Steam + \( \text{H}_3\text{PO}_4 \)): Ethene reacts directly with steam in the presence of a phosphoric acid catalyst to produce ethanol industrially.
Step-by-Step: The Electrophilic Addition Mechanism
Don't let the "curly arrows" scare you! They just show where electrons are moving. Here is how \( \text{HBr} \) adds to Ethene:
1. The Attack: The electron-rich double bond "attacks" the \( \text{H} \) in \( \text{H-Br} \). The \( \text{H-Br} \) bond breaks, and the electrons go to the \( \text{Br} \).
2. The Carbocation: Now, one Carbon is bonded to the new \( \text{H} \), but the other Carbon is missing an electron! This makes it positive, called a carbocation.
3. The Final Bond: The \( \text{Br}^- \) ion (which now has an extra lone pair of electrons) attacks the positive carbocation.
4. Result: You now have Bromoethane!
Common Mistake: When drawing mechanisms, always start your curly arrow from the bond or a lone pair, and point it exactly at the atom it is attacking.
Key Takeaway: Electrophilic addition involves an electrophile attacking the double bond, creating a carbocation intermediate, which then reacts with a negative ion.
4. Unsymmetrical Alkenes: Major and Minor Products
When an alkene like Propene reacts with \( \text{HBr} \), the \( \text{H} \) and \( \text{Br} \) have a choice: which Carbon do they go to? This leads to two possible products: a major product (most common) and a minor product (rare).
The Rule of Carbocation Stability
Whether a product is major or minor depends on the stability of the carbocation intermediate. Alkyl groups (like \( \text{-CH}_3 \)) push electrons toward the positive Carbon, helping to stabilize it.
• Primary (\( 1^\circ \)) Carbocation: Positive Carbon is attached to 1 other Carbon. (Least stable)
• Secondary (\( 2^\circ \)) Carbocation: Positive Carbon is attached to 2 other Carbons. (More stable)
• Tertiary (\( 3^\circ \)) Carbocation: Positive Carbon is attached to 3 other Carbons. (Most stable)
Memory Aid: "The Rich Get Richer"
In addition reactions, the Hydrogen atom will usually join the Carbon atom that already has the most Hydrogens attached to it. This creates the most stable carbocation intermediate and leads to the major product.
Key Takeaway: Tertiary carbocations are more stable than secondary, which are more stable than primary. The major product comes from the most stable intermediate.
5. Addition Polymers
Alkenes are the "bricks" used to build giant chains called polymers (plastics). This process is called addition polymerization.
Monomers to Polymers
The small alkene molecule is called a monomer. When thousands of them join together, they form a polymer. During this process, the double bond "opens up" to link with the next molecule.
Naming Polymers: It's simple! Just put "poly" in front of the monomer name in brackets.
Example: Ethene becomes poly(ethene). Chloroethene becomes poly(chloroethene).
Properties and Uses
• Reactivity: Unlike the alkenes they are made from, addition polymers are saturated and very unreactive. This makes them great for storage (like food wrap) but bad for the environment because they don't biodegrade easily.
• PVC (Poly(chloroethene)): Rigid and used for water pipes. However, we can add chemicals called plasticisers to make it flexible for use in electrical cable insulation or aprons.
• Intermolecular Forces: Polyalkenes are long chains held together by Van der Waals forces. The longer the chains and the closer they pack, the stronger the plastic.
Did you know? The "non-stick" coating on your frying pan is a polymer called PTFE, made from tetrafluoroethene!
How to draw a repeating unit:
1. Change the \( C=C \) to a \( C-C \) single bond.
2. Draw brackets around the Carbons.
3. Extend the side bonds through the brackets.
4. Add a little '\( n \)' at the bottom right corner.
Key Takeaway: Addition polymers are formed when many alkene monomers join up. They are unreactive, and their properties can be changed using plasticisers.
Quick Chapter Summary
1. Structure: Alkenes have a reactive \( C=C \) double bond with high electron density.
2. Stereoisomerism: Restricted rotation leads to \( E\text{–}Z \) isomers, determined by CIP priority rules.
3. Test: Bromine water turns from orange to colourless.
4. Mechanism: Electrophilic addition involves a carbocation intermediate.
5. Stability: Tertiary carbocations are the most stable; this decides the major product.
6. Polymers: Alkenes polymerise to form long, unreactive chains used in plastics.
Don't worry if the mechanisms feel a bit strange at first! Practice drawing the arrows for Ethene + \( \text{HBr} \) a few times, and you'll find they follow the same pattern every time. You've got this!