Welcome to the World of Alkenes!
In our last topic, we looked at Alkanes—the fairly "chill" and unreactive members of the hydrocarbon family. Now, things are getting exciting! Alkenes are much more reactive, and they are responsible for everything from the smell of lemons to the plastic bottle sitting on your desk. Don't worry if organic chemistry feels like a new language at first; we will break it down step-by-step.
1. What Makes an Alkene?
Alkenes are unsaturated hydrocarbons.
- Hydrocarbon: A molecule made of only Carbon and Hydrogen.
- Unsaturated: This means they contain at least one carbon-carbon double bond (C=C).
The General Formula
For straight-chain alkenes with one double bond, the formula is: \( C_nH_{2n} \).
Example: If an alkene has 3 carbons (Propene), it will have \( 3 \times 2 = 6 \) hydrogens. \( C_3H_6 \).
The Double Bond: Sigma (\(\sigma\)) and Pi (\(\pi\))
The double bond isn't just "two lines." It's made of two different types of overlaps:
1. Sigma (\(\sigma\)) bond: This is the first bond formed by the direct overlap of orbitals. It's very strong.
2. Pi (\(\pi\)) bond: This is the second bond, formed by the sideways overlap of p-orbitals. This bond sits above and below the plane of the carbon atoms.
Analogy: Think of the \(\sigma\) bond as a firm handshake. The \(\pi\) bond is like an extra hug over the top. The \(\pi\) bond is easier to break than the \(\sigma\) bond, which is why alkenes are so reactive!
Quick Review: Alkenes have the general formula \( C_nH_{2n} \) and are reactive because of the \(\pi\) bond in their double bond.
2. Geometric Isomerism (E-Z Isomers)
In alkanes, carbon atoms can rotate freely. But in alkenes, that double bond acts like a lock. It prevents the atoms from spinning. This leads to Stereoisomerism.
When does it happen?
You get geometric isomers if:
1. There is restricted rotation (the C=C bond).
2. Each carbon in the double bond is attached to two different groups.
Naming them: The E-Z System
We use the Cahn-Ingold-Prelog (CIP) priority rules. We look at the atoms directly attached to the C=C carbons. The atom with the higher atomic number gets higher priority.
Z-Isomer (Zusammen): The high-priority groups are on the "Zame Zide" (Same side) of the double bond.
E-Isomer (Entgegen): The high-priority groups are on "E-part" (Opposite sides/Apart).
Did you know? We used to use cis-trans naming. We still use it if two of the groups are the same (like two hydrogens), but the E-Z system is better because it works even when all four groups attached to the double bond are different!
Key Takeaway: The double bond prevents rotation, creating E (opposite) and Z (same side) isomers based on the priority of attached groups.
3. How Alkenes React: Addition Reactions
Because the \(\pi\) bond is relatively weak and "sticks out," it attracts atoms that want electrons. Most alkene reactions are Addition Reactions—the double bond opens up, and new atoms are added to the carbons.
Reaction 1: Hydrogenation (Making Alkanes)
Reagents: Hydrogen gas (\( H_2 \)).
Conditions: Nickel catalyst, \( 150^\circ C \).
Product: An Alkane.
Real-world use: This is how liquid vegetable oils are turned into solid margarine!
Reaction 2: Halogenation (The Test for Unsaturation)
Reagents: Bromine (\( Br_2 \)) or Chlorine (\( Cl_2 \)).
Observation: If you add orange bromine water to an alkene, it turns colourless. This is the standard laboratory test for a C=C bond!
Product: A di-substituted halogenoalkane (e.g., 1,2-dibromoethane).
Reaction 3: Hydrogen Halides
Reagents: \( HBr \) or \( HCl \).
Product: A mono-substituted halogenoalkane.
Reaction 4: Hydration (Making Alcohols)
Reagents: Steam (\( H_2O_{(g)} \)).
Conditions: Acid catalyst (usually Phosphoric acid, \( H_3PO_4 \)).
Product: An Alcohol.
Reaction 5: Oxidation to Diols
Reagents: Acidified Potassium Manganate(VII) (\( KMnO_4 \)).
Observation: The purple solution turns colourless.
Product: A diol (a molecule with two -OH groups).
Key Takeaway: Alkenes react by adding molecules across the double bond. Bromine water turning colourless is the classic test for an alkene.
4. The Mechanism: Electrophilic Addition
This is the "how" of the reaction. Alkenes are attacked by electrophiles.
Key Term: Electrophile - An electron-pair acceptor. They are "electron lovers" attracted to the negative charge of the double bond.
Step-by-Step Mechanism for Ethene + HBr:
1. The electron-rich double bond repels the electrons in the H-Br bond, creating a dipole (\( H^{\delta+} - Br^{\delta-} \)).
2. The First Arrow: A curly arrow starts from the double bond and points to the \( H^{\delta+} \).
3. The Second Arrow: The bond between H and Br breaks; the arrow goes from the bond to the \( Br \).
4. The Intermediate: The H attaches to one carbon. The other carbon is now missing electrons and has a positive charge. This is called a Carbocation.
5. The Third Arrow: The \( :Br^- \) ion uses its lone pair to attack the positive carbocation.
Markownikoff's Rule (For Propene + HBr)
When \( HBr \) adds to an unsymmetrical alkene like propene, you can get two products. The "major" product is the one formed from the most stable carbocation.
- Primary carbocation: C+ attached to 1 alkyl group (Least stable).
- Secondary carbocation: C+ attached to 2 alkyl groups.
- Tertiary carbocation: C+ attached to 3 alkyl groups (Most stable).
Memory Trick: "The rich get richer." The Carbon that already has more Hydrogens is the one that gets the new Hydrogen from the \( HBr \).
Common Mistake: Make sure your curly arrows start exactly from the double bond or a lone pair, and end exactly on an atom!
5. Addition Polymers
Alkenes can join together in long chains to make polymers (plastics).
- Monomer: The small alkene molecule (e.g., ethene).
- Polymer: The long chain (e.g., poly(ethene)).
Drawing the Repeat Unit:
1. Change the double bond to a single bond.
2. Draw brackets around the two carbons.
3. Draw bonds extending outside the brackets.
4. Add a small 'n' at the bottom right.
6. Sustainability and the Environment
Polymers are great, but they are hard to get rid of because they are non-biodegradable.
How chemists help:
1. Developing biodegradable polymers: Plastics that can be broken down by bacteria.
2. Incineration with Care: Burning plastics for energy, but using "scrubbers" to remove toxic gases (like \( HCl \)) produced during the process.
Quick Review: Polymers are made by opening the double bonds of monomers. We must manage their disposal carefully to protect the environment.
Congratulations! You've just covered the essentials of Alkenes. Keep practicing those mechanisms—they are the key to mastering Organic Chemistry!