Welcome to the World of Alkenes!
In this chapter, we are moving from the relatively "quiet" alkanes to the much more exciting alkenes. While alkanes are the stable foundations of organic chemistry, alkenes are the "doers." Because of their unique double bond, they can be transformed into everything from the plastic in your phone to the medicines in your cabinet. Don't worry if organic chemistry feels like a new language at first—we will break it down step-by-step!
1. Structure and Bonding: The "High-Five" Bond
Alkenes are unsaturated hydrocarbons. This simply means they contain at least one carbon-to-carbon double bond \( (C=C) \). Unlike the single bonds in alkanes, this double bond is made of two different parts:
The \(\sigma\)-bond (Sigma bond)
This is a single covalent bond formed by the direct overlap of orbitals between the carbon atoms. Think of this as a strong handshake directly between two people. It allows the atoms to rotate freely.
The \(\pi\)-bond (Pi bond)
This is the second part of the double bond. It is formed by the sideways overlap of adjacent p-orbitals above and below the line of the carbon atoms. Analogy: If the \(\sigma\)-bond is a handshake, the \(\pi\)-bond is like those two people trying to give each other a double high-five (one high, one low) at the same time. Because they are "linked" above and below, they can no longer rotate. This is called restricted rotation.
Shape and Angle
Because there are three areas of electron density around each carbon in the double bond, they repel each other as much as possible. This creates a trigonal planar shape with a bond angle of approximately \(120^\circ\).
Quick Review:
• Alkenes = Unsaturated (have a double bond).
• Double bond = 1 \(\sigma\)-bond + 1 \(\pi\)-bond.
• \(\pi\)-bond prevents the bond from twisting (restricted rotation).
• Shape: Trigonal Planar (\(120^\circ\)).
Key Takeaway: The \(\pi\)-bond is the "reactive" part of the molecule because the electrons are sitting outside the main body of the bond, making them easy pickings for other chemicals!
2. Stereoisomerism: Left, Right, Up, Down
Because that \(\pi\)-bond locks the molecule in place, we get stereoisomers: molecules with the same structural formula but a different arrangement of atoms in space.
E/Z Isomerism
For a molecule to have E/Z isomerism, it must have:
1. A \(C=C\) double bond (restricted rotation).
2. Two different groups attached to each carbon of the double bond.
How to tell them apart (CIP Priority Rules):
We look at the atomic number of the atoms directly attached to the double bond. The higher the atomic number, the higher the 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 Enemies (opposite sides).
Cis-Trans Isomerism
This is a special case of E/Z isomerism. We use "cis" (same side) and "trans" (opposite sides) specifically when one of the groups on each carbon is the same (usually Hydrogen).
Common Mistake to Avoid: If one of the carbons in the double bond has two identical groups (e.g., two hydrogens), it cannot have E/Z isomers. Always check each carbon separately!
3. Reactivity and Addition Reactions
Alkenes are much more reactive than alkanes. Why? Because the \(\pi\)-bond has a lower bond enthalpy (it is weaker) than the \(\sigma\)-bond and is easily broken.
Alkenes undergo Addition Reactions, where the \(\pi\)-bond breaks and new atoms are added to the carbons. Here are the four you need to know:
1. Hydrogenation: Adding \(H_2\).
• Condition: Nickel catalyst, \(150^\circ C\).
• Product: An alkane. (This is how margarine is made from vegetable oil!)
2. Halogenation: Adding \(Cl_2\) or \(Br_2\).
• Product: A dihaloalkane.
• Test for Unsaturation: When you add orange bromine water to an alkene, it turns colourless. This is the classic lab test to prove a double bond is present.
3. Addition of Hydrogen Halides: Adding \(HCl\) or \(HBr\).
• Product: A haloalkane.
4. Hydration: Adding steam (\(H_2O\)).
• Condition: Phosphoric acid catalyst (\(H_3PO_4\)).
• Product: An alcohol. (This is a major industrial way to make ethanol.)
Key Takeaway: In all these reactions, the double bond "opens up" like a gate to let new atoms in.
4. The Mechanism: Electrophilic Addition
How does the reaction actually happen? We use "curly arrows" to show the movement of electron pairs.
What is an Electrophile?
An electrophile is an electron-pair acceptor. Since the double bond is a cloud of negative electrons, it attracts things that are positive or "electron-hungry."
Step-by-Step Mechanism (e.g., \(HBr\) adding to Ethene):
1. The electron-rich \(\pi\)-bond is attracted to the \(\delta+\) Hydrogen in \(HBr\). A curly arrow goes from the double bond to the \(H\).
2. The \(H-Br\) bond breaks (heterolytic fission), and both electrons go to the \(Br\).
3. A new bond forms between one Carbon and the Hydrogen. The other Carbon is left with a positive charge—this is called a carbocation.
4. The \(Br^-\) ion (now with a lone pair) is attracted to the positive carbocation. A curly arrow goes from the lone pair on \(Br^-\) to the \(C^+\).
5. Result: Bromoethane!
Memory Aid: Curly arrows always start at a source of electrons (a bond or a lone pair) and point to where the electrons are going.
5. Markownikoff’s Rule: "The Rich Get Richer"
When adding \(H-X\) to an unsymmetrical alkene (like propene), you can get two different products. Which one forms most?
Markownikoff’s Rule: The Hydrogen atom will attach itself to the Carbon that already has the most Hydrogen atoms attached to it.
Analogy: Think of it like a "H-popularity contest." The Hydrogen wants to go where its friends already are.
Why does this happen? (Carbocation Stability)
The reaction follows the path that creates the most stable intermediate. Carbocations are stabilized by alkyl groups (methyl, ethyl, etc.) because they "push" electrons toward the positive charge.
• Tertiary (\(3^\circ\)): Most stable (connected to 3 carbons).
• Secondary (\(2^\circ\)): Quite stable (connected to 2 carbons).
• Primary (\(1^\circ\)): Least stable (connected to 1 carbon).
Key Takeaway: The major product is formed via the most stable carbocation.
6. Addition Polymers: Making Chains
Alkenes can join together in long chains to form addition polymers. The double bond "opens" and connects to the next molecule.
• Monomer: The single alkene unit (e.g., ethene).
• Repeat Unit: The specific arrangement of atoms that repeats. (Always draw this with square brackets and an '\(n\)' outside).
• Polymer: The long chain (e.g., poly(ethene)).
Did you know? Poly(ethene) is the most common plastic in the world, used for everything from carrier bags to shampoo bottles!
Sustainability and the Environment
Plastics are useful but stay in the environment for hundreds of years. We manage waste in several ways:
• Combustion: Burning for energy (but we must remove toxic gases like \(HCl\) produced from burning PVC).
• Feedstock Recycling: Breaking polymers back down into monomers to make new plastics.
• Biodegradable/Photodegradable Polymers: New plastics designed to break down naturally in sunlight or by bacteria.
Quick Review:
• Monomer \(\rightarrow\) Polymer.
• Change the \(C=C\) to a \(C-C\) and add "tentacles" out the sides for the repeat unit.
• Recycling and biodegradable options help reduce the environmental impact of plastic waste.
Final Encouragement: You've just covered the core of alkene chemistry! If the mechanisms feel tough, try drawing them out three times—it's all about muscle memory. You’ve got this!