Welcome to the World of Alkenes!
Hello! Today, we are diving into the world of Alkenes. If you’ve ever wondered how we get plastic bags from oil, or why some fats are "unsaturated," you’re looking at the right chapter. Alkenes are hydrocarbons (compounds made of only carbon and hydrogen) that contain at least one Carbon-Carbon double bond (C=C).
Don't worry if Organic Chemistry feels like a different language at first. We will break it down piece by piece. Think of the double bond as a "reactive hotspot" that makes these molecules much more exciting than their cousins, the alkanes!
1. The Structure of Alkenes: Why are they special?
In an alkene, the carbon atoms at the double bond are sp² hybridised. This creates a specific shape and set of bonds:
• The Sigma (σ) Bond: This is a strong, single bond formed by the direct overlap of orbitals between the carbon atoms.
• The Pi (π) Bond: This is formed by the "sideways" overlap of p-orbitals. It sits above and below the plane of the carbon atoms. This bond is much weaker than the sigma bond and is very "electron-rich."
Analogy: Imagine two people holding hands. A sigma bond is like a firm handshake. A pi bond is like them trying to balance a tray of water between their forearms above the handshake. It’s easier to "break" or react with that tray of water than it is to break the handshake!
Key Structural Facts:
• Shape: The area around the C=C bond is planar (flat) with bond angles of approximately 120°.
• Restricted Rotation: Unlike single bonds, you cannot "twist" a double bond without breaking the pi bond. This leads to stereoisomerism (specifically cis-trans or E/Z isomerism).
• Unsaturated: Because they have a double bond, they have fewer hydrogens than the maximum possible. We call this being unsaturated.
Quick Takeaway: The pi bond is the reason alkenes react the way they do. It is a cloud of electrons just waiting for something "electron-seeking" to come along!
2. How to Make Alkenes (Production)
According to your syllabus, there are three main ways you need to know to create an alkene:
A. Elimination of HX from a Halogenoalkane
Reagents: Sodium hydroxide (\(NaOH\)) or Potassium hydroxide (\(KOH\)) dissolved in ethanol.
Conditions: Heat under reflux.
What happens: The "H" and the "X" (halogen) are "eliminated" from the molecule, leaving a double bond behind.
B. Dehydration of Alcohols
Dehydration means "removing water." We take an \(H\) and an \(OH\) off the alcohol molecule.
Option 1: Pass alcohol vapor over a heated Catalyst of Aluminum Oxide (\(Al_{2}O_{3}\)).
Option 2: Warm the alcohol with Concentrated Acid (like \(H_{2}SO_{4}\) or \(H_{3}PO_{4}\)).
C. Cracking of Alkanes
Large, useless alkane molecules from crude oil are broken down into smaller, useful alkanes and alkenes using high heat and a catalyst.
Key Takeaway: To make an alkene, you usually have to "take something away" (eliminate) from a saturated molecule.
3. The Big Reaction: Electrophilic Addition
Because the pi bond is a big cloud of negative electrons, it attracts Electrophiles ("electron-lovers"). These are species that are looking for electrons.
The Four Main Addition Reactions:
1. Hydrogenation (Adding \(H_{2}\)):
Reagent: \(H_{2}(g)\)
Conditions: Platinum (\(Pt\)) or Nickel (\(Ni\)) catalyst and heat.
Result: The alkene becomes an alkane.
2. Hydration (Adding Steam/Water):
Reagent: Steam (\(H_{2}O(g)\))
Conditions: Phosphoric(V) acid catalyst (\(H_{3}PO_{4}\)), high temperature, and high pressure.
Result: The alkene becomes an alcohol.
3. Adding Hydrogen Halides (Adding \(HX\)):
Reagent: \(HCl, HBr,\) or \(HI\) gas.
Conditions: Room temperature.
Result: Forms a halogenoalkane.
4. Halogenation (Adding \(X_{2}\)):
Reagent: \(Cl_{2}, Br_{2},\) or \(I_{2}\).
Result: Forms a dihalogenoalkane.
Did you know? This is the standard Test for Unsaturation. If you add orange Bromine Water to an alkene, it turns colorless immediately!
4. Markovnikov’s Rule: Which Carbon gets the Hydrogen?
When adding \(HBr\) to an asymmetrical alkene (like propene), the \(H\) can go to two different carbons. Where does it go?
Mnemonic: "The Rich Get Richer"
The Hydrogen atom will join the Carbon atom that already has the most Hydrogen atoms attached to it.
Why? It’s all about Carbocations!
During the reaction, a carbocation (a carbon with a positive charge) is formed. Some are more stable than others because of the Inductive Effect of alkyl (\(R\)) groups.
• Primary carbocation: Least stable (only 1 \(R\) group helping out).
• Secondary carbocation: More stable (2 \(R\) groups helping).
• Tertiary carbocation: Most stable (3 \(R\) groups helping).
Analogy: Imagine the positive charge is a heavy weight. Alkyl groups are like "supportive friends" who help carry the weight. The more friends you have (tertiary), the more stable you are!
Quick Review: Markovnikov's rule helps us predict the Major Product. The reaction goes through the most stable path!
5. Oxidation of Alkenes
Alkenes react with acidified Potassium Manganate(VII) (\(KMnO_{4}\)). The result depends on the temperature.
A. Cold, Dilute \(KMnO_{4}\) (Mild Oxidation)
The purple color of \(KMnO_{4}\) disappears, and a Diol (a molecule with two \(OH\) groups) is formed. This is another test for the C=C bond.
B. Hot, Concentrated \(KMnO_{4}\) (Harsh Oxidation)
This reaction is so strong it breaks the C=C bond entirely! This is useful for figuring out where the double bond was in a mystery molecule. Look at the fragments:
• If a carbon has 2 Hydrogens (\(=CH_{2}\)): It turns into Carbon Dioxide (\(CO_{2}\)).
• If a carbon has 1 Hydrogen (\(=CH-R\)): It turns into a Carboxylic Acid (\(R-COOH\)).
• If a carbon has 0 Hydrogens (\(=C-R_{2}\)): It turns into a Ketone (\(R-CO-R\)).
Key Takeaway: Cold = adding two \(OH\)s. Hot = cutting the molecule in half and adding oxygens!
6. Addition Polymerisation
Because of that reactive pi bond, alkenes can join together in long chains. This is how we make plastics!
• Monomer: The single alkene molecule (e.g., ethene).
• Polymer: The long chain (e.g., poly(ethene)).
To draw the repeat unit, simply change the double bond to a single bond and draw "extension bonds" out through the brackets.
Environmental Concerns:
Polymers are great because they are strong and unreactive, but that’s also the problem!
1. Non-biodegradable: They don't rot away, filling up landfills.
2. Harmful Combustion: If you burn them, they can release toxic gases like \(HCl\).
Final Encouragement
You’ve just covered the core of the Alkenes chapter! The most important things to practice are the Electrophilic Addition mechanism (with the curly arrows) and predicting oxidation products. Don't worry if the mechanisms look like "chemical spaghetti" at first; once you realize the electrons are just moving from where they are (the pi bond) to where they are needed (the electrophile), it all starts to make sense!