Introduction to Alkenes
Welcome to the world of Alkenes! You’ve already met Alkanes, the "steady and stable" members of the hydrocarbon family. Alkenes are their more exciting, reactive cousins. In this chapter, we will explore why that little double bond makes such a huge difference, how they react with other chemicals, and how they are used to make the plastics we use every day.
Don't worry if organic chemistry feels like a puzzle at first—we’re going to break it down piece by piece!
1. Structure, Bonding, and Reactivity
Alkenes are unsaturated hydrocarbons. "Unsaturated" simply means they contain at least one carbon-carbon double bond (\( C=C \)). Because they have this double bond, they have fewer hydrogen atoms than an alkane with the same number of carbons.
The Nature of the Double Bond
The \( C=C \) double bond is more than just two lines on paper. It is a centre of high electron density. Think of it like a concentrated "cloud" of negative charge sitting between the two carbon atoms. Because electrons are negative, they naturally attract anything that is looking for electrons (positive things).
Why are Alkenes Reactive?
Because of that high electron density, alkenes are very attractive to electrophiles.
Memory Aid: An Electrophile is an "Electron-lover" (phile = lover). They are electron-pair acceptors that are attracted to the negative double bond.
Quick Review:
• Alkanes: Saturated (single bonds), relatively unreactive.
• Alkenes: Unsaturated (double bonds), highly reactive due to high electron density.
Key Takeaway: The \( C=C \) double bond is a "reactive hotspot" that attracts electron-seeking chemicals called electrophiles.
2. Addition Reactions of Alkenes
The most common type of reaction for an alkene is an electrophilic addition. In these reactions, the double bond "opens up" to allow new atoms to join the carbon atoms.
Testing for Unsaturation
How do we know if a liquid is an alkene or an alkane? We use Bromine Water (\( Br_2 \)).
1. Add orange bromine water to the sample.
2. If it is an alkene, the bromine water turns colourless (it decolourises).
3. If it is an alkane, it stays orange.
Electrophilic Addition Mechanisms
You need to know how alkenes react with three main things: Bromine (\( Br_2 \)), Hydrogen Bromide (\( HBr \)), and Sulfuric Acid (\( H_2SO_4 \)).
Step-by-Step: The General Mechanism
1. The electron-rich double bond attacks the positive part of the electrophile.
2. A bond forms between one of the carbons and the electrophile.
3. The other carbon is left with a positive charge—this is called a carbocation.
4. The remaining negative ion (like \( Br^- \)) attacks the positive carbocation to finish the molecule.
Common Mistake to Avoid: When drawing curly arrows, always start the arrow from the double bond or a lone pair, and point it to the atom or bond being formed. Never start an arrow from a positive charge!
Unsymmetrical Alkenes: Major and Minor Products
If you react \( HBr \) with an unsymmetrical alkene (like propene), you can get two different products. One is formed more often (the major product) than the other (the minor product).
This is decided by carbocation stability. Carbocations are more stable if they are surrounded by more alkyl groups (carbon chains).
• Tertiary (\( 3^\circ \)) carbocation: Most stable (connected to 3 carbons).
• Secondary (\( 2^\circ \)) carbocation: Moderately stable (connected to 2 carbons).
• Primary (\( 1^\circ \)) carbocation: Least stable (connected to 1 carbon).
Analogy: Think of the alkyl groups as "supportive friends" who help share the burden of the positive charge. The more friends (alkyl groups) the carbocation has, the happier (more stable) it is!
Did you know? This rule is often called Markovnikov's Rule. It basically says the hydrogen atom will usually join the carbon that already has the most hydrogens attached to it.
Key Takeaway: The major product is formed via the most stable carbocation intermediate.
3. Addition Polymers
Alkenes can join together in long chains to form addition polymers. This is how we make most of our plastics!
From Monomer to Polymer
The individual alkene molecules are called monomers. When they join up, they form a polymer.
• To name them, just put "poly" in front of the monomer name (e.g., ethene becomes poly(ethene)).
• Drawing: When drawing the repeating unit, change the double bond to a single bond, extend the "arms" out through the brackets, and add an '\( n \)' at the bottom right.
Properties and Uses
Addition polymers are unreactive because the main chain is made of strong, non-polar \( C-C \) and \( C-H \) bonds. This makes them great for food storage but bad for the environment because they don't biodegrade easily.
Poly(chloroethene), also known as PVC, is a very common polymer. On its own, it is hard and brittle (used for water pipes). However, we can add chemicals called plasticisers to it.
Analogy: Plasticisers act like a lubricant between the polymer chains, allowing them to slide over each other. This makes the PVC flexible enough to be used for electrical cable insulation or even "fake leather" clothing!
Intermolecular Forces
Polyalkenes are held together by van der Waals forces. Longer chains and chains that are packed closely together have stronger van der Waals forces, making the plastic stronger and harder.
Key Takeaway: Addition polymers are made by "opening" the double bonds of monomers to create long, unreactive chains.
4. Connections to Alcohols
It’s important to see how alkenes fit into the bigger organic chemistry picture. You can move between alkenes and alcohols quite easily!
1. Hydration (Making Alcohol):
Alkenes can react with steam (\( H_2O_{(g)} \)) in the presence of an acid catalyst (usually phosphoric acid) to form an alcohol. This is a common industrial way to make ethanol.
2. Elimination (Making Alkenes):
You can also go the other way! By heating an alcohol with a concentrated acid catalyst (like \( H_2SO_4 \) or \( H_3PO_4 \)), you can remove a water molecule to create an alkene. This is called a dehydration or elimination reaction.
Key Takeaway: Alkenes and alcohols are linked. Adding water (hydration) makes an alcohol; removing water (elimination) makes an alkene.
Final Quick Review Box
Key Points to Remember:
• Alkenes have a \( C=C \) bond with high electron density.
• They react via electrophilic addition.
• Use bromine water to test for them (orange to colourless).
• Tertiary carbocations are the most stable, leading to major products.
• Addition polymers are unreactive chains of monomers.
• Plasticisers make polymers like PVC more flexible.