Welcome to the World of Alkenes!
In this chapter, we are moving from the relatively "quiet" alkanes to the much more exciting alkenes. If organic chemistry was a city, alkanes would be the sturdy brick buildings, while alkenes would be the busy construction sites where all the action happens! Alkenes are essential because they allow us to create everything from the plastic in your phone to the antifreeze in a car engine. Let's dive in!
1. Structure, Bonding, and Reactivity
Alkenes are unsaturated hydrocarbons. But what does "unsaturated" actually mean? While alkanes are "full" of hydrogen (saturated), alkenes contain at least one carbon-carbon double bond \( (C=C) \). This double bond is the "hotspot" of the molecule.
The Double Bond: A Center of High Electron Density
The double bond consists of a high concentration of electrons. In chemistry, electrons are negative, and they act like a magnet for anything that loves negative charge. We call these "electron-lovers" electrophiles.
Quick Review:
• Alkenes: Hydrocarbons with \( C=C \) bonds.
• General Formula: \( C_nH_{2n} \).
• Reactivity: Much higher than alkanes because of the high electron density in the double bond.
2. Addition Reactions: How Alkenes React
Because the double bond is "eager" to react, alkenes undergo electrophilic addition. Imagine the double bond as two people holding both hands. To grab something new, they must let go of one pair of hands (the second bond) to reach out and catch a new atom.
A. Reaction with Bromine \( (Br_2) \)
This is the classic "Test for Unsaturation."
• The Test: Add orange bromine water to a liquid.
• The Result: If it's an alkene, the solution turns colorless.
• The Logic: The \( Br_2 \) molecule adds across the double bond, breaking it and forming a colorless dibromoalkane.
B. Reaction with Hydrogen Bromide \( (HBr) \)
When \( HBr \) reacts with an unsymmetrical alkene (like propene), we can get two different products. This is where students sometimes get confused, but don't worry! There is a simple rule.
The "Rich Get Richer" Rule (Markownikoff’s Rule):
In the addition of \( HX \) to an alkene, the Hydrogen atom \( (H) \) usually attaches to the Carbon atom that already has the most hydrogens attached to it.
Why does this happen? Carbocation Stability!
During the reaction, a carbocation (a carbon with a positive charge) is formed as an intermediate.
• Tertiary (\( 3^\circ \)) carbocations: Most stable (surrounded by 3 alkyl groups).
• Secondary (\( 2^\circ \)) carbocations: Moderately stable.
• Primary (\( 1^\circ \)) carbocations: Least stable.
Analogy: A carbocation is like a person carrying a heavy box. It’s much easier to stay stable if you have more friends (alkyl groups) around you to help share the load!
C. Reaction with Sulfuric Acid \( (H_2SO_4) \)
Alkenes react with concentrated sulfuric acid at room temperature to form alkyl hydrogensulfates. If you then add water and warm it, you get an alcohol. This is an industrial way to make alcohols!
Key Takeaway: Alkenes react by electrophilic addition. The stability of the carbocation intermediate determines which product is "Major" (more likely) and which is "Minor."
3. Addition Polymers
Alkenes are the "LEGO bricks" of the chemical world. Through addition polymerization, thousands of alkene molecules (monomers) join together to form long chains called polymers.
How to draw a repeating unit:
1. Change the \( C=C \) to a \( C-C \) bond.
2. Draw brackets around the two carbons.
3. Extend the "empty" bonds through the brackets.
4. Put an '\( n \)' at the bottom right.
Properties and Uses:
• Unreactivity: Polymers are very unreactive because they are now saturated (like alkanes) and have very strong \( C-C \) and \( C-H \) bonds. This makes them great for storage but bad for the environment (they don't biodegrade!).
• PVC (Poly(chloroethene)): Used for pipes and window frames. It is normally hard and brittle, but we can add plasticisers to make it flexible for things like aprons or cable insulation.
• Intermolecular Forces: Polyalkenes are held together by Van der Waals forces. Longer, straighter chains have stronger forces and make stronger plastics.
Did you know? The plastic bags you use are likely made of poly(ethene), which is just thousands of ethene molecules linked in a chain!
4. Epoxyethane: A Special Case
Oxford AQA focuses on a specific, very reactive molecule called epoxyethane \( (C_2H_4O) \). It looks like a triangle with an oxygen atom at the top.
Why is it so reactive?
The "triangle" shape is very strained. The bond angles are forced to be \( 60^\circ \), but the atoms would much rather be at larger angles. This ring strain makes the molecule "pop" open very easily when it reacts.
Production:
It is made by the partial oxidation of ethene using a silver catalyst:
\( CH_2=CH_2 + \frac{1}{2}O_2 \rightarrow C_2H_4O \)
Important Reactions:
1. With Water: Forms ethane-1,2-diol (used as antifreeze in cars).
2. With Alcohols: Forms molecules used to make surfactants (detergents and soaps).
Key Takeaway: Epoxyethane is highly reactive due to ring strain. Its products are economically vital for the automotive and cleaning industries.
5. Making Alkenes: Elimination from Alcohols
We can also go backwards! If we take an alcohol and remove a water molecule (dehydration), we get an alkene. This is an elimination reaction.
• Conditions: Concentrated sulfuric acid or phosphoric acid catalyst.
• Importance: This allows us to make plastics from plants (ethanol from fermentation) rather than just relying on crude oil!
Summary Quick-Check
Common Mistakes to Avoid:
• Arrows: In mechanisms, curly arrows must start from a lone pair or a bond, and end exactly where the electrons are going.
• Test for Unsaturation: Remember that bromine water turns colorless, not "clear" (water is clear, but it is also colorless!).
• Repeating Units: Never draw a double bond inside the brackets of a polymer repeating unit.
Mnemonic for Carbocations:
"Tertiary is Top" (Most stable).
Don't worry if the mechanisms seem like a lot of arrows at first. Just remember: the arrows show the "flow" of electrons from where they are (the double bond) to where they want to be (the electrophile). You've got this!