1. Structure, Bonding and Stereoisomerism
Alkenes are unsaturated hydrocarbons containing at least one carbon-carbon double bond (\(\text{C}=\text{C}\)):
- \(\sigma\) (sigma) bond: Formed by the direct end-on overlap of hybridized orbitals along the internuclear axis. Strong and stable.
- \(\pi\) (pi) bond: Formed by the sideways overlap of adjacent unhybridized \(p\) orbitals above and below the carbon-carbon plane. The \(\pi\) bond prevents rotation about the double bond and has high electron density, making alkenes open to attack by electrophiles.
- Trigonal Planar Geometry: Each carbon in the double bond is surrounded by 3 bonding regions, producing a planar arrangement with bond angles of \(120^\circ\).
E/Z Stereoisomerism
Restricted rotation around the \(\text{C}=\text{C}\) bond gives rise to \(E/Z\) isomerism when each carbon atom of the double bond is bonded to two different groups:
- Cahn-Ingold-Prelog (CIP) Priority Rules: Assign priority to atoms directly attached to each double-bonded carbon based on their atomic number (\(Z\)). Higher atomic number = higher priority.
- Z-isomer (zusammen / together): High priority groups are on the same side of the double bond plane.
- E-isomer (entgegen / opposite): High priority groups are on opposite sides of the double bond plane.
2. Electrophilic Addition Mechanisms
An electron-pair acceptor. Attracted to regions of high electron density, such as the \(\pi\) cloud of a \(\text{C}=\text{C}\) double bond.
| Electrophile | Reagent & Conditions | Intermediate | Product |
|---|---|---|---|
| HBr / HCl | Room temperature, gaseous or concentrated aqueous | Carbocation | Halogenoalkane |
| Bromine (\(\text{Br}_2\)) | Aqueous bromine water, room temperature | Carbocation | Vicinal dihalogenoalkane (turns orange bromine water colourless) |
| Conc \(\text{H}_2\text{SO}_4\) | Cold concentrated sulfuric acid | Carbocation | Alkyl hydrogensulfate (hydrolysed by warm water to form alcohol) |
3. Carbocation Stability and Markovnikov's Rule
When an unsymmetrical alkene (such as propene) reacts with an unsymmetrical electrophile (such as \(\text{HBr}\)), two different carbocation intermediates can form:
Carbocation Hierarchy
\[ \text{Tertiary } (3^\circ) > \text{Secondary } (2^\circ) > \text{Primary } (1^\circ) \]
Positive Inductive Effect: Alkyl groups are electron-releasing. They push electron density towards the positively charged carbon, dispersing the positive charge and stabilising the carbocation.
Markovnikov's Rule
In electrophilic addition to an unsymmetrical alkene, the hydrogen atom attaches to the carbon that already has the greater number of hydrogen atoms.
The reaction proceeds via the more stable carbocation intermediate, yielding the major product.
Propene (\(\text{CH}_3\text{CH}=\text{CH}_2\)) can add \(\text{H}^+\) in two ways:
- Route A: \(\text{H}^+\) adds to \(\text{CH}_2\), forming a secondary carbocation (\(\text{CH}_3\text{CH}^+\text{CH}_3\)) attached to two electron-releasing methyl groups.
- Route B: \(\text{H}^+\) adds to \(\text{CH}\), forming a primary carbocation (\(\text{CH}_3\text{CH}_2\text{CH}_2^+\)) attached to only one alkyl group.
The secondary carbocation is more stable due to greater positive inductive charge dispersal. Attack by \(\text{Br}^-\) produces 2-bromopropane as the major product (1-bromopropane is the minor product).
4. Addition Polymers
Alkenes undergo addition polymerisation under high pressure and catalyst conditions. The \(\pi\) bond breaks, and thousands of monomer units join to form long saturated polymer chains.
Environmental Inertness
Addition polymers (such as poly(ethene) and poly(chloroethene)) contain only strong, non-polar \(\text{C}-\text{C}\) and \(\text{C}-\text{H}\) bonds. They are chemically inert and non-biodegradable, creating long-term landfill disposal challenges. Waste polymer management relies on sorting, mechanical recycling, feedstock recycling, and controlled incineration for energy recovery.
5. Practice Questions
Repeating Unit: \(-[\text{CH}(\text{CH}_3)-\text{CH}_2]_n-\) (1 mark)
Explanation:
- The polymer chain contains only strong, saturated \(\text{C}-\text{C}\) and \(\text{C}-\text{H}\) bonds. (1 mark)
- The bonds are non-polar and lack electrophilic or nucleophilic centres, so they cannot be broken down by biological organisms or chemical hydrolysis. (1 mark)