Alkenes: properties, reactions, mechanisms
Alkenes: Properties, Reactions & Mechanisms
What are Alkenes?
Alkenes are hydrocarbons that contain at least one carbon–carbon double bond. Think of the double bond as a handshake between two carbon atoms, giving them a “flexible” connection that can react in many ways.
- General formula: $\ce{C_nH_{2n}}$
- Unsaturated: they have fewer hydrogen atoms than alkanes.
- Common examples: ethene ($\ce{C2H4}$), propene ($\ce{C3H6}$), butene ($\ce{C4H8}$).
Key Properties
- Reactivity – the double bond is a site of high electron density, making alkenes good electrophilic addition substrates.
- Geometric Isomerism – cis/trans (or E/Z) configurations arise because the two substituents on each carbon cannot rotate freely.
- Polarity – the C=C bond is slightly polar, but overall alkenes are non‑polar.
Exam tip: Remember that the presence of a double bond means the alkene can undergo addition reactions, not substitution.
Common Reactions
| Reaction Type | Typical Reagents | General Mechanism |
|---|---|---|
| Electrophilic Addition | $\ce{HBr}$, $\ce{H2O}$, $\ce{H2SO4}$, $\ce{Cl2}$ | Electrophile adds to one carbon, forming a carbocation; nucleophile attacks. |
| Hydroboration‑Oxidation | $\ce{BH3}$, $\ce{H2O2}$, $\ce{NaOH}$ | Anti‑syn addition giving anti‑Markovnikov alcohol. |
| Polymerisation | Radical initiator (e.g., $\ce{AIBN}$), cationic initiator (e.g., $\ce{BF3}$) | Chain growth via radical or cationic intermediates. |
| Oxidation | $\ce{KMnO4}$, $\ce{NaOCl}$ | Cleavage of the double bond to give diols or carboxylic acids. |
Exam tip: When you see a question about “addition of $\ce{HBr}$ to $\ce{C2H4}$”, write the mechanism step‑by‑step: (1) protonation, (2) carbocation formation, (3) nucleophilic attack.
Mechanism Example: Hydrohalogenation of Ethene
Step 1: Protonation of the double bond by $\ce{HBr}$.
$$\ce{CH2=CH2 + HBr -> [CH3-CH2-CH2^+]}$$
Step 2: Bromide ion attacks the more stable (secondary) carbocation.
$$\ce{[CH3-CH2-CH2^+] + Br^- -> CH3-CH2-CH2Br}$$
Key point: Markovnikov’s rule – the proton adds to the carbon with more hydrogens.
Exam tip: Highlight the carbocation intermediate and note why it is the most stable.
Exam Tip Box
When answering reaction questions:
- Write the reactants clearly.
- Show the intermediate (carbocation, radical, etc.).
- Indicate the product with proper stereochemistry.
- Use arrows to show electron flow.
Remember: “If you can’t draw the mechanism, you can’t answer the question.” 😊
Geometric Isomerism Quick Reference
| Isomer | Notation | Example |
|---|---|---|
| Cis | $\ce{(Z)}$ | $\ce{CH3-CH=CH-CH3}$ (both methyls on same side) |
| Trans | $\ce{(E)}$ | $\ce{CH3-CH=CH-CH3}$ (methyls on opposite sides) |
Exam tip: When a question asks for the “most stable isomer”, think about steric hindrance – trans (E) is usually more stable.
Common Mistakes to Avoid
- Forgetting to show the carbocation intermediate in addition reactions.
- Mixing up Markovnikov vs. anti‑Markovnikov additions.
- Ignoring stereochemistry in elimination reactions.
- Assuming all alkenes behave the same – remember that substitution patterns affect reactivity.
Keep these in mind and you’ll ace the questions! 🚀
Revision
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