Chapter 10: Alkenes
Chapter 10: Alkenes
Chapter 10
Alkenes
Understanding the Essence of Alkenes:
The Carbon-Carbon Double Bond
Most chemical and physical properties of Alkenes can be attributed to it's signature
carbon-carbon double bond. Hence it is essential to understand how the bond is formed.
Each C atom of the double bond will form a bond each with 3 other atoms. Hence the molecule has a trigonal planar shape with respect to each C atom of the double bond, and has a bond angle of 120°. As we have learnt in alkanes, the C atoms usually form 4 bonds with 4 other atoms. But in alkenes, one s orbital is combined with two p orbitals forming three hybrid sp² orbitals, which are used to form three 𝛔-bonds.
The unhybridized 2p orbital is left for π-bonding.
Hence the sp² hybrid orbital of one carbon atom ‘head-on’ overlaps with the sp² hybrid orbital of another carbon atom to form a C-C 𝛔-bond. The unhybridized 2p orbital of the same carbon atom ‘side-ways’ overlaps with the unhybridized 2p orbital of the other carbon atom to form the C=C π-bond.
Preparation of alkenes (how to obtain alkenes from other organic compounds) :
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Elimination |
|---|
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Elimination of H₂O from alcohols: REAGENTS AND CONDITIONS : Excess concentrated H₂SO₄ at 170°C 
Note: the “H” and “OH” to be eliminated MUST be on adjacent carbons
- Reagents and conditions for industrial method : Al₂O₃ catalyst, 350°C
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Elimination of HX from halogenoalkanes (where X is a halide, ie Cl, Br etc): REAGENTS AND CONDITIONS: Ethanolic NaOH, heat 
Note: the “H” and “X” to be eliminated MUST be on adjacent carbon atoms |
The primary chemical reaction that alkenes undergo is electrophilic addition. This is because the high electron density in the π bond of the C=C would react with the electrophiles (electron-seeking particles). Hence, this results in electrophilic addition reactions. There are many different reagents and conditions for electrophilic addition reactions that would result in different types of products formed. The table below highlights such:
| Reagents and conditions | Products |
|---|---|
| Cl₂ / Br₂ / I₂ in CCl₄ in the dark | Di-halogenoalkane |
| Cl₂ (aq) / Br₂ (aq ) / I₂ (aq) | Halohydrin (haloalcohol) |
| HCl (g) / HBr (g) / HI(g) | Halogenoalkane |
| H₂O (g), H₃PO₄ (aq), high temperature | Alcohol |
| Reagents and conditions |
|---|
| Cl₂ / Br₂ / I₂ in CCl₄ in the dark |
| Cl₂ (aq) / Br₂ (aq ) / I₂ (aq) |
| HCl (g) / HBr (g) / HI(g) |
| H₂O (g), H₃PO₄ (aq), high temperature |
| Product |
|---|
| Di-halogenoalkane |
| Halohydrin (haloalcohol) |
| Halogenoalkane |
| Alcohol |
Name of mechanism: Electrophilic addition
Checklist when drawing electrophilic addition mechanism
❏ Name of mechanism
❏ Steps in sequential order (Attack of electrophile by π electrons on C=C, attack of C⁺ by
nucleophile to form product)
❏ Curly full arrows (from C=C to electrophile)
❏ Labels (slow, fast, lone pair of electrons on X⁻, δ⁻,δ⁺
❏ If there are more than one possible product, always draw the major product
The Markovnikov’s Rule
When electrophilic additions occur in an asymmetrical alkene, the major product of the reaction involves the formation of the most stable carbocation as an intermediate. Students are reminded to always check the stability of the carbocation first before drawing the final product.
The stability of carbocations is as such:
A carbocation bonded to more alkyl (‘R’) groups is more stable as alkyl groups are electron-donating groups, which disperses the positive charge on the carbon atom. Hence, this stabilises the carbocation.
Worked Example 1:
Draw the major product in the reaction of propene with HBr.
Solution:
Firstly, students have to be reminded that in the reaction of propene with HBr, two products are
possible as propene is an asymmetrical alkene.
Hence, B is the major product as it is formed through a more stable carbocation intermediate.
Other than electrophilic addition, alkenes can undergo both oxidation and reduction reactions.
The table below will illustrate both reactions clearly:
| Oxidation | Reduction |
|---|---|
|
Mild oxidation
REAGENTS AND CONDITIONS: KMnO₄
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REAGENTS AND CONDITIONS:
|
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Strong oxidation
REAGENTS AND CONDITIONS: KMnO₄
|
| Oxidation |
|---|
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Mild oxidation
REAGENTS AND CONDITIONS: KMnO₄
|
|
Strong oxidation
REAGENTS AND CONDITIONS: KMnO₄
|
| Reduction |
|---|
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REAGENTS AND CONDITIONS:
|
For strong oxidation, products formed on each side of the C=C depend on the different substituents attached. Students are expected to correctly draw the products for strong oxidation.
There are a few general rules to follow:
- Add O to every C=C broken to get C=O on both sides
- Add O to every H directly bonded to the C=C
- When an alkaline medium is used, any acidic products formed (eg carboxylic acid,
carbon dioxide) will further react with NaOH in a neutralisation reaction
Distinguishing test for alkenes
This section of organic chemistry is extremely easy to secure marks as students would just need to follow a few basic steps to easily guarantee 2-3 marks in the examinations. Below is a list of tests that can be used to determine if a particular organic compound is an alkene.
| Reagents and conditions | Observations |
|---|---|
| Br₂ (aq) | Orange solution decolourised |
| KMnO₄ (aq), NaOH (aq), cold | Purple KMnO₄ decolourised and brown precipitate of MnO₂ |
| KMnO₄ (aq), H₂SO₄ (aq), heat | Purple KMnO₄ decolourised If terminal alkene (=CH₂) is present, effervescence seen and gas (CO₂) evolved forms a white precipitate in Ca(OH)₂ (aq) |
| Reagents and conditions |
|---|
| Br₂ (aq) |
| KMnO₄ (aq), NaOH (aq), cold |
| KMnO₄ (aq), H₂SO₄ (aq), heat |
| Observations |
| Orange solution decolourised |
| Purple KMnO₄ decolourised and brown precipitate of MnO₂ |
| Purple KMnO₄ decolourised If terminal alkene (=CH₂) is present, effervescence seen and gas (CO₂) evolved forms a white precipitate in Ca(OH)₂ (aq) |
Checklist when drawing electrophilic addition mechanism
❏ Reagents and conditions for the chemical test
❏ Observation for the substance that provides a positive test (state colour change of
solution/color of ppt/gas test etc)
❏ Observation for the substance that does not provide a positive test
Continue Reading on Chapter 11: Arenes