Alkenes & Saturation – Comprehensive Study Notes

Page 1 – Chapter Identification

Chapter 16: Hydrocarbons (Discover Chemistry for GCE ‘O’-Level / Normal (A) Level Science).
Copyright © 2023 Marshall Cavendish Education Pte Ltd.

Page 2 – Lesson Label

Lesson segment: 16.3 (double period).
Corporate branding: Marshall Cavendish Education.

Page 3 – Chapter Road Map

Sub-sections within Chapter 16:
- 16.1 What is a Homologous Series?
- 16.2 What Are Alkanes?
- 16.3 What Are Alkenes? ← current focus.
- 16.4 How Do Saturated and Unsaturated Compounds Differ?

Page 4 – Learning Outcomes for 16.3 “Alkenes”

Describe alkenes as a homologous series of unsaturated hydrocarbons with general formula $CnH{2n}$ .
Draw unbranched alkenes (2–3 C atoms) and name them.
Explain manufacture of alkenes + hydrogen by cracking.
State why cracking is essential: matches demand for smaller-molecule fractions from crude-oil refining.
Describe reactions of alkenes:
1. Combustion.
2. Polymerisation.
3. Addition with bromine (test for unsaturation).
4. Addition with hydrogen (hydrogenation).

Page 5 – Natural Occurrence & Nutritional Relevance

Trace biogenic production: Ripening apples & tomatoes emit ethene and propene.
Plant essential oils: Contain alkene-rich compounds such as β-carotene, vitamin A, natural rubber.
Vitamins with multiple alkene groups:
- Cholecalciferol (vitamin D₃).
- Retinol (vitamin A₁).
Alkenes render these vitamins prone to oxidation → over-cooking destroys them.

Page 6 – β-Carotene & Vision

Folk wisdom: carrots improve eyesight.
Chemical basis: carrots rich in β-carotene, a purple-orange pigment with 11 C=C bonds (highly unsaturated).
Illustrates biological importance of alkenes.

Page 7 – Fundamental Definition of Alkenes

Covalent molecular compounds.
Hydrocarbons → contain only C & H atoms.
Each molecule has one carbon–carbon double bond, $C=C$ .
Functional group: double bond itself (common to entire homologous series).

Page 8 – General Formula & Terminology

General formula: $CnH{2n}$ (valid for unbranched, open-chain alkenes; $n≥2$ ).
Classified as unsaturated because of ≥1 double bond.
Naming rule: root + “-ene”.

Page 9 – Textbook Cross-Reference

Confirms definition & general formula, textbook p. 246.

Page 10 – Physical-Property Trends (Parallel to Alkanes)

Melting/boiling point ↑ with molecular mass.
Viscosity ↑ with chain length.
Flammability ↓ (longer chains burn less readily).
Overall trends mirror those seen in alkanes due to similar London dispersion forces, albeit slightly lower m.p./b.p. per mass because unsaturation introduces kinks.

Page 11 – FIVE-Step Method: Drawing Full Structural Formulae (Unbranched)

Step	Instruction
1	Draw carbon skeleton horizontally.
2	Add functional group on extreme right (e.g. $C=C$ for alkene).
3	Complete each carbon’s valency to 4 single bonds (or 1 double + 2 single).
4	Add hydrogens (each forms 1 single bond).
5	Verify atom count matches molecular formula.

Page 12 – Mini-Table of First Two Alkenes

No. C atoms	Name	Molecular Formula	$M_r$	Physical State (r.t.p.)
2	Ethene (ethylene)	$C<em>2H</em>4$	28	Gas
3	Propene (propylene)	$C<em>3H</em>6$	42	Gas
Full structural formulae must show the double bond between C₁ & C₂.

Page 13 – Commercial Use of Ethene Gas

Natural & controlled ripening agent for climacteric fruits/vegetables.
Industrial practice: enclose produce in ethene-rich atmosphere for uniform, timed ripening.
Household tip: place fruit in sealed bag; add citrus (lemon/lime) → boosts ethene concentration → faster ripening.

Page 14 – Addition of Hydrogen (Hydrogenation)

Reaction type: addition.
Equation (generic): $\text{alkene} + H_2 \xrightarrow[\text{Ni}]{200^{\circ}C} \text{alkane}$
Example: $C2H4 + H2 \rightarrow C2H_6$ (ethene → ethane).
Conditions:
- Temperature ≈ $200^{\circ}\text{C}$ .
- Nickel catalyst (solid surface for heterogeneous catalysis).

Page 15 – Industrial Significance of Hydrogenation

Margarine production:
- Feedstock = vegetable oil (polyunsaturated triglycerides).
- Hydrogenation decreases number of C=C bonds → raises melting point → semi-solid spread.
- Same conditions: $200^{\circ}C$ , Ni catalyst.
Health-science note: fewer double bonds → higher saturation → different nutritional profile.

Page 16 – Saturation vs. Melting Point (Fats & Oils)

Greater saturation ⇒ higher melting point.
Saturated chains align/stack well → stronger London forces → solid fats at r.t.p.
Unsaturated chains kink at C=C → poor packing → weaker forces → oils (liquids) at r.t.p.

Page 17 – Addition of Bromine (Bromination & Chemical Test)

Reaction: $\text{alkene} + Br_2 \rightarrow \text{bromoalkane}$ (no catalyst, r.t.p.).
Example: $C2H4 + Br2 \rightarrow C2H4Br2$ (1,2-dibromoethane).
Qualitative test for unsaturation.

Page 18 – Correct Structural Outcome of Bromination

Both Br atoms add across the original double bond → each C formerly in $C=C$ receives one Br.
Misconception: attaching both Br to single carbon is wrong.

Page 19 – Common Error Highlighted

Reminder: product must lack a double bond; new C–Br single bonds appear on both carbons.

Page 20 – Observations & Conclusions of Bromine Test

Observation (aq. Br₂)	Conclusion
Solution changes from reddish-brown → colourless (decolourised)	Compound is unsaturated (e.g. alkene).
Solution remains reddish-brown	Compound is saturated (e.g. alkane).

Experimental details:

Reagent: aqueous bromine/bromine water.
Conditions: room temperature, no UV needed.

Page 21 – Practical Execution of Bromine Test

Liquid sample: Add a few drops of Br₂(aq), shake.
Gaseous sample: Bubble gas through Br₂(aq) (analogous to CO₂–limewater test apparatus, but with bromine water).

Page 22 – Summary Table: Saturated vs. Unsaturated Hydrocarbons

Feature	Saturated (Alkanes)	Unsaturated (Alkenes/Alkynes)
Bonding	Only C–C single bonds	At least one C=C (or C≡C)
Typical reactions	Substitution	Addition
Bromine water test	No colour change (remains brown)	Decolourised

Page 23 – Textbook Prompt

Question: In bromine test, what reaction occurs? → Electrophilic addition (bromination) at the double bond.

Page 24 – Practice Recommendations

Do Practice 2 (Questions 1–3) in notes.
Attempt Textbook Workbook (TWB) exercises 16B & 16C on p. 252.

Cross-Topic Connections & Exam Tips

Homologous series concept (16.1): Same functional group, consecutive members differ by $CH_2$ , exhibit gradual physical-property change but similar chemical reactivity.
Comparing alkanes vs. alkenes (16.2 ⇄ 16.3):
- Formula difference: $CnH{2n+2}$ (alkane) vs. $CnH{2n}$ (alkene).
- Test for unsaturation distinguishes them.
Cracking (industrial context): Supplies shorter-chain alkanes + alkenes needed for fuels & feedstocks; endothermic, catalysts/steam used.
Polymerisation of alkenes: Double bonds open to form long chains (e.g. polyethylene); link hydrogen-ation vs. addition-polymerisation – both consume C=C.
Ethical/health considerations: Hydrogenation can produce trans-fats if incomplete; nutritional impact of saturation levels.

Key Equations (LaTeX Ready)

General alkene formula: $CnH{2n}$
Hydrogenation: $CnH{2n} + H2 \xrightarrow[\text{Ni}]{200^{\circ}C} CnH_{2n+2}$
Bromination: $CnH{2n} + Br2 \rightarrow CnH{2n}Br2$
Ethene specific: $C2H4 + Br2 \rightarrow C2H4Br2$
Ethene hydrogenation: $C2H4 + H2 \rightarrow C2H_6$

End of page-by-page study notes.