Edexcel International A Level Chemistry: Alkenes (YCH11) – Revision Notes

These notes condense Topic 5: Alkenes of the Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry specification (YCH11), Issue 1, September 2017, outcomes 5.1 to 5.8. Topic 5 is part of Unit 1: Structure, Bonding and Introduction to Organic Chemistry, an IAS unit. The alkenes study guide has the full teaching and worked examples.

Use them with the course hub, the printable checklist and the alkenes practice questions. Bonding background is in the Unit 1 bonding revision notes. To find weak spots quickly, take a free diagnostic.

Key definitions (5.1)

Term	Meaning
Alkene	Hydrocarbon with at least one C=C; one C=C, open chain: CₙH₂ₙ
Cycloalkene	Ring hydrocarbon with a C=C; one ring and one C=C: CₙH₂ₙ₋₂
Unsaturated	Contains a C=C, so atoms can be added across it
σ bond	Head-on overlap of orbitals on the axis between the two nuclei
π bond	Sideways overlap of p orbitals; electron density above and below the plane
Electrophile	Species that accepts an electron pair
Carbocation	Ion with a positively charged carbon atom

Key idea in one line: the π bond is weaker and its electrons are exposed, so alkenes undergo addition, and electrophiles attack them.

Shape: each C of the C=C is trigonal planar, bond angle about 120°.

Geometric isomerism (5.2) and E–Z naming (5.3)

Two conditions, both needed

1. A C=C, which has restricted rotation because rotating would break the π overlap.
2. Two different atoms or groups on each carbon of the C=C.

E–Z method in steps

1. For each C=C carbon, compare the atomic numbers of the two atoms bonded directly to it; higher wins.
2. Tie? Step out one atom and compare again until something differs.
3. Winners together = Z; winners across = E.

Why E–Z is needed: cis–trans relies on a matching group on each carbon. When three or four of the groups are different, cis–trans is ambiguous. E–Z always gives one answer.

Quick reminder: in CH₃CH=C(Br)CH₂CH₃, carbon 2 ranks CH₃ over H, and carbon 3 ranks Br (35) over the ethyl carbon (6). So the isomer with CH₃ and Br on the same side is Z.

Reactions table (5.4)

Reagent	Conditions	Type	Product type
H₂	Ni catalyst, about 150 °C	addition (hydrogenation)	alkane
Br₂ / Cl₂	room temperature	electrophilic addition	di-substituted halogenoalkane
HBr / HCl / HI	room temperature	electrophilic addition	mono-substituted halogenoalkane
H₂O (steam)	acid catalyst (phosphoric acid), high T and P	addition (hydration)	alcohol
KMnO₄ / H⁺	room temperature	oxidation	diol (purple → colourless)

Small worked reminders:

• But-2-ene + H₂ → butane.
• Propene + Br₂ → 1,2-dibromopropane, CH₃CHBrCH₂Br.
• Propene + steam → mainly propan-2-ol.
• But-2-ene + [O] + H₂O → butane-2,3-diol, CH₃CH(OH)CH(OH)CH₃.

Test for C=C (5.5)

Add bromine water and shake. Orange-brown → colourless with an alkene. No change with an alkane in the dark.

Mechanisms (5.6)

Checklist for any electrophilic addition diagram

1. Draw the C=C and the electrophile with its dipole (δ+ on the atom that bonds first).
2. Arrow 1 starts on the C=C and ends on the δ+ atom.
3. Arrow 2 starts on the bond in the electrophile and ends on the δ− atom.
4. Draw the carbocation with a full + on the correct carbon, and the anion with a lone pair and − charge.
5. Arrow 3 starts on a lone pair of the anion and ends on C⁺.
6. Draw and name the product.

Specifics

• Br₂ + ethene: Br₂ is polarised by the π electrons (induced dipole). Intermediate: CH₂BrCH₂⁺.
• HBr + ethene: permanent dipole Hδ+–Brδ−. Intermediate: CH₃CH₂⁺.
• HBr + propene: secondary CH₃CH⁺CH₃ (major route) → 2-bromopropane; primary CH₃CH₂CH₂⁺ (minor) → 1-bromopropane.

Carbocation stability: tertiary > secondary > primary. More alkyl groups push more electron density onto C⁺ (positive inductive effect), stabilising it.

Evidence for a two-step mechanism: ethene with bromine in sodium chloride solution gives BrCH₂CH₂Cl as well as BrCH₂CH₂Br. A carbocation must form first and then be attacked by any available nucleophile. No ClCH₂CH₂Cl appears, so Cl⁻ does not start the attack.

Addition polymerisation (5.7)

Monomer	Repeat unit	Polymer
CH₂=CH₂	–[CH₂–CH₂]ₙ–	poly(ethene)
CH₂=CHCH₃	–[CH₂–CH(CH₃)]ₙ–	poly(propene)
CH₂=CHCl	–[CH₂–CHCl]ₙ–	poly(chloroethene), PVC
CH₂=C(CH₃)COOCH₃	–[CH₂–C(CH₃)(COOCH₃)]ₙ–	Perspex

Rules: two carbons in the main chain of the repeat unit; single bond between them; side groups kept; bonds through both brackets; n outside. Reverse the process to find the monomer. Atom economy is 100% because only one product forms.

Further suggested practicals (Topic 5)

These are suggested in the specification, not Core Practicals: comparing alkanes and alkenes (combustion, bromine water, acidified potassium manganate(VII)); making cyclohexene from cyclohexanol; extracting limonene from orange peel by steam distillation; making Perspex from methyl 2-methylpropenoate. Know the observations: alkenes decolourise both bromine water and the purple manganate(VII) solution, and burn with a smokier flame than alkanes of similar size.

Polymer disposal (5.8)

• Poly(alkenes) are saturated and their C–C and C–H bonds are strong, so microorganisms cannot break them down.
• Biodegradable polymers are designed to be broken down by microorganisms, for example polymers based on starch or lactic acid. Many need composting conditions to degrade quickly.
• Incineration recovers energy but can release toxic gases. PVC gives HCl. Scrubbers containing a base such as calcium hydroxide remove it: Ca(OH)₂ + 2HCl → CaCl₂ + 2H₂O.

Must-know distinctions

• σ vs π: head-on overlap on the axis vs sideways p overlap above and below; π is weaker.
• Di- vs mono-substituted: halogen (X₂) gives two halogen atoms; hydrogen halide (HX) gives one.
• Hydrogenation vs hydration: H₂ with Ni gives an alkane; H₂O with acid catalyst gives an alcohol.
• Major vs minor product: decided by the more stable carbocation, not by which carbon “looks” more crowded.
• Repeat unit vs monomer: repeat unit has no C=C; monomer does.
• Cis–trans vs E–Z: cis–trans needs a matching group; E–Z uses priority by atomic number.

Quick self-test

1. Give the molecular formula of the open-chain alkene with seven carbon atoms.
2. Give the molecular formula of cyclopentene.
3. State the two conditions needed for geometric isomerism.
4. Name the E or Z isomer of ClCH=CHBr in which Cl and Br are on opposite sides.
5. State the reagent and conditions to convert propene into propan-2-ol.
6. What volume of hydrogen, in cm³, reacts with 2.52 g of hex-1-ene, C₆H₁₂ (Mr 84.0)? Take the molar volume as 24.0 dm³ mol⁻¹.
7. What mass of bromine, Br₂ (Mr 159.8), reacts with 0.0150 mol of propene?
8. Which carbocation forms faster from 2-methylpropene and HBr, and why?
9. Draw the repeat unit of the polymer from CH₂=CHCN.
10. A poly(propene) chain has Mr 1.26 × 10⁵. How many monomer units does it contain? (Mr of propene 42.0)
11. Name the gas formed when PVC is burned and one substance used to remove it.

Answers

1. C₇H₁₄.
2. C₅H₈.
3. Restricted rotation from a C=C, plus two unlike groups on both of its carbons.
4. Carbon 1 ranks Cl over H; carbon 2 ranks Br over H. Opposite sides: (E)-1-bromo-2-chloroethene.
5. Steam with a phosphoric acid catalyst at high temperature and pressure.
6. n = 2.52 ÷ 84.0 = 0.0300 mol; one H₂ per C=C, so V = 0.0300 × 24 000 = 720 cm³.
7. 1 : 1 ratio, so 0.0150 × 159.8 = 2.40 g.
8. The tertiary carbocation, (CH₃)₃C⁺, because three alkyl groups release electron density towards C⁺ and stabilise it more than the primary alternative.
9. –[CH₂–CH(CN)]ₙ–.
10. 1.26 × 10⁵ ÷ 42.0 = 3000.
11. Hydrogen chloride; calcium hydroxide (slaked lime) or another base in a scrubber.

Where marks are usually lost

• Writing CₙH₂ₙ for a cycloalkene instead of CₙH₂ₙ₋₂.
• Describing a π bond as “a second σ bond” or omitting that it is above and below the plane.
• Naming only one condition for geometric isomerism.
• Ranking E–Z groups by size rather than atomic number of the attached atom.
• Drawing curly arrows that begin on an atom instead of on the C=C, a bond or a lone pair.
• Missing the dipole on Br₂, or the lone pair and charge on Br⁻.
• Drawing the primary carbocation for propene and HBr, or explaining stability without mentioning electron release by alkyl groups.
• Forgetting the acid catalyst for hydration or the nickel catalyst for hydrogenation.
• Writing “goes clear” for bromine water instead of “orange-brown to colourless”.
• Leaving a C=C inside a polymer repeat unit.

Official syllabus

Pearson Edexcel International Advanced Subsidiary/Advanced Level in Chemistry (XCH11/YCH11) specification, Issue 1, September 2017, Pearson Education Limited – Unit 1, Topic 5: Alkenes, outcomes 5.1 to 5.8.