Alkenes

— Chemistry · AS Level (7405) · Crash Course —

★ Exam Tip: This chapter is a mark-collector: oxidation by KMnO₄ (cold dilute vs hot conc), electrophilic addition mechanism (curly arrows, lone pairs, charges!), Markovnikov / carbocation stability and addition polymers. Missing a lone pair or a δ+ in the mechanism = marks cut. Learn the reagents & conditions — they’re free marks.

1. What are Alkenes?

Definition: Alkenes are unsaturated hydrocarbons containing a C=C bond. Obtained from the cracking of alkanes.

General formula: C_nH_2n

Name	Structural formula
but-1-ene	CH₂=CHCH₂CH₃
cis-but-2-ene	CH₃CH=CHCH₃ (CH₃ same side)
trans-but-2-ene	CH₃CH=CHCH₃ (CH₃ opposite sides)
2-methylpropene	CH₂=C(CH₃)₂

Property	Alkenes
Intermolecular forces	temporary dipoles ⇒ low m.p./b.p.
Polar / non-polar	non-polar ⇒ insoluble in water (soluble in alkanes — both non-polar)

Why are alkenes reactive?

The C=C = one σ bond + one π bond. The π bond’s electron density lies above and below the internuclear axis (sideways overlap of p orbitals) ⇒ it is weaker than a σ bond and has high electron density that attracts electrophiles. The π bond can break and form 2 strong σ bonds ⇒ addition is favourable (unlike alkanes, where substitution happens).

Memory: Electrophile = “electron-loving” = a species that accepts a pair of electrons. Nucleophile = “nucleus-loving” = donates a pair of electrons.

σ = strong, along the internuclear axis; π = weaker, so it is the one that breaks

2. Reactions of Alkenes

① Combustion: already discussed (alkane chapter) — CO₂ + H₂O. e.g. C₂H₄ + 3O₂ → 2CO₂ + 2H₂O.

3. Oxidation by KMnO₄

Remember: Oxidising agents are always used acidified (H⁺ from dilute H₂SO₄) — otherwise reactions are slow/messy. The full ionic equation is long and not needed — just show [O]. H₃PO₄ is not an oxidising acid, which is why it is used as the catalyst in hydration.

(a) Cold, dilute, acidified KMnO₄ → a DIOL

CH₂=CH₂ + [O] + H₂O → HOCH₂CH₂OH (ethane-1,2-diol)

Observation: purple solution turns colourless (MnO₄⁻ → Mn²⁺) — used as a test for alkenes (but not the most specific: other things reduce MnO₄⁻ too).
Both C of the C=C get an –OH; the C–C σ bond stays. No balancing needed: one H and one OH are added across — it is balanced.

(b) Hot, concentrated, acidified KMnO₄ → C=C is RUPTURED

Each carbon of the C=C is oxidised depending on its substituents:

each half of the broken double bond is judged on its own

Alkene	Products with hot conc. acidified KMnO₄
(CH₃)₂C=C(CH₃)₂	2 × propanone (ketone, ketone)
CH₃CH=C(CH₃)₂	ethanoic acid CH₃COOH + propanone
CH₂=C(CH₃)₂	CO₂ + H₂O + propanone
CH₃CH=CHCH₃ (but-2-ene)	2 × ethanoic acid
cyclohexene (ring)	hexanedioic acid HOOC(CH₂)₄COOH (ring opens — chain, not two molecules)

Sir’s warning: Students slip here — draw the C=C, cut it, then fix each end. If the examiner says “draw it a particular way” (e.g. skeletal / displayed), follow it exactly. Always sketch the displayed formula first.

4. Addition Polymerisation

Alkenes undergo addition polymerisation: many monomers (each with C=C) join; nothing is lost (same empirical formula as monomer). Unsaturated monomer → saturated polymer (no C=C left).

Polymer	Monomer (name & formula)	Repeat unit	Uses
polyethene	ethene CH₂=CH₂	–CH₂–CH₂–	plastic bags
polypropene	propene CH₂=CHCH₃	–CH₂–CH(CH₃)–	plastic boxes, crates, waterproof sheets
poly(chloroethene) PVC (polyvinyl chloride)	chloroethene CH₂=CHCl	–CH₂–CHCl–	water pipes
poly(phenylethene) polystyrene	phenylethene CH₂=CH–C₆H₅	–CH₂–CH(C₆H₅)–	coffee cups, packaging
poly(tetrafluoroethene) PTFE / Teflon	tetrafluoroethene CF₂=CF₂	–CF₂–CF₂–	non-stick coating on cookware

n CH₂=CHX → [ –CH₂–CH(X)– ]_n (X = H, CH₃, Cl, C₆H₅ …)

Exam rule (sir): Repeat unit must be complete on its own (both carbons of the old C=C, with bonds extending through the brackets). If the examiner asks for just one repeat unit, give one unit in brackets with n. If he says “draw the polymer” / “three repeat units” show the units joined, with the chain continued by “–” at each end. A carbon only has 4 bonds — substituents (CH₃, Cl) go above/below the chain, nowhere else.

Environment: Addition polymers are non-biodegradable. Burning releases harmful gases (CO, CO₂; PVC gives chlorine-containing gases e.g. HCl/Cl₂) and, if not burned, they accumulate in landfills (fruit peels decompose; plastic doesn’t). They should be reused and recycled.

5. Electrophilic Addition Reactions

Reaction	Reagent & conditions	Product	Note
Hydrogenation	H₂ + Ni catalyst + 180 °C	alkane (e.g. ethene → ethane)	used in making margarine
Hydration	H₂O(g) + 350 °C + 60–70 atm + conc. H₃PO₄ catalyst	alcohol (ethene → ethanol)	manufacture of ethanol
Halogenation	Cl₂ or Br₂ — no other conditions	dihalogenoalkane (1,2-dibromoethane)	test: Br₂(aq) brown ⇒ colourless
Hydrogen halide	HCl or HBr — no other conditions	halogenoalkane (chloroethane)	—

Sir’s notes: Test for alkenes: add Br₂(aq) — brown/orange solution → colourless. If the examiner gives “unknown X + aqueous Br₂ ⇒ decolourised”, that indicates a C=C. (Iodine solution is also brown — read the question.) With H₂SO₄ instead of H₃PO₄ you’d get messy products — so use H₃PO₄. If you forget temperature/pressure, write “high temperature and pressure” — but learn the numbers.

Mechanism: electrophilic addition

Heterolytic fission: breaking of a bond so that one atom takes both electrons — shown by a full curly arrow (↷). Carbocation: an alkyl group carrying a single positive charge on one of its carbons (a carbon with only 3 bonds, so no “quaternary” carbocation).

Positive inductive effect (+I): pushing of electron density by a group (alkyl groups are electron-donating). Alkyl groups push electron density towards the C⁺, reducing the magnitude of the positive charge ⇒ stabilises the carbocation.

stability: tertiary > secondary > primary carbocation

A. Symmetrical alkenes: ethene + Br₂

Br–Br is polarised (induced dipole) by the high electron density of C=C; every curly arrow starts at a π bond / bond / lone pair

Exam marking points: Marks: ① arrow from the π bond to Brδ⁺; ② δ+ / δ– on Br–Br; ③ arrow showing Br–Br breaking heterolytically to Br⁻; ④ the carbocation with + charge; ⑤ :Br⁻ with lone pair and arrow from the lone pair to C⁺; ⑥ product. Forget a lone pair, charge or partial charge ⇒ you lose marks. Bromine is “induced” polar: “temporary dipole induced in Br₂ due to the high electron density in C=C”.

Same mechanism for Cl₂ with but-2-ene (CH₃CH=CHCH₃ → CH₃CHClCHClCH₃) and for the hydrogen halides (HBr: H is δ+, Br δ–; π bond attacks Hδ⁺).

B. Unsymmetrical alkenes — which carbon does H go to?

Rule (Markovnikov): Hydrogen goes to the carbon that already has MORE hydrogens. The halogen ends up on the carbon with fewer H. This is because the more stable (more substituted) carbocation forms the major product.

Reaction	Carbocation formed	Major product
propene + HCl	H adds to CH₂ (more H) ⇒ secondary carbocation CH₃CH⁺CH₃	2-chloropropane (major); 1-chloropropane only trace
2-methylbut-2-ene + HCl	H adds to CH(CH₃) ⇒ tertiary carbocation (CH₃)₂C⁺CH₂CH₃	2-chloro-2-methylbutane (major)
2-methylpropene + HBr	H adds to CH₂ ⇒ tertiary carbocation (CH₃)₃C⁺	2-bromo-2-methylpropane (major); 1-bromo… trace

Exam answer: Why is the tertiary/secondary product major? “The tertiary carbocation is more stable than the primary carbocation formed otherwise, as the alkyl groups on it push electron density (positive inductive effect) towards the C⁺, reducing the magnitude of its positive charge, stabilising it.” In MCQ 56 (HBr + propene): the intermediate formed in greatest amount = CH₃CH⁺CH₃ ⇒ A.

6. Past-Paper Questions (board)

Q	Question	Working / answer
2	PVC (chloroethene) — C–C–C bond angles along the chain	all C are saturated sp³ ⇒ all 109° ⇒ A ✓
6	cyclohexa-1,4-diene + Br₂ in CCl₄	both C=C are alkenes ⇒ 2 Br₂ add ⇒ tetrabromo, Br on four adjacent carbon pairs ⇒ B ✓ (C = only one C=C reacted ✗; D = gem-dibromo ✗)
15	which hydrocarbon + hot acidified MnO₄⁻ gives ethanoic acid only	but-2-ene: CH₃CH=CHCH₃ → 2 CH₃COOH ⇒ B ✓. A gives CH₃COOH + CO₂ + H₂O; C: propanone + acid; D: 2 propanone
19	1,2-dichloroethene (ClCH=CHCl): incorrect statement?	can be hydrogenated ✓; planar ✓ (120°); shows cis–trans ✓; optical isomerism ✗ (no chiral C: C=C carbons can’t be chiral) ⇒ D ✓ (the incorrect one)
22	limonene + molecular Br₂ at room T in the dark	electrophilic addition at both C=C ⇒ tetrabromo ⇒ D ✓ (not free-radical: no UV)
27	1,1-dichloroethene polymer chain	CH₂=CCl₂ ⇒ –CH₂–CCl₂–CH₂–CCl₂– ⇒ A ✓
28	polymer from monomer (empirical formula same)	–CH₂–CH(CO₂H)– ⇒ monomer CH₂=CH–CO₂H ⇒ C ✓

Q	Question	Working / answer
30	C₄H₈Br₂ made by alkene addition	Br goes on adjacent carbons ⇒ 1,2-dibromobutane ⇒ B ✓
31	skeletal monomer of PVF (repeat unit –CH₂–CHF–)	CH₂=CHF (skeletal: vinyl–F) ⇒ B ✓
33	3-methylhex-2-enoic acid + cold dilute MnO₄⁻ — chiral centres produced?	both C=C carbons become C–OH: C3 (CH₃, OH, C₃H₇, C) chiral ✓; C2 (H, OH, COOH, C3) chiral ✓ ⇒ 2 ⇒ C ✓
37	monomer of polymer … –CH₂–CHCl–…	repeat unit CH₂–CHCl ⇒ CH₂=CHCl ⇒ B ✓
56	greatest-amount intermediate: HBr + propene	secondary carbocation CH₃CH⁺CH₃ ⇒ A ✓
63	cycloalkene C₇H₁₂ → only 2-methylhexane-1,6-dioic acid	ring opens at C=C: HOOC–CH(CH₃)–(CH₂)₃–COOH ⇒ 1-methylcyclohexene ⇒ A ✓
79	termite secretion + hot conc. MnO₄⁻ — products?	terminal =CH₂ ⇒ CO₂ + H₂O ✓; (CH₃)₂C= ⇒ CH₃COCH₃ ✓; CH₃COOH ✗ ⇒ 1 and 2 only

7. Long CIE-style Questions (board)

CIE Q4 · C₂H₄Br₂ / C₂H₂Br₂isomers and hydrogenation

(a)(i) two structural isomers of C₂H₄Br₂: D = BrCH₂CH₂Br (1,2) and E = CH₃CHBr₂ (1,1). (ii) cis- and trans-1,2-dibromoethene (BrCH=CHBr: both Br same side / opposite sides).

(b)(i) cis-isomer → structural isomer: H₂ gas, Ni catalyst, 180 °C. (ii) product is D (BrCH₂CH₂Br): “bromines are on separate carbon atoms” (hydrogenation just saturates the C=C, keeping 1 Br on each C).

CIE Q6 · dodec-1-eneCH₃(CH₂)₉CH=CH₂

(a) products: (i) Br₂ ⇒ CH₃(CH₂)₉CHBrCH₂Br · (ii) HBr ⇒ CH₃(CH₂)₉CHBrCH₃ (Br on C2) · (iii) hot conc. MnO₄⁻ ⇒ CH₃(CH₂)₉COOH (+ CO₂ + H₂O from =CH₂) · (iv) steam ⇒ CH₃(CH₂)₉CH(OH)CH₃.

(b) (i) products (i), (ii), (iv) show optical isomerism (they contain a chiral C*). (ii) draw the two mirror-image forms of one of them (wedge/dash through a mirror line). (c) dodec-2-ene shows cis–trans (geometric) isomerism: draw cis and trans, labelled.

CIE Q7 · ethane / propene / cyclohexenestructures and equations

(a)(i) ethane + Br₂, UV ⇒ CH₃CH₂Br · (ii) propene polymerisation ⇒ [–CH₂–CH(CH₃)–]_n · (iii) propene + cold acidified MnO₄⁻ ⇒ CH₃CH(OH)CH₂OH (propane-1,2-diol) · (iv) cyclohexene + HBr ⇒ bromocyclohexane · (v) cyclohexene + hot conc. MnO₄⁻ ⇒ HOOC(CH₂)₄COOH.

(b)(i) C₂H₆ + 7/2 O₂ → 2CO₂ + 3H₂O(ii) CH₃CH=CH₂ + H₂O(g) →[conc. H₃PO₄, high T & P] CH₃CH(OH)CH₃(iii) cyclohexene + H₂ →[Ni, 180 °C] cyclohexane

(c)(i) cracking is useful: larger alkanes ⇒ smaller ones, e.g. octane (motor fuel); also alkenes for synthesising more organic products. (ii) C₁₆H₃₄ → C₂H₄ + C₆H₁₂ + C₈H₁₈ (C: 2+6+8 = 16; H: 4+12+18 = 34 ✓).

CIE Q15 · but-2-enecracking, reactions, polymer

(a) thermal cracking: high T (~450–700 °C), ~70 atm; catalytic: SiO₂ / Al₂O₃ + heat. (b) C₁₂H₂₆ → C₄H₈ + C₈H₁₈. (c) hot conc. MnO₄⁻/H⁺ ⇒ A = CH₃COOH; Cl₂ ⇒ B = CH₃CHClCHClCH₃; cold dilute MnO₄⁻/H⁺ ⇒ C = CH₃CH(OH)CH(OH)CH₃. (e) poly(butene): –CH(CH₃)–CH(CH₃)–CH(CH₃)–CH(CH₃)– (two repeat units = four carbons in chain).

CIE Q19 · (CH₃)₂C=C(CH₃)₂ and (CH₃)₂C=CH₂name, oxidation, polymer, mechanism

(b)(i) 2,3-dimethylbut-2-ene. (ii) cold dilute acidified MnO₄⁻ ⇒ 2,3-dimethylbutane-2,3-diol (skeletal: two C–OH on adjacent C). (iii) hot conc. ⇒ propanone (CH₃COCH₃, two molecules). (iv) polymer: three repeat units, each C carrying two CH₃ (4 CH₃ per C–C unit).

(c)(i) 2-methylpropene. (ii) mechanism with HBr: π bond → Hδ⁺, H–Br breaks heterolytically → Br⁻; tertiary carbocation (CH₃)₃C⁺; lone pair on :Br⁻ attacks C⁺ ⇒ 2-bromo-2-methylpropane. (iii) tertiary carbocation is more stable than the primary one (alkyl groups push electron density, +I, reducing the + charge) ⇒ major product.

✓ Quick Recap — Alkenes

C_nH_2n, unsaturated (C=C = σ + π); π is weaker, electron-rich ⇒ attracts electrophiles ⇒ addition.
Cold dilute acidified KMnO₄: diol, purple → colourless (test). Hot conc.: C=C cut — R,R′ ⇒ ketone; R,H ⇒ COOH; H,H ⇒ CO₂ + H₂O.
Addition polymers: monomer C=C → repeat unit [–C–C–]_n; non-biodegradable; recycle.
Addition: H₂ (Ni, 180 °C) ; H₂O(g) (350 °C, 60–70 atm, conc. H₃PO₄) ; Cl₂/Br₂ (no conditions; Br₂(aq) brown → colourless) ; HCl/HBr (no conditions).
Mechanism: π bond attacks δ+ end ⇒ heterolytic fission ⇒ carbocation ⇒ Br⁻/Cl⁻ attacks. Show lone pairs, charges, δ+/δ–.
Unsymmetrical: H to the C with more H; tertiary > secondary > primary carbocation (+I effect).