OC Ch 03 · Unsaturated · Day Before

Day-Before Sheet

🌙 45–60 min ← Back 📝 Full Notes 🏠 All Units
Day-before. Chapter 3 dominates Part V "Reaction products" (28/100 pts). Master the reagent-product map for alkenes, alkynes and aromatic EAS. Then a walk-in algorithm.

1 · Family overview + DU

Foundation

Alkene CnH2n (one C=C), alkyne CnH2n−2 (one C≡C), aromatic (benzene ring with delocalised π cloud).

Degree of unsaturation (DU) = (2C + 2 + N − H − X)/2. Every multiple bond or ring = 1 DU. Benzene C₆H₆ = DU 4. A useful first step before drawing structures from a formula.

2 · Alkene structure + cis/trans + Z/E

★★

Each alkene C is sp², trigonal planar (120°). C=C = 1σ + 1π; total ~147 kcal/mol; π alone ~64 kcal/mol — the weaker, more reactive component. C=C length 134 pm (shorter than 154 pm of C–C). Rotation locked (would break π) → cis/trans isomers are isolable.

cis/trans: identical groups on same side of C=C = cis; opposite = trans. Z/E (CIP rules): rank substituents on each alkene C by atomic number; higher-priority pair same side → Z (zusammen), opposite → E (entgegen). Z/E supersedes cis/trans when substituents are mixed.

Isomerb.p.Net dipole
cis-2-butene3.7 °CSmall, nonzero
trans-2-butene0.9 °CZero
⚔ Biology
11-cis-retinal → all-trans-retinal at photon hit drives vision. cis double bonds in fatty acids put kinks → vegetable oils are liquid; trans fats behave like saturated fats.

3 · IUPAC naming of alkenes

★
  1. Find the longest chain containing the C=C (not always the longest chain overall).
  2. Replace "-ane" with "-ene".
  3. Number from whichever end gives the C=C the lowest locant.
  4. List substituents alphabetically with their locants.
  5. Add cis/trans or Z/E.
  6. Cycloalkenes: numbering starts at the C=C (C1 + C2) and proceeds to give substituents the lowest locant.
  7. Two C=C → "-diene" (e.g. 1,3-butadiene, 1,4-cyclohexadiene).

4 · Alkene reactions — the engine of Part V

★★★ 28 pts

Memorise as: reagent → product + rule. Every reaction breaks π but preserves the C–C σ.

ReagentProductNotes
H₂ / Pt or Pd or NiAlkanesyn addition from catalyst surface. Used industrially for hydrogenated oils → cis-trans issue (trans fats).
Br₂ / CCl₄ (or Cl₂)vic-1,2-dihalideVia 3-membered bromonium ion → anti addition. Br₂ decolourises — classic test for C=C/C≡C.
HX (no peroxide)Markovnikov alkyl halideVia the more stable carbocation. Propene + HCl → 2-chloropropane.
HBr + peroxideAnti-Markovnikov 1° alkyl bromideRadical chain. Only HBr does this. Propene → 1-bromopropane.
H₂O / dilute H₂SO₄Markovnikov alcohol2-methylpropene → t-butanol.
Cold dilute KMnO₄ (neutral/basic)cis-1,2-diol (vicinal diol)syn dihydroxylation. Cyclohexene → cis-1,2-cyclohexanediol. Purple KMnO₄ decolourises — Baeyer's test.
Hot conc. KMnO₄ / H⁺C=C cleaved into ketone + carboxylic acidR₂C= → ketone. RCH= → RCOOH. =CH₂ → CO₂ + H₂O. Example: 2-methyl-2-butene → acetone + acetic acid.
O₃ then Zn / H₃O⁺ (or Me₂S)C=C cleaved into two carbonylsEach sp² C becomes C=O. RCH= → RCHO. R₂C= → R₂CO. =CH₂ → HCHO. Ozonolysis is the structure-determination workhorse.
◆ Markovnikov mechanism (must explain)
(1) Alkene π attacks H⁺ → carbocation. (2) Two cations possible; the more stable one (3°>2°>1°>Me) forms preferentially via lower-energy TS. (3) Halide adds to that cation. Modern statement: HX adds to give the more stable carbocation intermediate.
◆ Peroxide effect (HBr only)
(1) Peroxide initiates Br·. (2) Br· adds to less-substituted alkene C, giving the more stable 2° (or 3°) radical at the other C. (3) The radical abstracts H from HBr → product + new Br· (chain). Net: anti-Markovnikov. HCl/HI don't do this (bond energies don't fit).

5 · Alkynes

Tests

C≡C: sp hybridised, linear 180°, 1σ + 2π, length 120 pm. Both addition reactions (1 or 2 equiv) and oxidation parallel alkenes but with characteristic differences.

Reagent1 equivExcess
H₂ / PtAlkeneAlkane
Br₂ / CCl₄1,2-dibromoalkene1,1,2,2-tetrabromoalkane
HBr (Markovnikov twice)2-bromoalkene2,2-dibromoalkane (gem)
Hot KMnO₄ or O₃Cleaves C≡C → two carboxylic acids. Terminal alkyne → RCOOH + CO₂.
◆ Terminal alkyne acidity
R–C≡C–H has pKa ~25 (acetylide carbanion has lots of s-character). Internal alkynes are not acidic. Tests:
  • NaNH₂ in liq NH₃ → R–C≡C–Na (sodium acetylide).
  • [Ag(NH₃)₂]NO₃ → white silver acetylide ppt.
  • [Cu(NH₃)₂]Cl → red copper acetylide ppt.
Used to distinguish terminal vs internal alkynes — favourite Part-VI exam question.

6 · Aromatic structure + nomenclature

Hückel

Benzene: planar regular hexagon. All 6 C sp². All 6 C–C bond lengths equal at 139 pm. 6 p orbitals overlap to form a delocalised π cloud (3 bonding MOs filled with 6 π electrons).

Hückel's rule: aromaticity requires (1) cyclic, (2) planar, (3) fully conjugated, (4) (4n+2) π electrons. Aromatic stabilisation ~36 kcal/mol — the energy "tax" on disrupting it is why benzene resists addition and prefers substitution.

Names to memorise: toluene (Me), phenol (OH), aniline (NH₂), benzoic acid (COOH), benzaldehyde (CHO), benzenesulfonic acid (SO₃H), styrene (vinyl), phenylacetylene (C≡CH). When the benzene ring is a substituent on another chain it's called phenyl (C₆H₅–).

Disubstituted: o (1,2), m (1,3), p (1,4). Higher substituted → number for lowest locants.

⚠ 2019 PP III.6
"Benzene is unsaturated, so addition reactions far exceed substitution." FALSE. The aromatic π system is too stable to give up — substitution wins.

7 · Electrophilic Aromatic Substitution (EAS)

★★★ Part V

Mechanism: E⁺ attacks the π cloud → cationic sigma-complex (arenium ion, charge delocalised over the ring) → loss of H⁺ restores aromaticity → Ar–E + HX.

ReactionReagentElectrophileProduct
HalogenationBr₂/FeBr₃ or Cl₂/FeCl₃Br⁺ or Cl⁺PhBr or PhCl
NitrationHNO₃/H₂SO₄NO₂⁺PhNO₂
SulfonationFuming H₂SO₄ (SO₃ in H₂SO₄)SO₃H⁺PhSO₃H
F-C alkylationRCl/AlCl₃R⁺ (rearranges → more stable cation)Alkylbenzene PhR
F-C acylationRCOCl/AlCl₃RCO⁺ (acylium, no rearrangement)Aryl ketone PhCOR

Directing effects on a monosubstituted ring:

GroupTypeActivationDirects
–OH, –OR, –NH₂, –NR₂Strong π-donorStrongly activatingo/p
–R (alkyl), –ArWeak donorWeakly activatingo/p
–X (halogen)Mixed (resonance+/inductive−)Weakly deactivatingo/p (exception!)
–NO₂, –CN, –CHO, –COR, –COOH, –SO₃Hπ-acceptorStrongly deactivatingm
✨ Memory shortcut
"If the existing group gives electrons to the ring (lone pair or alkyl), it activates AND o/p directs. If it takes electrons (C=O/C≡N/SO₂/NO₂), it deactivates AND m directs. Halogens are the odd ones: take inductively, give by resonance → deactivate but still o/p-direct."

🎯 Walk-in algorithm

Use this
  1. Calculate DU from formula (sanity check: is it an alkene/alkyne/aromatic?).
  2. Name an alkene: longest chain with C=C → "-ene" suffix → lowest C=C locant → substituents → cis/trans (or Z/E).
  3. HX product on an alkene: identify which C of the C=C gives the more stable carbocation when protonated → X goes there. Default = Markovnikov. Switch if HBr + peroxide.
  4. KMnO₄ product: cold dilute → cis-diol. Hot/H⁺ → cleave into ketone + acid (terminal CH₂ → CO₂).
  5. Ozonolysis product: split at C=C, put C=O on each sp² C. H still attached → aldehyde. Two C-subs → ketone.
  6. Distinguishing tests: Br₂/CCl₄ for unsaturation; [Ag(NH₃)₂]⁺ or [Cu(NH₃)₂]⁺ for terminal alkyne.
  7. EAS product: pick the right electrophile from the 5 canonical reagent sets; check the existing substituent → o/p or m directing.
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McMurry & Ballantine 8e Ch 14 · TMU Slide pp.1–106 · Past Papers 2019–2022 · Built by Goperamanan Thirusenthivel, MSc · gtdigital.tech