OC Ch 03 — 15-Minute Revision

Last hour. Seven cards = the slide's seven sections. Read straight through.

1 · Types of unsaturated + DU

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Families

Alkene C_nH_2n, alkyne C_nH_2n−2, aromatic (benzene C₆H₆, DU 4).

Degree of unsaturation

DU = (2C + 2 + N − H − X) / 2. Every ring or π bond = 1 DU. Benzene = 4 DU (1 ring + 3 C=C).

2 · Alkene structure & isomerism

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C=C

Each C is sp², planar, 120°. C=C = 1σ + 1π. Length 134 pm. No rotation.

cis vs trans

Identical groups on same side = cis; opposite = trans. Different physical & biological properties.

Z/E

Rank substituents by CIP priority. Higher-priority groups same side = Z; opposite = E.

⚠ 2019 PP III.4

"C=C is two identical π bonds." FALSE — 1σ + 1π.

3 · IUPAC naming of alkenes

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Rules

Longest chain containing C=C → "-ene" suffix → number to give C=C lowest locant → add substituents & cis/trans (or Z/E).

Cycloalkenes

Numbering starts at C=C: one C is 1, the other 2, then around to give substituents lowest locant.

Diene/triene

Two C=C → "-diene"; e.g. 1,3-butadiene, 1,4-cyclohexadiene.

4 · Alkene reactions — Part V engine

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Reagent	Product / rule
H₂ / Pt, Pd, Ni	Alkane (syn addition)
Br₂ (or Cl₂) / CCl₄	vic-dihalide via bromonium ion (anti). Br₂ test for C=C/C≡C.
HX (no peroxide)	Markovnikov: H to C with more H's, X to more substituted C (via more stable C⁺).
HBr + peroxide	Anti-Markovnikov: radical chain, Br on less-substituted C. Only HBr.
H₂O / H₂SO₄	Markovnikov alcohol. 2-methylpropene → t-butanol.
Cold dilute KMnO₄	cis-1,2-diol (syn). Cyclohexene → cis-1,2-cyclohexanediol. Baeyer's test.
Hot conc. KMnO₄ / H⁺	Cleavage. R₂C= → ketone. RCH= → RCOOH. =CH₂ → CO₂.
O₃ then Zn / H₃O⁺	Two carbonyls. =CH₂ → HCHO. RCH= → RCHO. R₂C= → R₂CO.

✨ Markovnikov shortcut

"Rich get richer" — H joins the H-rich C; X joins the C-rich C.

5 · Alkynes

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C≡C

sp, linear 180°, 1σ + 2π, length 120 pm.

Addition

Adds H₂, X₂, HX twice (Markovnikov). With excess HBr terminal alkyne → gem-dibromide.

Oxidation

Hot KMnO₄ or O₃ cleaves C≡C → two RCOOH; terminal → RCOOH + CO₂.

✨ Terminal alkyne tests

[Ag(NH₃)₂]⁺ → white ppt. [Cu(NH₃)₂]⁺ → red ppt. Internal alkyne → no ppt.

6 · Aromatic structure & nomenclature

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Aromaticity

Cyclic + planar + fully conjugated + (4n+2) π e−. Benzene = 6 π e− (n=1). ~36 kcal/mol stabilisation → resists addition.

Common names

Toluene = methylbenzene · phenol = OH · aniline = NH₂ · benzoic acid = COOH · benzaldehyde = CHO · styrene = vinyl.

o/m/p

ortho = 1,2 · meta = 1,3 · para = 1,4. The ring as substituent on a chain = "phenyl".

⚠ 2019 PP III.6

"Benzene is unsaturated so undergoes addition > substitution." FALSE — substitution wins.

7 · EAS reactions & directing effects

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Reaction	Reagent	Electrophile	Product
Halogenation	Br₂ / FeBr₃	Br⁺	PhBr
Nitration	HNO₃ / H₂SO₄	NO₂⁺	PhNO₂
Sulfonation	SO₃ / H₂SO₄	SO₃H⁺	PhSO₃H
F-C alkylation	RCl / AlCl₃	R⁺ (rearranges!)	PhR
F-C acylation	RCOCl / AlCl₃	RCO⁺	PhCOR (clean)

✨ Directors

o/p: –OH, –NH₂, –OR, –R, halogens (activators except halogens). m: –NO₂, –CN, –CHO, –COOH, –SO₃H (deactivators). Halogen exception: deactivates but o/p directs.

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McMurry & Ballantine 8e Ch 14 · TMU Slide pp.1–106 · Past Papers 2019–2022