Lesson 13 of 13 · 15 min
Chapter review
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Must-know facts
20 facts
- 1Haloalkane: X on sp³ carbon; haloarene: X on sp² carbon of an aromatic ring. Allylic and benzylic halides are sp³; vinylic and aryl halides are sp².
- 2gem-dihalide = both X on one carbon (alkylidene halide); vic-dihalide = X on adjacent carbons (alkylene dihalide).
- 3C₅H₁₁Br has 8 structural isomers: 4 primary, 3 secondary and 1 tertiary (2-bromo-2-methylbutane).
- 4C–X bond length: 139, 178, 193, 214 pm (F → I); bond enthalpy 452, 351, 293, 234 kJ mol⁻¹; dipole moment largest for CH₃Cl (1.860 D).
- 5SOCl₂ gives pure alkyl chlorides because SO₂ and HCl escape as gases.
- 6Alcohol reactivity with HX is 3° > 2° > 1°; 1° and 2° need ZnCl₂ with HCl.
- 7Finkelstein: RCl/RBr + NaI in dry acetone → RI (NaCl/NaBr precipitates). Swarts: RCl/RBr + AgF, Hg₂F₂, CoF₂ or SbF₃ → RF.
- 8Sandmeyer: ArN₂⁺ + CuCl/CuBr → ArCl/ArBr; ArN₂⁺ + KI → ArI without copper.
- 9Boiling point: RI > RBr > RCl > RF; branching lowers it. p-Dihalobenzene has the highest melting point of its isomers (symmetry).
- 10KCN → alkyl cyanide (RCN); AgCN → isocyanide (RNC). KNO₂ → alkyl nitrite; AgNO₂ → nitroalkane.
- 11SN2: rate = k[RX][Nu⁻], one step, backside attack, inversion; CH₃X > 1° > 2° > 3°.
- 12SN1: rate = k[RX], carbocation intermediate, polar protic solvent, racemisation; 3° > 2° > 1°; allylic and benzylic fast.
- 13Leaving group ability for both mechanisms: R–I > R–Br > R–Cl >> R–F.
- 14(−)-2-Bromooctane + NaOH (SN2) → (+)-octan-2-ol with inversion; optically active 2-bromobutane hydrolysis (SN1) → (±)-butan-2-ol.
- 15Racemic mixture = 50:50 enantiomers, zero rotation, written (±) or dl. Enantiomers differ only in the sign of rotation.
- 16Zaitsev: more substituted alkene is major; 2-bromopentane → pent-2-ene. Alcoholic KOH eliminates; aqueous KOH substitutes.
- 17Grignard RMgX made in dry ether; any proton source (water, alcohol, amine) turns it into RH.
- 18Wurtz: 2RX + 2Na → R–R; Wurtz-Fittig: RX + ArX + Na → Ar–R; Fittig: 2ArX + 2Na → Ar–Ar.
- 19Aryl C–Cl 169 pm vs alkyl C–Cl 177 pm; chlorobenzene → phenol needs NaOH, 623 K, 300 atm; o/p –NO₂ activates, m –NO₂ does not.
- 20Halogen on benzene: deactivating (–I) but ortho, para-directing (resonance). Freon 12 = CCl₂F₂; chloroform is stored in dark bottles filled to the top to stop phosgene forming.
Common traps
Where marks are lost
Calling benzyl chloride an aryl halide because it contains a benzene ring.
Assuming dipole moment rises steadily from CH₃I to CH₃F with electronegativity.
Using H₂SO₄ with KI to convert an alcohol to an alkyl iodide.
Writing RCN as the main product of a haloalkane with AgCN.
Expecting a tertiary halide to be fastest in SN2 because it forms the most stable carbocation.
Thinking the SN1 rate doubles when [OH⁻] is doubled.
Concluding that a (−) reactant must give a (−) product when configuration is retained.
Saying SN1 gives a product with retention because the halide leaves from the same side.
Predicting pent-1-ene as the major product from 2-bromopentane.
Believing a halogen on benzene is meta-directing because it withdraws electrons.
Formulas
5 to know
SN2 rate law
rate = k[RX][Nu⁻]
Second order: CH₃Cl + OH⁻. One step, inversion of configuration.
SN1 rate law
rate = k[RX]
First order: (CH₃)₃CBr + OH⁻. Rate set by the slow ionisation step.
Grignard reagent with water
RMgX + H₂O → RH + Mg(OH)X
Any proton source destroys RMgX, hence dry ether.
Wurtz reaction
2RX + 2Na → R–R + 2NaX
Dry ether; the alkane has twice the carbons of R.
Wurtz-Fittig and Fittig
RX + ArX + 2Na → Ar–R + 2NaX; 2ArX + 2Na → Ar–Ar + 2NaX
Both in dry ether.
Key terms
18 terms
- Haloalkane (alkyl halide)
- A compound with halogen on an sp³ carbon of an alkyl group.
- Haloarene (aryl halide)
- A compound with halogen bonded straight to an sp² carbon of an aromatic ring.
- Allylic / benzylic halide
- Halogen on an sp³ carbon next to a C=C / next to an aromatic ring.
- Vinylic halide
- Halogen on one of the sp² carbons of a C=C.
- gem- / vic-dihalide
- Both halogens on the same carbon / on neighbouring carbons.
- Nucleophile
- An electron-rich species that attacks an electron-poor centre.
- Leaving group
- The group, here a halide ion, that departs with the bonding pair.
- Ambident nucleophile
- A nucleophile with two different atoms that can attack, such as CN⁻ or NO₂⁻.
- Carbocation
- A planar, sp² carbon ion carrying a positive charge, formed in the slow step of SN1.
- Chiral
- Not superimposable on its own mirror image.
- Stereocentre (asymmetric carbon)
- A carbon joined to four different groups.
- Enantiomers
- A pair of stereoisomers that are non-superimposable mirror images.
- Racemic mixture
- A 50:50 mix of two enantiomers, with zero net optical rotation.
- Inversion of configuration
- The product's arrangement around the stereocentre is the mirror of the reactant's.
- Retention of configuration
- The arrangement around the stereocentre is unchanged by the reaction.
- β-Elimination
- Loss of H from the β-carbon and X from the α-carbon to give an alkene.
- Grignard reagent
- An alkyl or aryl magnesium halide, RMgX, made in dry ether.
- Freons
- Chlorofluorocarbons of methane and ethane, such as CCl₂F₂.