Official JEE Chemistry Revision Material: JEE Chemistry Haloalkanes and Haloarenes Notes Set 02
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Haloalkanes: Preparation
| Method | Detail |
|---|---|
| Free radical halogenation of alkanes | Cl2 or Br2 with light; gives a mixture, so poor for synthesis |
| Addition of HX to alkenes | Markovnikov; HBr with peroxide gives anti-Markovnikov |
| From alcohols | HX with ZnCl2 (Lucas), \(\ce{PX3}\), \(\ce{PCl5}\) or \(\ce{SOCl2}\) (best, since the by-products are gases) |
| Borodin-Hunsdiecker | Silver salt of an acid with Br2 in \(\ce{CCl4}\) gives R-Br, one carbon fewer |
| Halide exchange | Finkelstein: R-Cl or R-Br with NaI in dry acetone gives R-I. Swarts: R-Cl with AgF or \(\ce{Hg2F2}\) gives R-F |
| Allylic halogenation | NBS or Cl2/Br2 at high temperature |
Haloalkanes: Physical Properties
1. Boiling point rises with mass: RI > RBr > RCl > RF, and falls with branching.
2. Alkyl halides are polar but insoluble in water, since they cannot form hydrogen bonds with it.
3. Density rises from chlorides to iodides; bromides and iodides are denser than water.
4. Bond strength falls in the order C-F > C-Cl > C-Br > C-I, so reactivity rises in the reverse order.
Haloalkanes: Chemical Reactions
| Reaction | Conditions and result |
|---|---|
| SN2 | One step, inversion, rate = k[RX][Nu]; methyl > 1 degree > 2 degree > 3 degree; favoured by strong nucleophile and polar aprotic solvent |
| SN1 | Two steps through a carbocation, racemisation; 3 degree > 2 degree > 1 degree; favoured by polar protic solvent; benzyl and allyl halides are fast |
| Elimination (E2) | Alcoholic KOH or a bulky base; Saytzeff gives the more substituted alkene; tertiary halides eliminate most |
| Wurtz | 2 R-X + 2 Na in dry ether gives R-R |
| Grignard reagent | R-X + Mg in dry ether gives RMgX; it is destroyed by water, alcohols and amines to give R-H |
| Friedel-Crafts alkylation | Arene with R-X and anhydrous \(\ce{AlCl3}\) gives alkylarene |
| With KCN, AgCN | KCN gives nitrile R-CN; AgCN gives isocyanide R-NC (ambident nucleophile) |
| With \(\ce{NaNO2}\), \(\ce{AgNO2}\) | NaNO2 gives R-ONO (nitrite); AgNO2 gives R-NO2 (nitro) |
Dihalides
1. Gem dihalides carry both halogens on one carbon; vicinal dihalides on adjacent carbons.
2. Hydrolysis of a gem dihalide gives a carbonyl compound (aldehyde or ketone).
3. Two moles of base remove two HX from either type, giving an alkyne.
4. Zn dust removes both halogens of a vicinal dihalide, giving an alkene.
Haloforms
| Compound | Key facts |
|---|---|
| Chloroform \(\ce{CHCl3}\) | Made from ethanol or acetone with bleaching powder; oxidises in light and air to poisonous phosgene, so stored in dark full bottles with a little ethanol; test for impurity with \(\ce{AgNO3}\) |
| Reactions of chloroform | Carbylamine test with a primary amine and KOH; Reimer-Tiemann with phenol; with acetone gives chloretone |
| Iodoform \(\ce{CHI3}\) | Yellow solid with antiseptic smell, from the iodoform test: I2 with NaOH on a methyl ketone, ethanol, ethanal or a CH3CH(OH) compound |
| Carbon tetrachloride | Solvent and fire extinguisher; with phenol and base gives salicylic acid |
| Freons | Chlorofluorocarbons such as \(\ce{CF2Cl2}\), used as refrigerants; they damage the ozone layer |
| D.D.T. | Made from chloral and chlorobenzene with sulphuric acid; an insecticide |
Haloarenes
1. The C-X bond is shorter and stronger than in alkyl halides because of resonance and sp2 hybridisation, so haloarenes resist nucleophilic substitution.
2. Halogens deactivate the ring through -I but direct ortho and para through +R.
3. Nucleophilic substitution needs electron-withdrawing groups such as nitro at the ortho and para positions; more such groups mean faster reaction.
4. Without activation, extreme conditions are needed, or elimination-addition through benzyne with a very strong base such as \(\ce{NaNH2}\).
5. Haloarenes form Grignard reagents, take part in Wurtz-Fittig and Fittig reactions, and undergo Friedel-Crafts reactions at the para position mainly.
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