JEE Chemistry Haloalkanes and Haloarenes Notes Set 01

Classification

1. Halogen derivatives are formed when one or more hydrogen atoms of a hydrocarbon are replaced by halogen atoms. They are grouped as alkyl, alkenyl, alkynyl and aryl halides, depending on the parent hydrocarbon.
2. Alkyl halides are further grouped as mono-, di-, tri- and tetrahalides, and monohalides are primary, secondary or tertiary according to the carbon that bears the halogen.
3. Gem (geminal) dihalides have both halogens on the same carbon, vicinal dihalides have them on adjacent carbons, and α,ω-dihalides have them on the two terminal carbons of a chain. Gem and vicinal dihalides are position isomers.

Preparation of Haloalkanes

1. Free-radical halogenation of alkanes with Cl₂ in UV light gives a mixture of products, so it is not a good laboratory method.
2. Addition of HX to alkenes follows Markovnikov's rule: CH₃CH=CH₂ + HBr → CH₃CHBrCH₃.
3. From alcohols: R–OH + HX (ZnCl₂ catalyst for HCl), or with PCl₅, PCl₃ or SOCl₂. Thionyl chloride (Darzens' method) is best since SO₂ and HCl escape as gases and the product is pure. Bromides and iodides are made with red P + Br₂ or red P + I₂.
4. Borodine-Hunsdiecker reaction: RCOOAg + Br₂ (in CCl₄, heat) → R–Br + CO₂ + AgBr. The product has one carbon fewer than the acid.
5. Halide exchange: Finkelstein reaction (R–Cl or R–Br + NaI in dry acetone → R–I) and Swarts reaction (R–Cl + AgF or Hg₂F₂ → R–F).

Physical Properties of Haloalkanes

1. Boiling points are higher than those of alkanes of similar mass. For the same alkyl group the order is R–I > R–Br > R–Cl > R–F. Among isomeric halides the boiling point falls from primary to secondary to tertiary as branching reduces surface area.
2. Density rises R–F < R–Cl < R–Br < R–I. Fluorides and chlorides are lighter than water, while bromides and iodides are heavier.
3. Haloalkanes are polar but insoluble in water because they cannot form hydrogen bonds with it. They dissolve in organic solvents.
4. Beilstein's test: a compound containing Cl, Br or I (not F) colours a flame green when heated on a clean copper wire.
5. Dipole moment order of the methyl halides: CH₃Cl > CH₃F > CH₃Br > CH₃I. Chlorobenzene has a lower dipole moment than chlorocyclohexane because of delocalisation and the sp² carbon.

Nucleophilic Substitution Reactions

1. Replacing X by OH gives alcohols (aqueous KOH or moist Ag₂O), by SH gives thiols (KSH), by SR′ gives thioethers, by OR′ gives ethers (Williamson synthesis) and by R′COO gives esters (R′COOAg).
2. Ambident nucleophiles give two products. KCN (ionic) gives mainly the cyanide RCN, and AgCN (covalent) gives mainly the isocyanide RNC. KNO₂ gives mainly the alkyl nitrite R–O–N=O, and AgNO₂ gives mainly the nitroalkane R–NO₂.
3. With ammonia, repeated alkylation gives primary, secondary and tertiary amines and finally a quaternary ammonium salt.
4. S_N2: one step, second order, inversion of configuration, favoured by methyl and primary halides, strong nucleophiles and polar aprotic solvents (DMF, DMSO, acetone). Reactivity falls CH₃ > 1° > 2° > 3° because of steric hindrance.
5. S_N1: two steps through a carbocation, first order, racemisation, favoured by tertiary, allylic and benzylic halides and polar protic solvents. Reactivity rises 1° < 2° < 3° and with the stability of the carbocation.
6. Leaving-group ability: I^- > Br^- > Cl^- > F^-.

Elimination, Coupling and Organometallic Reactions

1. Dehydrohalogenation: alcoholic KOH or NaNH₂ removes HX from a haloalkane to give an alkene. When two alkenes are possible, the more substituted one is the major product (Saytzeff or Zaitsev rule), e.g. 2-bromobutane gives mainly but-2-ene. Bulky bases such as tert-butoxide, and poor leaving groups such as fluoride, favour the less substituted alkene (Hofmann product).
2. Tertiary halides with strong bases give mainly elimination, and primary halides with good nucleophiles give mainly substitution.
3. Wurtz reaction: 2R–X + 2Na in dry ether gives R–R. Fittig reaction couples two aryl halides to a biaryl, and the Wurtz-Fittig reaction couples an aryl halide with an alkyl halide.
4. Grignard reagent: R–X + Mg (dry ether) → R–MgX. Ease of formation is R–I > R–Br > R–Cl. Ether solvates and stabilises the reagent, and any compound with an active hydrogen (water, alcohol, amine, terminal alkyne) destroys it to give an alkane RH.
5. Grignard reagents add to carbonyl compounds: HCHO gives a primary alcohol, other aldehydes give secondary alcohols, ketones give tertiary alcohols and CO₂ gives a carboxylic acid (after H₃O⁺).
6. Other organometallic compounds: (C₂H₅)₂Zn (Frankland reagent) and tetraethyl lead, made from C₂H₅Cl and a sodium-lead alloy and once used as an antiknock additive.

Dihalides

1. General formula C_nH_2nX₂. Gem dihalides are made by PCl₅ on aldehydes or ketones (CH₃CHO gives CH₃CHCl₂, acetone gives 2,2-dichloropropane) or by adding two moles of HX to an alkyne.
2. Vicinal dihalides are made by adding X₂ to alkenes or by PCl₅ on glycols.
3. Aqueous KOH distinguishes the two types. Vicinal dihalides give glycols (diols), and gem dihalides give aldehydes or ketones through the unstable gem-diol.
4. Alcoholic KOH removes two molecules of HX and gives an alkyne. Zinc dust (dehalogenation) converts a vicinal dihalide to an alkene and an α,ω-dihalide such as 1,3-dibromopropane to cyclopropane.
5. With KCN and then hydrolysis, a vicinal dihalide gives a dicarboxylic acid (succinic acid, which forms an anhydride on heating), and a gem dihalide gives an acid with one carbon fewer after loss of CO₂.

Chloroform (Trihalide)

1. Preparation: from ethanol or acetone with bleaching powder (haloform reaction). Bleaching powder supplies Cl₂ and Ca(OH)₂. For ethanol the steps are oxidation to CH₃CHO, chlorination to CCl₃CHO and hydrolysis to CHCl₃ and calcium formate. Pure chloroform is made by heating chloral hydrate with NaOH.
2. Chloroform is a colourless, sweet-smelling liquid with boiling point 61 °C. It is heavier than water and insoluble in it, and it was used as an anaesthetic.
3. In air and light it is oxidised to poisonous phosgene, COCl₂. It is therefore stored in dark, full bottles with about 1 percent ethanol, which converts phosgene to harmless diethyl carbonate. Impure chloroform turns blue litmus red, gives a white AgCl precipitate with AgNO₃ and turns conc. H₂SO₄ yellow.
4. Carbylamine reaction: a primary amine (aliphatic or aromatic) + CHCl₃ + 3KOH gives an isocyanide, RNC, with a foul smell. Secondary and tertiary amines do not react. The electrophile is dichlorocarbene, :CCl₂.
5. Reimer-Tiemann reaction: phenol + CHCl₃ + KOH gives salicylaldehyde mainly. With CCl₄ the product is salicylic acid.
6. Other reactions: with acetone and a base it gives chloretone (a hypnotic), with HNO₃ it gives chloropicrin (tear gas), and with benzene and AlCl₃ it gives triphenylmethane. Chloral reacts with chlorobenzene in conc. H₂SO₄ to give DDT.

Iodoform Test

1. Compounds containing a CH₃CO– group (acetaldehyde and methyl ketones such as acetone and acetophenone) or a CH₃CH(OH)– group (ethanol, propan-2-ol, butan-2-ol, 1-phenylethanol) give a yellow precipitate of CHI₃ on warming with I₂ and NaOH.
2. Methanol, formaldehyde, other aldehydes, esters, amides and ketones such as pentan-3-one and benzophenone do not respond.
3. Iodoform gives a yellow precipitate of AgI on warming with AgNO₃, since the C–I bond is weak, while chloroform does not give AgCl.
4. Use: it distinguishes ethanol from methanol, acetaldehyde from propanal, pentan-2-ol from pentan-3-ol, and acetophenone from benzophenone.

Freons

1. Chlorofluorocarbons such as CF₂Cl₂ (Freon-12) are non-toxic, non-flammable gases used as refrigerants and aerosol propellants. They destroy the ozone layer in the stratosphere.
2. The code of a freon is written as the three digits (c)(b)(a), where c = carbon atoms − 1, b = hydrogen atoms + 1 and a = fluorine atoms. For CF₂Cl₂: c = 0, b = 1, a = 2, so it is Freon-12.

Haloarenes

1. Preparation: direct halogenation of benzene with Cl₂ or Br₂ and a Lewis acid catalyst (FeCl₃, AlCl₃ or Fe), and from phenol with PCl₅ in poor yield.
2. The C–X bond is shorter and stronger than in haloalkanes because of delocalisation of the halogen lone pair with the ring and because the carbon is sp². Therefore aryl halides do not undergo S_N1 or S_N2 reactions under ordinary conditions. Chlorobenzene gives phenol only with NaOH at 623 K and 300 atm.
3. Nucleophilic aromatic substitution is helped by electron-withdrawing groups (especially NO₂) at the ortho and para positions, which stabilise the negatively charged intermediate. The order is 2,4,6-trinitrochlorobenzene > 2,4-dinitrochlorobenzene > 4-nitrochlorobenzene > chlorobenzene. Meta nitro groups help only slightly.
4. Halogens deactivate the ring through the –I effect but direct incoming electrophiles ortho and para through the +R effect. Chlorobenzene gives a mixture of o- and p-products on nitration, sulphonation, halogenation and Friedel-Crafts reactions, and the para product is major.
5. Fittig reaction: 2C₆H₅Cl + 2Na (dry ether) gives biphenyl. Wurtz-Fittig reaction with an alkyl halide gives an alkylbenzene.
6. Benzylic halides such as C₆H₅CH₂Cl are very reactive in S_N1 and S_N2 reactions. A –CCl₃ group on a ring is meta directing.