Haloalkanes and Haloarenes

A haloalkane is an alkane in which one or more hydrogens are replaced by halogen; the halogen sits on an sp³ carbon of an aliphatic chain. A haloarene has the halogen bonded directly to an sp² carbon of an aromatic ring. Keep this distinction in mind from the first page, because everything about reactivity in this chapter follows from whether the C–X carbon is sp³ or sp².

NCERT classifies the compounds in two ways. First, by the number of halogen atoms: mono-, di-, tri- and polyhalogen compounds. Dihalides get two more names that the board likes. A geminal dihalide (gem-dihalide) has both halogens on the same carbon, for example ethylidene chloride, CH₃CHCl₂, which is 1,1-dichloroethane. A vicinal dihalide (vic-dihalide) has them on adjacent carbons, for example ethylene dichloride, ClCH₂CH₂Cl, which is 1,2-dichloroethane.

Second, by the hybridisation of the carbon carrying the halogen. Compounds with an sp³ C–X bond are of three types. Alkyl halides (R–X) are primary (1°), secondary (2°) or tertiary (3°) according to whether the halogen-bearing carbon is joined to one, two or three other carbons. Allylic halides have the halogen on an sp³ carbon that is next to a C=C double bond, as in CH₂=CH–CH₂Cl (3-chloropropene). Benzylic halides have the halogen on an sp³ carbon attached to a benzene ring, as in C₆H₅CH₂Cl. Compounds with an sp² C–X bond are of two types: vinylic halides, where the halogen is on a carbon of a C=C double bond (CH₂=CHCl), and aryl halides, where the halogen is on a benzene ring carbon (C₆H₅Cl).

The trap: benzyl chloride is not an aryl halide. The chlorine is on the CH₂, an sp³ carbon, so it behaves like a very reactive alkyl halide. Chlorobenzene, with chlorine on the ring, is the unreactive one.

In the common system an alkyl halide is named as alkyl + halide (methyl chloride, isopropyl bromide, tert-butyl chloride). In the IUPAC system the halogen is a substituent, so the name is halo + alkane with the position number: CH₃CH₂CH₂Cl is 1-chloropropane, (CH₃)₂CHBr is 2-bromopropane and (CH₃)₃CCl is 2-chloro-2-methylpropane. Two rules earn most of the marks: number the chain so the halogen and other substituents get the lowest set of locants, and when there is a tie, list substituents alphabetically and give the lower number to the one that comes first in the alphabet (bromo before chloro before iodo before methyl).

Dihalides: the common names ethylene dichloride (1,2-dichloroethane) and ethylidene chloride (1,1-dichloroethane) are asked directly. Haloarenes: C₆H₅Cl is chlorobenzene in both systems; for disubstituted rings the common prefixes o-, m-, p- correspond to 1,2-, 1,3- and 1,4- in IUPAC, so p-dichlorobenzene is 1,4-dichlorobenzene. For haloalkenes the double bond gets priority in numbering, so CH₂=CH–CH₂Br is 3-bromoprop-1-ene and CH₂=CHCl is chloroethene (vinyl chloride).

Some NCERT names the board reuses: CH₃CH(Cl)CH(Br)CH₃ is 2-bromo-3-chlorobutane (bromo is alphabetically first so the chain is numbered from its side); CHF₂CBrClF is 1-bromo-1-chloro-1,2,2-trifluoroethane; ClCH₂C≡CCH₂Br is 1-bromo-4-chlorobut-2-yne; (CH₃)₂CHCH(Cl)CH₃ is 2-chloro-3-methylbutane; CH₃CH=CH–C(Br)(CH₃)₂ is 4-bromo-4-methylpent-2-ene. Practise the reverse direction too: 4-tert-butyl-3-iodoheptane, 1-chloro-4-ethylcyclohexane and 1-bromo-4-sec-butyl-2-methylbenzene have all been set as structure-drawing questions.

Halogens are more electronegative than carbon, so the C–X bond is polar: carbon carries a partial positive charge and the halogen a partial negative charge. This is why haloalkanes are attacked by nucleophiles at carbon and why the halide ion is a good leaving group.

Down the group the halogen atom gets bigger, so the C–X bond becomes longer and weaker. NCERT gives the numbers and the examiner accepts them as they are: bond lengths C–F 139 pm, C–Cl 178 pm, C–Br 193 pm, C–I 214 pm; bond enthalpies C–F 452, C–Cl 351, C–Br 293, C–I 234 kJ mol⁻¹. So the C–I bond breaks most easily, which makes iodides the most reactive in substitution and elimination, and the C–F bond hardly breaks at all.

Dipole moments do not follow a simple trend. The values for the methyl halides are CH₃F 1.847 D, CH₃Cl 1.860 D, CH₃Br 1.830 D and CH₃I 1.636 D. Fluorine is the most electronegative, but the C–F bond is so short that the product of charge and distance is slightly smaller than for C–Cl. This is why CH₃Cl has the highest dipole moment among the methyl halides, a favourite one-mark question.

Two comparisons from this section recur in give-reason form. Among CH₂Cl₂, CHCl₃ and CCl₄, dichloromethane has the highest dipole moment (about 1.60 D) because its two C–Cl dipoles reinforce each other, in CHCl₃ the three C–Cl dipoles partly cancel (about 1.08 D), and in the tetrahedral CCl₄ they cancel completely to give zero. And the C–Cl bond in chlorobenzene is shorter (about 169 pm) and stronger than in an alkyl chloride because the carbon is sp² and the bond has partial double-bond character from resonance; that point is developed in section 6.7.2.

From alcohols. The –OH group is replaced by halogen. With halogen acids: primary and secondary alcohols need HCl with anhydrous ZnCl₂ (Lucas reagent), tertiary alcohols react with concentrated HCl alone by shaking. Bromides are made with constant boiling HBr (48%) or with NaBr and concentrated H₂SO₄. Iodides are made with KI and 95% orthophosphoric acid, never with H₂SO₄, because sulphuric acid oxidises the HI formed to I₂. With phosphorus halides: 3ROH + PX₃ → 3RX + H₃PO₃ (X = Cl, Br), ROH + PCl₅ → RCl + POCl₃ + HCl, and for bromides and iodides red phosphorus with Br₂ or I₂ generates PBr₃ or PI₃ in situ. With thionyl chloride: ROH + SOCl₂ → RCl + SO₂ + HCl. The SOCl₂ method is preferred for chlorides because both by-products are gases and escape, leaving pure alkyl chloride. Reactivity of alcohols towards these reagents is 3° > 2° > 1°.

From hydrocarbons. Free radical chlorination or bromination of an alkane gives a mixture of isomeric mono- and polyhaloalkanes that is hard to separate, so it is a poor route to a single product; it is still asked as 'how many monochloro products' from a given alkane. Electrophilic substitution of arenes with Cl₂ or Br₂ in the presence of a Lewis acid (FeCl₃, FeBr₃) gives ortho and para haloarenes; the isomers boil close together but the para isomer melts much higher, so they are separated by fractional crystallisation. Iodination is reversible because HI reduces the iodoarene back, so an oxidising agent such as HNO₃ or HIO₄ is added to destroy HI. Fluoro compounds are not made this way because fluorine is too reactive.

Sandmeyer's reaction. A primary aromatic amine is diazotised with NaNO₂ and HCl at 273–278 K to give the benzenediazonium salt; treatment with Cu₂Cl₂/HCl or Cu₂Br₂/HBr replaces –N₂⁺ by Cl or Br. Iodine needs no copper salt: shaking the diazonium solution with KI gives iodobenzene.

From alkenes. HX adds across C=C by Markovnikov's rule (propene + HBr → 2-bromopropane). In the presence of peroxide, HBr alone adds anti-Markovnikov (1-bromopropane); HCl and HI do not show the peroxide effect. X₂ in CCl₄ adds to give vic-dihalides, and the discharge of the reddish-brown colour of bromine is the test for unsaturation.

Halogen exchange. Finkelstein reaction: R–Cl or R–Br with NaI in dry acetone gives R–I; NaCl and NaBr are insoluble in acetone and precipitate, pulling the equilibrium forward. Swarts reaction: an alkyl chloride or bromide heated with a metallic fluoride such as AgF, Hg₂F₂, CoF₂ or SbF₃ gives the alkyl fluoride, e.g. CH₃Br + AgF → CH₃F + AgBr.

NCERT gives this as a short separate section, and it is really two of the routes above applied to the aromatic ring. Aryl halides cannot be made from phenols by replacing –OH with halogen, because the C–O bond in phenol has partial double-bond character and is not broken by HX or PX₃; this is an assertion-reason favourite, so hold on to the reason.

Electrophilic substitution. Benzene or a substituted benzene reacts with Cl₂ or Br₂ in the presence of the Lewis acid catalyst FeCl₃ or FeBr₃ (in the dark, at ordinary temperature). The halogen is polarised by the catalyst, the ring attacks the positive end, and the product is the haloarene. With toluene, the methyl group directs to ortho and para, giving 2-chlorotoluene and 4-chlorotoluene, which are separated by exploiting the large difference in melting point of the para isomer. The examiner expects the note that the reaction must be done in the absence of sunlight; with sunlight and no catalyst chlorine attacks the side chain instead and gives benzyl chloride.

From diazonium salts. Aniline is diazotised with NaNO₂ and HCl at 273–278 K to give benzenediazonium chloride. Sandmeyer's reaction with Cu₂Cl₂/HCl or Cu₂Br₂/HBr replaces the diazonium group by Cl or Br with loss of N₂. For iodobenzene, the diazonium salt is simply warmed with potassium iodide. This is the route to use whenever a conversion asks you to place a halogen at a specific position that direct halogenation would not give, for example aniline to chlorobenzene, or benzene to 4-bromonitrobenzene (nitrate first, then brominate; the nitro group is meta-directing, so instead brominate first and then nitrate, which puts the nitro para to bromine).

The Fittig and Wurtz-Fittig reactions, which use haloarenes to build biphenyl and alkylbenzenes, are covered under the reactions of haloarenes in section 6.7.2.

Pure haloalkanes are colourless, but bromides and iodides develop colour on exposure to light because a little halogen is liberated. Many volatile halogen compounds have a sweet smell.

Boiling points. The molecules are polar, so dipole-dipole attractions add to van der Waals forces and the boiling points are well above those of the parent hydrocarbons. For the same alkyl group the order is RI > RBr > RCl > RF, because the size and mass of the halogen, and hence the strength of the van der Waals forces, increase down the group. Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Boiling point rises with the number of carbon atoms, and with the number of halogen atoms (CH₃Cl < CH₂Cl₂ < CHCl₃ < CCl₄). For isomers, boiling point falls with branching because the molecule becomes more compact and its surface area drops: 1-bromobutane > 2-bromobutane > 2-bromo-2-methylpropane. Hence an NCERT order: chloromethane < bromomethane < dibromomethane < bromoform, and isopropyl chloride < 1-chloropropane < 1-chlorobutane.

Dihalobenzenes. The ortho, meta and para isomers have nearly the same boiling points, but the para isomer has a much higher melting point and a lower solubility. The reason the examiner wants: the para isomer is symmetrical, fits better into the crystal lattice, and so has stronger intermolecular forces in the solid; more energy is needed to melt it or to pull it apart to dissolve it.

Density. Bromo, iodo and polychloro derivatives are heavier than water. Density increases with the number of carbon atoms, the number of halogen atoms and the atomic mass of the halogen.

Solubility. Haloalkanes are only very slightly soluble in water although they are polar. To dissolve, the attractions between haloalkane molecules and the hydrogen bonds between water molecules must both be broken, and the new attractions between haloalkane and water are not strong enough to make up for this. They dissolve readily in organic solvents because the new forces are about as strong as those broken.

The carbon of the C–X bond is electron deficient, so a nucleophile attacks it and the halide leaves: R–X + Nu⁻ → R–Nu + X⁻. Products depend on the nucleophile: OH⁻ (aqueous KOH) gives an alcohol, R'O⁻ gives an ether (Williamson synthesis), KCN gives a nitrile, AgCN gives an isocyanide, KNO₂ gives an alkyl nitrite R–O–N=O, AgNO₂ gives a nitroalkane R–NO₂, NH₃ gives an amine, RCOOAg gives an ester, and LiAlH₄ reduces R–X to the alkane. CN⁻ and NO₂⁻ are ambident nucleophiles: they can attack through either of two atoms. KCN is ionic and the cyanide ion attacks through carbon (the C–C bond is stronger than C–N), while in covalent AgCN the carbon is bonded to silver, so nitrogen attacks and the isocyanide forms.

S_N2 (substitution nucleophilic bimolecular). Studied by Hughes and Ingold in 1937 with CH₃Cl + OH⁻ → CH₃OH + Cl⁻. The rate depends on both reactants: rate = k[CH₃Cl][OH⁻], second order. It is a one-step process. The nucleophile attacks the carbon from the side opposite to the halogen, and in the transition state the carbon is bonded partially to both the incoming nucleophile and the leaving halide, with the three other groups in a plane. As the new bond forms the old one breaks and the three groups flip like an umbrella in the wind: the configuration is inverted (Walden inversion). Bulky groups around the carbon obstruct the attack, so reactivity is CH₃X > 1° > 2° > 3°.

S_N1 (substitution nucleophilic unimolecular). The model reaction is (CH₃)₃C–Br + OH⁻ → (CH₃)₃C–OH + Br⁻ in a polar protic solvent such as water, ethanol or acetic acid. Rate = k[(CH₃)₃CBr], first order; it depends only on the halide. The reaction is in two steps. Step 1, slow and rate determining: the C–Br bond breaks heterolytically to give a planar tert-butyl carbocation and Br⁻, helped by solvation. Step 2, fast: the nucleophile attacks the carbocation. Since the rate depends on how easily the carbocation forms, reactivity is 3° > 2° > 1° > CH₃X, the reverse of S_N2. Allylic and benzylic halides react fast by S_N1 because their carbocations are stabilised by resonance.

For both mechanisms the leaving group matters: R–I > R–Br > R–Cl > R–F, following the weakening of the C–X bond. Between C₆H₅CH₂Cl and C₆H₅CH(Cl)C₆H₅, the second is hydrolysed faster because its carbocation is stabilised by two phenyl rings.

Plane polarised light and optical activity. Ordinary light vibrates in all planes perpendicular to its direction; passing it through a Nicol prism gives plane polarised light. A compound that rotates the plane of polarised light is optically active. If it rotates the plane clockwise (as seen by an observer looking towards the light) it is dextrorotatory, shown as d or (+); anticlockwise is laevorotatory, l or (−). The instrument is a polarimeter.

Chirality. Objects that are not superimposable on their mirror images are chiral (like your hands); those that are superimposable are achiral. A molecule with a carbon bonded to four different groups (an asymmetric or stereocentre) is chiral: 2-chlorobutane, 2-bromobutane, butan-2-ol and bromochloroiodomethane are chiral; 1-chlorobutane, propan-2-ol and 2-bromopropane are achiral. The two non-superimposable mirror-image forms are enantiomers. They have identical physical properties except that they rotate polarised light by equal amounts in opposite directions. An equimolar mixture of the two enantiomers is a racemic mixture, shown as (±) or dl; it is optically inactive because the rotations cancel (external compensation). Converting an enantiomer into a racemic mixture is called racemisation.

Retention, inversion and racemisation. If the spatial arrangement of the groups around the stereocentre is kept, the reaction proceeds with retention of configuration. If the product's configuration is the mirror image of the starting compound's, there is inversion. If equal amounts of both are formed, the product is racemic.

What the mechanisms do. In S_N2 the nucleophile attacks from the side opposite the leaving group, so an optically active halide gives a product of inverted configuration: (−)-2-bromooctane with OH⁻ gives (+)-octan-2-ol. In S_N1 the intermediate carbocation is planar and sp², so the nucleophile can attack from either face with equal probability; 50% inversion and 50% retention give a racemic mixture. So an optically active tertiary or secondary halide hydrolysed by the S_N1 path gives a racemic (optically inactive) alcohol. The board asks this both ways: 'which mechanism gives inversion' and 'why does the product show no optical rotation'.

β-Elimination (dehydrohalogenation). When a haloalkane with a β-hydrogen is heated with alcoholic KOH, the halogen is removed from the α-carbon and a hydrogen from the β-carbon, and an alkene forms. If more than one β-hydrogen is available, more than one alkene can form. Saytzeff's rule decides the major product: the more highly substituted alkene (the one with more alkyl groups on the double-bond carbons) is preferred. So 2-bromopentane with alcoholic KOH gives pent-2-ene (major, 81%) and pent-1-ene (minor, 19%). 2-Bromobutane gives but-2-ene as the major product; 2-chloro-2-methylbutane gives 2-methylbut-2-ene rather than 2-methylbut-1-ene.

Aqueous versus alcoholic KOH. In water KOH ionises to give hydroxide ions, a strong nucleophile that attacks the carbon and gives an alcohol by substitution. In alcohol the ethoxide ion (from KOH + C₂H₅OH) is a stronger base than a nucleophile, so it removes the β-hydrogen and gives the alkene by elimination. This is a two-mark give-reason that appears almost every second year. Elimination is also favoured by higher temperature, a bulky base and a tertiary halide; substitution by a lower temperature, a small strong nucleophile and a primary halide.

Reaction with magnesium: Grignard reagents. A haloalkane reacts with magnesium metal in dry ether to give an alkyl magnesium halide, RMgX, discovered by Victor Grignard in 1900. The C–Mg bond is covalent but highly polar, with carbon towards the negative end, so the alkyl group behaves as a carbanion and adds to carbonyl compounds to build C–C bonds (used in the Alcohols chapter). Grignard reagents are extremely reactive towards any compound with an acidic hydrogen, even water: RMgX + H₂O → R–H + Mg(OH)X. That is why they must be prepared under anhydrous conditions, and why the reaction with D₂O is used to put a deuterium atom onto a carbon.

Wurtz reaction. Two molecules of alkyl halide react with sodium in dry ether to give an alkane with twice the number of carbon atoms: 2R–X + 2Na → R–R + 2NaX. It gives symmetrical alkanes; 1-chlorobutane with sodium gives n-octane, and isobutyl bromide gives 2,5-dimethylhexane.

Nucleophilic substitution. Aryl halides are far less reactive than haloalkanes towards nucleophiles. NCERT gives four reasons and the examiner wants at least two, ideally with the resonance structures. (1) Resonance: the lone pairs on chlorine delocalise into the ring, so the C–Cl bond acquires partial double-bond character and is harder to break. (2) Hybridisation: the ring carbon is sp², more electronegative than an sp³ carbon, so it holds the shared pair more tightly and the bond is shorter (169 pm against 177 pm in haloalkanes) and stronger. (3) Instability of the phenyl cation: an S_N1 path would need a phenyl cation, which cannot be stabilised by resonance, so it does not form. (4) Repulsion: the electron-rich ring repels an approaching electron-rich nucleophile. Chlorobenzene is converted to phenol only by heating with aqueous NaOH at 623 K and 300 atm (the Dow's process), followed by acidification.

Effect of substituents. Electron-withdrawing groups at the ortho and para positions make nucleophilic substitution easier, because the negative charge of the intermediate carbanion is delocalised onto the nitro group. The conditions for hydrolysis to phenol fall steadily: chlorobenzene needs 623 K and 300 atm; 4-nitrochlorobenzene needs 15% NaOH at 443 K; 2,4-dinitrochlorobenzene needs 15% NaOH at 368 K; 2,4,6-trinitrochlorobenzene is hydrolysed by warm water alone. A nitro group at the meta position has no such effect because none of the resonance structures puts the negative charge on the carbon bearing it.

Electrophilic substitution. Haloarenes undergo the usual reactions of benzene: halogenation (Cl₂/FeCl₃ gives 1,2- and 1,4-dichlorobenzene), nitration (conc. HNO₃ + conc. H₂SO₄ gives mainly 1-chloro-4-nitrobenzene), sulphonation (conc. H₂SO₄ gives 4-chlorobenzenesulphonic acid), Friedel-Crafts alkylation (CH₃Cl/anhyd. AlCl₃ gives 4-chlorotoluene) and acylation (CH₃COCl/AlCl₃ gives 4-chloroacetophenone). The halogen is deactivating but ortho- and para-directing. Its −I effect withdraws electrons and deactivates the ring; but resonance places the electron density at ortho and para positions, so the attack happens there. The para product predominates because of steric hindrance at the ortho positions.

Reaction with metals. Wurtz-Fittig reaction: an aryl halide and an alkyl halide with sodium in dry ether give an alkylarene (chlorobenzene + CH₃Cl + 2Na → toluene + 2NaCl). Fittig reaction: two aryl halides with sodium give a biaryl (2C₆H₅Cl + 2Na → C₆H₅–C₆H₅ + 2NaCl). Bromobenzene with Mg in dry ether gives phenylmagnesium bromide.

Carbon compounds with more than one halogen atom are useful in industry and agriculture, and the board asks their uses and their harmful effects in one- or two-mark bits.

Dichloromethane (methylene chloride, CH₂Cl₂) is a solvent for paint removers, a propellant in aerosols, a process solvent in drug manufacture and a metal-cleaning and finishing solvent. It harms the human central nervous system; low exposure causes slightly impaired hearing and vision, higher doses dizziness, nausea and tingling of the fingers and toes. Skin contact causes burning and redness; direct contact with the eye damages the cornea.

Trichloromethane (chloroform, CHCl₃) is a solvent for fats, alkaloids and iodine, and is used mainly to make the refrigerant R-22 (chlorodifluoromethane). It was once an anaesthetic in surgery but has been replaced by safer ones such as ether. Inhaling it depresses the central nervous system; about 900 parts per million for a short time causes dizziness, fatigue and headache, and long exposure damages the liver (where it is metabolised to phosgene) and kidneys and causes skin sores. In air and light it is slowly oxidised to the poisonous gas phosgene, CCl₂O, so it is stored in closed, dark-coloured bottles filled to the brim to keep air out.

Triiodomethane (iodoform, CHI₃) was used as an antiseptic; the action is due to the free iodine it liberates, not to the compound itself. It has been replaced by other iodine formulations because of its objectionable smell.

Tetrachloromethane (carbon tetrachloride, CCl₄) is used to make refrigerants and aerosol propellants, as a feedstock for chlorofluorocarbons, and as a cleaning fluid, degreasing agent, spot remover and fire extinguisher. It causes liver cancer, and exposure brings dizziness, nausea and vomiting, nerve damage and in severe cases coma or death. Released into the air it reaches the stratosphere and depletes the ozone layer, increasing UV exposure, skin cancer and eye disease.

Freons are chlorofluorocarbons of methane and ethane: stable, unreactive, non-toxic, non-corrosive and easily liquefied. Freon 12 (CCl₂F₂), the commonest, is made from CCl₄ by the Swarts reaction. They are used as aerosol propellants and in refrigeration and air conditioning, but they diffuse unchanged into the stratosphere and start radical chain reactions that break down ozone.

DDT (p,p'-dichlorodiphenyltrichloroethane) was the first chlorinated organic insecticide, synthesised in 1873; Paul Müller discovered its insecticidal action in 1939 and won the Nobel Prize in 1948. It was used against the mosquito that spreads malaria and the lice that spread typhus. Insects developed resistance, it is highly toxic to fish, and because it is chemically stable and not metabolised rapidly it accumulates in fatty tissues. Its use was banned in the United States in 1973.