Class 12 Chemistry · Chapter 6 NotesHaloalkanes and Haloarenes

Halogenated compounds are first classified by the number of halogen atoms they contain as mono-, di- or polyhalogen compounds. Monohalogen compounds are then grouped by the hybridisation of the carbon that carries the halogen. When the halogen is attached to an sp3 hybridised carbon, the compound may be an alkyl halide (haloalkane), an allylic halide in which the halogen is on a carbon next to a carbon-carbon double bond, or a benzylic halide in which the halogen is on a carbon attached to an aromatic ring. When the halogen is attached to an sp2 hybridised carbon, the compound is either a vinylic halide, where the halogen is on a doubly bonded carbon, or an aryl halide (haloarene), where the halogen is directly attached to an aromatic ring. Alkyl halides are further described as primary, secondary or tertiary depending on the nature of the carbon bearing the halogen.

• Haloalkanehalogen bonded to sp3 hybridised carbon of an alkyl group
• Haloarenehalogen bonded to sp2 hybridised carbon of an aryl group
• Allylic halidehalogen on a carbon adjacent to a C=C bond
• Benzylic halidehalogen on a carbon attached to an aromatic ring
• Vinylic halidehalogen on a carbon of a C=C bond

Common names of alkyl halides are written by naming the alkyl group first and then the halide, for example ethyl chloride. In the IUPAC system they are named as halogen-substituted hydrocarbons, so ethyl chloride becomes chloroethane. For monohalogen derivatives of benzene the common and IUPAC names are the same. For dihalogen derivatives of benzene, the common system uses the prefixes o-, m- and p-, while the IUPAC system uses the numerals 1,2; 1,3 and 1,4. Dihaloalkanes that contain the same halogen on the same carbon are called geminal or gem-dihalides, and those with halogens on adjacent carbons are called vicinal or vic-dihalides. In IUPAC nomenclature both are simply named as dihaloalkanes.

• Common namealkyl group + halide, e.g. sec-butyl chloride
• IUPAC namehalosubstituted hydrocarbon, e.g. 2-chlorobutane
• gem-dihalideboth halogens on the same carbon atom
• vic-dihalidehalogens on adjacent carbon atoms

Halogen atoms are more electronegative than carbon, so the carbon-halogen bond is polarised: carbon carries a partial positive charge and the halogen carries a partial negative charge. Moving down the halogen group, atomic size increases from fluorine to iodine, so the C-X bond length increases from C-F to C-I while bond enthalpy decreases. Alkyl halides are generally colourless when pure, though bromides and iodides develop colour on exposure to light, and many volatile halogen compounds have a sweet smell. Because the molecules are polar and have higher molecular mass than the parent hydrocarbons, dipole-dipole and van der Waals forces are stronger, giving higher boiling points. For the same alkyl group the boiling points follow the order RI > RBr > RCl > RF, and branching lowers the boiling point. Para-isomers of dihalobenzenes have higher melting points than ortho- and meta-isomers because their symmetry allows better packing in the crystal lattice. Haloalkanes are only very slightly soluble in water but dissolve readily in organic solvents.

• C-X bond is polar; carbon is partially positive, halogen partially negative
• Bond lengthC-F < C-Cl < C-Br < C-I
• Boiling point order for the same alkyl groupRI > RBr > RCl > RF
• Bromo, iodo and polychloro derivatives are denser than water

Alkyl halides are best prepared from alcohols, whose hydroxyl group is replaced by halogen using concentrated halogen acids, phosphorus halides or thionyl chloride. Thionyl chloride is preferred because the by-products SO2 and HCl are gases and escape, leaving pure alkyl halide. Primary and secondary alcohols need a ZnCl2 catalyst with HCl, while tertiary alcohols react on simply shaking with concentrated HCl at room temperature. The reactivity of alcohols with a given haloacid is 3° > 2° > 1°. Alkyl bromides are made using constant boiling 48% HBr, and alkyl iodides by heating alcohols with sodium or potassium iodide in 95% orthophosphoric acid. Alkanes give mixtures of mono- and polyhaloalkanes on free radical halogenation, while alkenes add hydrogen halides according to Markovnikov's rule or add halogens to form vic-dihalides. Halogen exchange methods include the Finkelstein reaction, in which alkyl chlorides or bromides react with NaI in dry acetone, and the Swarts reaction, in which alkyl chlorides or bromides are heated with metallic fluorides such as AgF, Hg2F2, CoF2 or SbF3.

• R-OH + SOCl2 gives R-Cl + SO2 + HCl
• Alcohol reactivity with a haloacid3° > 2° > 1°
• Finkelstein reactionR-Cl/R-Br + NaI in dry acetone gives R-I
• Swarts reactionR-Cl/R-Br + metallic fluoride gives R-F

Aryl chlorides and bromides are conveniently prepared by electrophilic substitution of arenes with chlorine and bromine in the presence of Lewis acid catalysts such as iron or iron(III) chloride. The ortho and para isomers can be separated because their melting points differ widely. Iodination is reversible and needs an oxidising agent such as HNO3 or HIO4 to oxidise the HI formed, and fluoro compounds are not prepared this way because fluorine is too reactive. Aryl halides are also prepared from primary aromatic amines by the Sandmeyer reaction: the amine is treated with sodium nitrite in cold aqueous mineral acid to form a diazonium salt, which is then mixed with cuprous chloride or cuprous bromide to replace the diazonium group by -Cl or -Br. Replacement by iodine needs no cuprous halide and is done by shaking the diazonium salt with potassium iodide.

• Arenes + Cl2/Br2 with Fe or FeCl3 give aryl chlorides or bromides
• Iodination requires an oxidising agent such as HNO3 or HIO4
• Sandmeyer reactiondiazonium salt + Cu2Cl2 or Cu2Br2 gives aryl halide
• Diazonium salt + KI gives aryl iodide

The polar C-X bond makes haloalkanes susceptible to attack by nucleophiles. In nucleophilic substitution the halogen leaves as a halide ion and is replaced by a nucleophile such as OH-, OR'-, I-, NH3, CN-, NO2- or R'COO-. Cyanide and nitrite ions are ambident nucleophiles because they can link through two different atoms. Two mechanisms operate: SN2 is a one-step bimolecular reaction in which the nucleophile attacks from the side opposite the leaving group, giving inversion of configuration, and its rate depends on both reactants; bulky groups slow it down, so the reactivity order is primary > secondary > tertiary. SN1 is a two-step unimolecular reaction in which the C-X bond first breaks to give a carbocation, which is then attacked by the nucleophile; its rate depends only on the alkyl halide, and the reactivity order is tertiary > secondary > primary. SN1 reactions of optically active halides give racemisation because the planar carbocation can be attacked from either side. Haloalkanes with a β-hydrogen atom undergo β-elimination when heated with alcoholic KOH, giving an alkene; according to the Saytzeff rule, the more substituted alkene is the major product. Haloalkanes also react with metals: with magnesium in dry ether they form Grignard reagents (RMgX), and with sodium in dry ether they undergo the Wurtz reaction to give a hydrocarbon with double the number of carbon atoms.

• SN2one step, bimolecular, inversion of configuration, primary > secondary > tertiary
• SN1two steps via carbocation, unimolecular, racemisation, tertiary > secondary > primary
• β-elimination with alcoholic KOH gives an alkene; Saytzeff rule gives the more substituted alkene
• Grignard reagentRMgX, prepared in dry ether
• Wurtz reaction2R-X + 2Na in dry ether gives R-R + 2NaX

Haloarenes are far less reactive than haloalkanes towards nucleophilic substitution. The lone electron pairs on the halogen conjugate with the ring π electrons, giving the C-X bond partial double bond character, and the sp2 hybridised carbon holds the bond more tightly because of its greater s-character, making the bond shorter and harder to break. The phenyl cation is not stabilised by resonance, so SN1 is ruled out, and an electron-rich nucleophile finds it difficult to approach the electron-rich ring. Chlorobenzene can still be converted to phenol by heating with aqueous sodium hydroxide at 623 K and 300 atmospheres. An electron withdrawing group such as -NO2 at the ortho or para position increases reactivity by stabilising the intermediate carbanion, but a meta -NO2 has no such effect. Haloarenes also undergo the usual electrophilic substitution reactions of benzene, such as halogenation, nitration, sulphonation and Friedel-Crafts reactions. The halogen is slightly deactivating but ortho- and para-directing, so further substitution occurs at ortho and para positions, though more drastic conditions are needed than for benzene. With metals, a mixture of an alkyl halide and an aryl halide and sodium in dry ether gives an alkylarene in the Wurtz-Fittig reaction, while two aryl halides give a biaryl in the Fittig reaction.

• Haloarenes are less reactive than haloalkanes towards nucleophilic substitution
• Chlorobenzene + aqueous NaOH at 623 K and 300 atm gives phenol
• o- and p-NO2 groups increase reactivity; m-NO2 has no effect
• Halogen is o,p-directing but slightly deactivating in electrophilic substitution
• Wurtz-Fittig reaction joins an alkyl group and an aryl group

Compounds containing more than one halogen atom have many industrial uses but also raise environmental concerns. Dichloromethane is used as a solvent, paint remover and propellant, but it harms the central nervous system. Chloroform is a solvent for fats, alkaloids and iodine and is used in making freon R-22; it was once a general anaesthetic but is now replaced by safer substances, and it is slowly oxidised by air in light to the poisonous gas phosgene, so it is stored in closed dark bottles. Iodoform was used as an antiseptic because it liberates free iodine. Carbon tetrachloride is used in making refrigerants and propellants and was once a cleaning fluid and fire extinguisher, but exposure can damage the liver, nerves and heart, and its release depletes the ozone layer. Freons, the chlorofluorocarbons of methane and ethane, are stable, unreactive, non-toxic and easily liquefied; freon 12 (CCl2F2) is made from tetrachloromethane by the Swarts reaction, and in the stratosphere freons initiate radical chain reactions that upset the natural ozone balance. DDT, the first chlorinated organic insecticide, was effective against the mosquito that spreads malaria and lice that carry typhus, but insects developed resistance, it is highly toxic to fish, and being fat soluble and chemically stable it accumulates in animal fatty tissues.

• Chloroform is oxidised by air and light to phosgene (carbonyl chloride)
• Freon 12 (CCl2F2) is manufactured from tetrachloromethane by the Swarts reaction
• Carbon tetrachloride and freons contribute to ozone depletion
• DDT is fat soluble, chemically stable and accumulates in fatty tissues