Aromatic halogens are the compounds which are obtained by the replacement of one or more hydrogen atoms of aromatic hydrocarbons by an equal number of halogen atoms. These are two types:
i) Aryl halides (Haloarenes) in which the halogen is bonded to a Sp2 hybridized carbon of the benzene ring. For examples, Chlorobenzene (C6H5—Cl), Bromobenzene(C6H5—Br), Iodobenzene (C6H5—I), etc.
ii) Arylalkyl halides (Side-chain halides):- Aryl-alkyl halides are those in which a halogen is bonded to a carbon of the side-chain. For examples, Benzyl chloride (C6H5 CH2 —Cl), Benzal chloride (C6H5 CH—Cl2), Benzo trichloride (C6H5 C—Cl3), etc.
Naming of Haloarenes:
Haloarenes are named by adding the prefix halo (fluoro, chloro, bromo, iodo) before the name of the aromatic hydrocarbon. In case of disubstituted compounds, the relative positions of the substituents 1, 2; 1, 3 and 1, 4 are indicating by the prefixes othro (o), meta (m) and para (p) respectively. e.g.
Preparation of Haloarenes:-
1) Direct Halogenation: – Aryl chloride and aryl bromides are generally prepared by direct chlorination or bromination of benzene. These reactions are carried out in dark, at ordinary temperature in the presence FeCl3, FeBr3 or AlCl3.
2) From Diazonium salts: – Chloro and bromobenzene are prepared by treating a freshly prepared diazonium salt solution with cuprous chloride or cuprous bromide dissolved in corresponding halogen acids.
The Sandmeyer reaction has been modified by using copper powder in the presence of corresponding halogen acid (HCl, HBr) in place of cuprous halide (CuCl, CuBr). The modified reaction is called Gattermann reaction.
Iodobenzene is prepared by warming the diazonium salt solution with aqueous KI solution.
Properties of haloarenes:-
Haloarenes are generally colourless crystalline solids or liquids. They are heavier than water and insoluble in it but soluble in organic solvents. The m.p and b.p. of haloarenes are nearly the same as those of alkyl halides containing the same number of carbon atoms. The b.p. of monohalogen derivatives of benzene are in the order:
Note: The b.p. of isomeric dihaloarenes are nearly the same but their m.p. are quite different as shown below:
- o-dichlorobenzene (b.p. 453, m.p. 256)
- m-dichlorobenzene (b.p. 446, m.p. 249)
- p-dichlorobenzene (b.p. 448, m.p. 325)
The m.p. of para isomer is quite higher than that of ortho and meta is due to the fact that it has symmetrical structure and therefore, its molecule can easily pack closely in the crystal lattice. As a result, intermolecular force of attraction is stronger and therefore, greater energy is required to break its lattice and it melts at higher temperature.
Chemical properties of Haloarenes:-
Haloarenes are chemically less reactive than haloalkanes. The low reactivity of haloarenes in comparison to haloalkanes is due to the following reasons:
(1) Resonance Effect: – In haloarenes, there is delocalization of electrons due to resonance. For examples, chlorobenzene is considered to be a resonance hybrid of the following structures:
It is evident that the contribution of structures III, IV and V imparts a partial double bond character to the carbon-chlorine bond. This is confirmed by X-ray analysis which shows that the C—Cl bond length in chlorobenzene is 169 pm while the C—Cl bond length in ethyl chloride molecule is 182 pm. The shortening of bond length imparts stability to aryl halides and as a result, the bond cleavage becomes rather difficult. The aryl halides are, therefore, less reactive than alkyl halides.
Different hybridisation states of carbon atom: – In haloalkanes, the carbon atom of the C—X bond is sp3 hybridised while in haloarenes, the carbon atom is sp2 hybridised. The sp2 hybridised carbon atom with a greater s- character is more electronegative. It can hold the electron pair of the bond more tightly than the sp3 hybridised carbon atom in alkyl halides. Therefore, it has less tendency to release electrons to the halogen. As a result, the bond cleavage in aryl halides is somewhat more difficult than in alkyl halides.
Polarity of carbon-halogen bond: – The sp2 hybridized carbon atom involved in C—X bond in haloarene molecule is more electronegative than the sp3 hybrid carbon atom in alkyl halide. Therefore, this sp2 hybridised carbon has less tendency to release electrons to the Cl-atom and the C—Cl bond in aryl halides is less polar than in alkyl halides. This is supported by the fact that the dipole moment of chlorobenzene is 1.73 D while the dipole moment of chloroethane is 2.05 D. Thus, lesser the polarity of C—Cl bond, lesser is the reactivity.
Instability of phenyl cation: – In case of haloarenes, the phenyl cation formed as a result of self ionisation will not be stabilised by resonance and therefore, SN1 mechanism cannot occur.
Repulsion between the electron rich attacking nucleophiles and electron rich arenas: – Because electron rich arenes, electron rich nucleophile will not approach closely for the attack because of repulsion. Thus, haloarenes are less reactive towards the substitution reactions than haloalkanes. However, under drastic conditions, aryl halides undergo substitution reactions as discussed below.
Electrophilic Substitution Reactions:
Haloarenes undergo electrophilic substitution reactions in the benzene ring. The halogen atom is ortho and para directing and therefore, substitution takes place at ortho and para positions. Some important reactions of chlorobenzene are as follows-
1) Halogenation: – When chlorobenzene is treated with chlorine in presence of ferric salt as catalyst, it gives ortho & para dichlorobenzene.
2) Nitration: – When chlorobenzene is treated with conc. HNO3 in presence of conc. H2SO4, it gives ortho & para chloronitrobenzene.
3) Sulphonation: – When chlorobenzene is heated with conc.H2SO4, it gives ortho & para chlorobenzene sulphonic acid.
4) Alkylation and acylation:- The reaction, known as Friedel-Craft reaction, is carried out by treating haloarene with alkyl chloride or acyl chloride in the presence of anhydrous aluminium chloride acting as a catalyst. For examples,
Reaction with Mg metals:
Like alkyl halides, aryl bromides and iodides also reacts with magnesium in dry ether to form Grignard reagent.
Chlorobenzene reacts with Mg in presence of tetrahydrofuran (THF) solvent, to give phenyl magnesium chloride.
Reaction with Lithium:
Bromo and iodoarenes reacts with lithium metal in the presence of dry ether to form corresponding organometallic compounds.
Reaction with sodium:
When aryl halide is heated with alkyl halide in presence of sodium & ether, halogen atom is replaced by alkyl group. This reaction is called Wurtz Fitting reaction.
Reaction with Cu powder:
Iodobenzene when heated with Cu powder in a sealed tube, it gives diphenyl. This reaction is called Ullmann reaction.
Nucleophilic Substitution Reaction:
i) Reaction with NaOH: – When aryl halide is heated with aqueous solution of NaOH at 623 K under pressure (about 300 atm), the halogen atom is replaced by hydroxyl group forming phenol.
ii) Reaction with Ammonia: – Chlorobenzene on heating with aqueous solution of ammonia in the presence of cuprous oxide as a catalyst at 475 K and under pressure of 60 atmospheres, the halogen atom is replaced by amino group giving aniline.
iii) Reaction with CuCN: – Chlorobenzene on heating with anhydrous cuprous cyanide in the presence of pyridine or DMF (dimethylformamide), the halogen atom is replaced by cyano group giving cyanobenzene (Benzonitile).
Comparison of reactivity of alkyl, vinyl and aryl halides:
Vinyl halides and aryl halides are less reactive than alkyl halides. This is mainly because the vinyl and aryl halides have partial double bond character between carbon and halogen atoms. As a result, carbon halogen bond of a vinyl or aryl halide is stronger than that of an alkyl halide and the bond cleavage is difficult. As shown below, the p-orbital of chlorine atom having a lone pair of electrons overlaps with the unhybridised p-orbital of carbon (in vinyl and aryl halide) resulting partial double bond character to C—Cl bond. As a result Cl atom becomes firmly attached to C atom and cannot be easily substituted.
However, there is no possibility of double bond character in C—X bond in alkyl halides. In benzyl halides also, there is a —CH2— group separating the halogen atom from a sp2 hybridised carbon. Therefore, the filled p-orbital of halogen atom (e.g. Cl) cannot overlap sidewise with the unhybridised p-orbital of C-atom. Therefore, the C—Cl bond is not very strong and can be easily cleaved. Thus, both alkyl halides and benzyl halides are more reactive than vinyl and aryl halides.
