Aryl Halides

ARYL HALIDES

Aryl halides are the compounds that contain halogen atom directly attached to the benzene ring. They have general formula ArX.

Any halogen compound that contains a benzene ring is not classified as aryl halide. e.g. Benzyl chloride is not an aryl halide, but is a substituted alkyl halide

Nomenclature of aryl halide

Aryl halides are compounds where a halogen atom is directly attached to an aromatic ring. The nomenclature of aryl halides follows the same IUPAC rules as other organic compounds with a few additional rules:

1.     Identify the parent hydrocarbon by selecting the longest chain that contains the halogen atom. The ring is then named as a substituent using the prefix "phenyl-" for benzene.

2.     Number the carbon atoms in the ring starting from the carbon atom directly attached to the halogen atom. If there is more than one halogen atom, the numbering should be such that the halogen atoms are assigned the lowest possible numbers.

3.     Name the halogen substituent as a prefix before the name of the parent hydrocarbon. The prefixes used are fluoro-, chloro-, bromo- and iodo-. If there are more than one halogen atom, the prefixes are used in alphabetical order.

4.     If the compound contains other functional groups, they are named using the appropriate prefixes and suffixes.

5.     If the compound contains more than one substituent, the substituents are named in alphabetical order.

Preparation of Haloarenes

 

Aryl halide From hydrocarbons by electrophilic substitution

This involves the direct halogenation of benzene ring in the presence of Lewis acid catalysts like iron or iron (III) chloride.

Sandmeyer’s reaction

Aniline is treated with sodium nitrite to give a diazonium salt which is then treated with cuprous chloride or cuprous bromide to produce the corresponding aryl halide:

Physical Properties of aryl halides

1.     Aryl halides are insoluble in water but soluble in organic solvents like alcohol, ether, benzene, etc. due to the non-polar nature of the aryl group.

2.     Aryl halides have higher boiling points than corresponding alkyl halides due to the presence of the aromatic ring which provides additional van der Waals forces of attraction.

3.     Aryl halides have a high melting point due to the presence of the polar halogen atom and the non-polar aromatic ring which results in strong intermolecular forces of attraction.

4.     Aryl halides are less reactive than alkyl halides due to the electron-withdrawing nature of the aromatic ring which decreases the reactivity of the halogen atom towards nucleophilic substitution reactions.

5.     The boiling point and melting point of aryl halides increase with the increase in the size of the halogen atom attached to the aryl group. For example, fluorobenzene has the lowest boiling point and melting point among the aryl halides, while iodobenzene has the highest.

6.     The reactivity of aryl halides towards nucleophilic substitution reactions increases with the decrease in the size of the halogen atom attached to the aryl group. For example, iodobenzene is more reactive towards nucleophilic substitution reactions than chlorobenzene.

 

Chemical Reactions of aryl halides

Nucleophilic substitution of aryl halides

Aryl halides are extremely less reactive towards nucleophilic substitution reactions due to the following reasons:

 (i) Resonance effect : In haloarenes, the electron pairs on halogen atom are in conjugation with π-electrons of the ring and the following resonating structures are possible.

C—Cl bond acquires a partial double bond character due to resonance. As a result, the bond cleavage in haloarene is difficult than haloalkane and therefore, they are less reactive towards nucleophilic substitution reaction.

 (ii) Difference in hybridisation of carbon atom in C—X bond: In haloalkane, the carbon atom attached to halogen is sp3 hybridised while in case of haloarene, the carbon atom attached to halogen is sp2-hybridised.



The sp2 hybridised carbon with a greater s-character is more electronegative and can hold the electron pair of C—X bond more tightly than sp3-hybridised carbon in haloalkane with less s-chararcter. Thus, C—Cl bond length in haloalkane is 177pm while in haloarene is 169 pm. Since it is difficult to break a shorter bond than a longer bond, therefore, haloarenes are less reactive than haloalkanes towards nucleophilic substitution reaction.

 (iii) 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 is ruled out.

(iv) Because of the possible repulsion, it is less likely for the electron rich nucleophile to approach electron rich arenes.

Replacement by hydroxyl group

 Chlorobenzene can be converted into phenol by heating in aqueous sodium hydroxide solution at a temperature of 623K and a pressure of 300 atmospheres.



The presence of an electron withdrawing group (-NO2) at ortho- and para-positions increases the reactivity of haloarenes.



 

 The effect is pronounced when (-NO2) group is introduced at ortho- and para- positions. However, no effect on reactivity of haloarenes is observed by the presence of electron withdrawing group at meta-position. Mechanism of the reaction is as depicted:

 

Electrophilic substitution reactions of aryl halide

  • Haloarenes can undergo electrophilic reactions of the benzene ring, such as halogenation, nitration, sulphonation, and Friedel-Crafts reactions.
  • The halogen atom in haloarenes is slightly deactivating and o, p-directing, meaning that further substitution occurs at ortho- and para-positions with respect to the halogen atom.
  • The o, p-directing influence of the halogen atom can be understood through the resonance structures of halobenzene, where electron density increases more at ortho- and para-positions due to resonance.
  • The halogen atom also has a -I effect, withdrawing electrons from the benzene ring and deactivating it compared to benzene.
  • Due to the deactivating effect of the halogen atom and its o, p-directing influence, electrophilic substitution reactions in haloarenes occur slowly and require more drastic conditions compared to those in benzene.

 

Halogenation of aryl halides

Nitration of aryl halides

Sulphonation of aryl halides

 

Friedel-Crafts reaction

Reaction of aryl halides with metals

Wurtz-Fittig reaction

A mixture of an alkyl halide and aryl halide gives an alkylarene when treated with sodium in dry ether and is called Wurtz-Fittig reaction.


Fittig reaction

Aryl halides also give analogous compounds when treated with sodium in dry ether, in which two aryl groups are joined together. It is called Fittig reaction.