Chemistry > Alkyl Halides and Aryl Halides > 12.0 Chemical Properties of Aryl Halides
Alkyl Halides and Aryl Halides
1.0 Alkyl halides
2.0 Halogen Derivatives of Unsaturated Hydrocarbons
3.0 Nomenclature
4.0 Isomerism
5.0 Nature of $C- X$ bond
6.0 Preparation
7.0 Physical Properties
8.0 Chemical Properties
9.0 Elimination Reactions
10.0 Reaction with Metals
11.0 Aryl Halides
12.0 Chemical Properties of Aryl Halides
13.0 Halogenation
14.0 Sulphonation
15.0 Nitration
16.0 Friedel-Craft's alkylation
17.0 Friedel Craft's acylation
12.1 Nucleophilic substitution reactions
1. Nucleophilic substitution
Aryl halides, unlike alkyl halides, do not easily undergo nucleophilic substitution reactions. This is because of the following reasons:
- Stabilisation by resonance
- Hybridization
- Polarity of $C-X$ bond
- Instability of phenyl cation
- Electron rich arenes
Stability by resonance:
Aryl halides can form the following resonant structures:
The resonant hybrid structure indicates that the $C-Cl$ bond has a partial double bond character. Hence, the $C-Cl$ bond in chlorobenzene is shorter and thus it is more stronger as compared to haloalkanes. Since it is stronger, the cleavage of the bond requires more energy, and thus nucleophilic substitution does not occur readily.
Hybridization
The hybridization of carbon atom (attached the halogen) is $sp^3$ while the carbon in aryl halides are $sp^2$ hybridized. The $sp^2$ hybridised carbon has a greater s-character and is more electronegative. Thus, it can hold the electron pair of C-X bond more tightly making it difficult for the cleavage of the bond to occur.
Polarity of $C-X$ bond
The $C-X$ bond in aryl halides is less polar than the bond in haloalkanes because the electron withdrawing inductive effect of halogen is opposed by the electron releasing resonance effect. Thus, aryl halides are less reactive.
Instability of phenyl cation
$SN_1$ reactions cannot occur since the phenyl cation is very unstable.
Electron rich arenes
- Replacement by hydroxyl group: Reaction of chlorobenzene with $NaOH$ at a temperature of ${350^ \circ }C$ and a pressure of $300\ atm$ forms sodium phenoxide. On acidification, sodium phenoxide produces phenol. This is known as Dow's process for the manufacture of phenol.
- Replacement by amino group: Reaction of haloarenes with aqueous ammonia at a temperature of ${200^ \circ }C$ and a pressure of $60\ atm$ replaces the halogen atom on haloarenes by an amino group. This reaction happens in the presence of cuprous oxide.
- Replacement by methoxy group: Reaction of chlorobenzene and sodium methoxide at ${200^ \circ }C$ in the presence of copper salts forms Anisole.
Reduction
Benzene is obtained when chlorobenzene is reduced in the presence of $NaOH$ solution containing a $Ni-Al$ alloy or lithium aluminum hydride.
Reaction with metals
- The treatment of a mixture of alkyl halides and aryl halides with sodium in the presence of dry ether forms alkylarene. This reaction is known as Wurtz-Fittig reaction.
- Through this reaction, toluene can be formed from chlorobenzene.
- Treatment of aryl halides with sodium in the presence of dry ether forms analogous compounds. This reaction is known as Fittig reaction.
