Aryl Halides

• What are 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

• Preparation Methods of Aryl Halides

1. Halogenation

For introducing only the halogen at para position, the Lewis acid thallium acetate is used.

2. From diazonium salts

• Properties of Aryl Halides

1. Reactivity of Aryl halides

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.

Structures III, IV and V stabilise chlorobenzene molecule and give a double bond character to the carbon-chlorine bond. Now because of this the carbon-chlorine bond has more strength and hence aryl halides are more stable towards Nucleophilic substitution reactions. In Alkyl halides the carbon atom attached to halogen is sp³ hybridized and in aryl halides it is sp², hybridized, as sp² hybridized carbon is more electronegative it does not permit the chlorine atom to get displaced with the bonded pair of electrons.

• Nucleophilic Substitution Reactions of Aryl Halides

Aryl halides undergo Nucleophilic substitution reactions when a strong Electron withdrawing group is present on the benzene ring. Electron withdrawing groups activate the benzene ring towards nucleophilic substitution in aryl halides whereas Electron donating groups deactivate the ring.

Mechanism: Bimolecular Displacement Mechanism

Any factor that stabilizes the carbanion will increase the rate of Nucleophilic substitution reaction by dispersing the charge present on resonating structures. An electron withdrawing group present at meta position does not activate the ring as much as it does from ortho and para position. This can be known by looking at following resonance structures

Elimination – Addition Mechanism

In the absence of an electron withdrawing group, nucleophilic substitution takes place in presence of very strong bases, but the mechanism is entirely different from what we have seen in bimolecular nucleophilic substitution reactions. These reaction proceed by a mechanism called benzyne mechanism.

Benzyne is a symmetrical intermediate and can be attacked by nucleophile at both the positions.

Isotopic labelling confirmed that there is an equal chance of abstraction from both carbons. An aryl halide which does not contain alpha hydrogen with respect to halogen does not undergo this reaction. In the reactions involving Benzyne intermediates, two factors affect the position of incoming group, the first one is direction of aryne formation. When there are groups ortho or para to the leaving group, then, the following intermediates should be formed.

when a meta group is present, aryne can form in two ways, In such cases 

more acidic hydrogen is removed, i.e., an electron attracting ‘Z’ favours removal of ortho hydrogen while an electron donating ‘Z’ favours removal of para hydrogen.

• Electrophilic Substitution Reactions of Aryl Halides

 1. Halogenation of Aryl halides

2. Nitration of Aryl halides

3. Sulphonation of Aryl halides

 4.  Friedel-Crafts reaction for Aryl halides

•  Reaction of Haloarenes With Metals

1. Wurtz-Fittig reaction

2. Fittig reaction

You can also refer to  Organic Chemistry Revision Notes and IIT JEE Chemistry Syllabus

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