Arenes
Arenes are aromatic hydrocarbons. The term "aromatic" originally referred to their pleasant smells, but now implies a particular sort of delocalised bonding. The simplest of them is benzene itself, \({ C }_{ 6 }{ H }_{ 6 }\).
The next simplest is methylbenzene(oldname:toluene) which has one of the hydrogen atoms attached to the ring replaced by a methyl group: \({ C }_{ 6 }{ H }_{ 5 }C{ H }_{ 3 }\).
Contents
Aryl Halides - Preparation
- From dizoniam salts:
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- Halogenation of Aromatic systems:
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Aryl Halides - Chemical Properties
Reaction occurs instantly with tertiary alkyl or benzly halides, and with in five minutes or so with primary and secondary halides, but halobenzenes or vinyl halides reacts very very slowly (heated for days with \(alc.AgN{ O }_{ 3 }\))
Aryl halides undergo nucleophilic substitution reaction only in extreme conditions, when conditions are feasible
Arly halides can't be used in FRIEDEL-CRAFT'S alkylation reaction
The presence of electron withdrawing groups like- \(N{ O }_{ 2 },C{ F }_{ 3 }\),e.t.c at ortho/para position to the halogen atom makes the arly halides more susceptible to nucleophilic attack.
Aryl Halides - Mechanisms
- \({ S }_{ N }\)Ar Mechanism
Nucleophilic aromatic substitution(Addition-Elimination)
Rate determining step(R.D.S): First step, when nucleophile \((O{ H }^{ - })\) attacks aryl halide
Nucleophile attacks the carbon having halogen because only that carbon has \({ \partial }^{ + }\) charge.
Important Points and Examples
- CINE substitution is followed when nucleophile is present on the carbon other than the one in which, the leaving group is present
- Presence of electron withdrawing group at ortho/para to the halide group, enhances reactivity.
- Arly halides can be distinguished from alkyl halides by decolorisation of bromine water:
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