Arenes

Arenes are aromatic hydrocarbons. The term "aromatic" originally referred to their pleasant smells, but now implies a particular sort of delocalized bonding.

The simplest of them is benzene itself, $\ce{C6H6}$.

The next simplest is methylbenzene (oldname: toluene) which has one of the hydrogen atoms attached to the ring replaced by a methyl group: $\ce{C6H5CH3}$.

1. From dizoniam salts:$$

$$
2. Halogenation of aromatic systems:

1. Reaction occurs instantly with tertiary alkyl or benzyl halides, and within five minutes or so with primary and secondary halides, but halobenzenes or vinyl halides react very slowly $\big($heated for days with $alc.\ce{AgNO3}\big).$

2. Aryl halides undergo nucleophilic substitution reaction only in extreme conditions, when conditions are feasible.

3. Aryl halides can't be used in Friedel–Crafts alkylation reactions.

4. The presence of electron withdrawing groups like $\ce{NO2, CF3},$ etc. at ortho/para position to the halogen atom makes the aryl halides more susceptible to nucleophilic attack.

• $\text{S}_{\text{N}}$ Ar Mechanism
  Nucleophilic aromatic substitution (Addition-Elimination)

• Rate determining step (R.D.S.): first step, when nucleophile $(\ce{O{ H }^{ - }})$ attacks aryl halide

• Nucleophile attacks the carbon having halogen because only that carbon has ${ \partial }^{ + }$ charge.

1. CINE substitution is followed when nucleophile is present on the carbon other than the one in which the leaving group is present.
2. Presence of electron withdrawing group at ortho/para to the halide group enhances reactivity.
3. Aryl halides can be distinguished from alkyl halides by decolorisation of bromine water: