Arenes:
- Arenes are aromatic compounds containing only carbons and hydrogens.
- They are quite stable due to the delocalisation of electrons i.e. resonance and undergo fewer reactions.
- The simplest of these arenes is benzene and is denoted by C₆H₆.
- Benzene is a planar molecule with delocalised electrons above and below the plane of the ring. It’s electron-rich.
- Benzene is highly attractive to electron-deficient species, or electrophiles. Therefore, it undergoes electrophilic substitution reactions very easily.
- Due to the presence of an electron cloud of delocalised electrons on the benzene ring, the nucleophilic attack is difficult. So, they are repelled by benzene. Benzene undergoes nucleophilic substitutions with difficulty.
- As electrophilic addition destabilises the ring, arenes prefer substitution reactions. In these substitution reactions, the aromaticity is restored by heterolytic cleavage of the C-H bond.
- In addition reactions, the aromaticity is not restored and in some cases completely lost.

Friedel‐Crafts alkylation:
- An alkyl group can be added to a benzene molecule by an electrophilic aromatic substitution reaction called the Friedel‐Crafts alkylation reaction of benzene.
- Methylbenzene or toluene is prepared by this process reacting benzene with halogenoalkane in the presence of a Lewis acid catalyst such as aluminium chloride where hydrogen on the benzene ring is replaced by a group like methyl or ethyl and so on.
- For example: benzene reacts at room temperature with chloroalkanes such as, chloromethane or chloroethane in the presence of aluminium chloride as a catalyst, the substitution of one of the hydrogen atoms by a methyl group gives methylbenzene.
- Multiple substitutions of the alkyl group in alkyl substituted benzene is possible but the position of the incoming alkyl group depends to a large extent on the temperature of the reaction since the reactivity of a benzene ring is governed by the electron density around the ring.
- For example, at 0 °C, when substituting methyl groups into methylbenzene, a mixture of the 2-, 3-, and 4- isomers in the proportion of 54%,17% and 29% respectively are obtained. But at 25 °C, the proportions change to 3%, 69% and 28%.
- Here, methyl groups tend to “push” electrons towards the ring, increasing the density at the 2 and 4 positions of benzene. Hence, making the ring more attractive to attacking reagents.

Friedel–Crafts acylation:
- Acylation means substituting an acyl group into something.
- An acyl group is an alkyl group attached to a carbon-oxygen double bond. If “R” represents any alkyl group, then an acyl group has the formula RCO-.
- When benzene is heated with a mixture of ethanoyl chloride, and aluminium chloride as the catalyst for about 60 °C for about 30 minutes, a ketone called phenylethanone (or 1-phenylethanone) is formed. This acylation reaction is also called ethonylation.
- In the case of Friedel–Crafts acylation of alkyl substituted benzene such as methyl benzene, the position where the acyl group attaches is relative to the alkyl group such as methyl group present. The methyl group in methylbenzene directs new groups into the 2- and 4- positions, assuming the methyl group is in the 1- position
- In acylation, though, virtually all the substitution happens in the 4- position.

Substitution reaction of benzene with halogens (Cl₂ and Br₂)
- The reaction of benzene with chlorine or bromine occurs only in the presence of a catalyst of aluminium chloride, or aluminium bromide.
- Chlorination:
  - The reaction between benzene and chlorine in the presence of aluminium chloride gives chlorobenzene.

Bromination:
- The reaction between benzene and bromine in the presence of aluminium bromide gives bromobenzene.

Nitration:
- In nitration of benzene, one or more of the hydrogen atoms on the benzene ring are replaced by a nitro group, NO₂.
- When benzene is heated with a mixture of concentrated nitric acid and concentrated sulfuric acid for half an hour at a temperature not exceeding 50 °C, yellow oily nitrobenzene is formed.
- At higher temperatures, the nitrobenzene formed reacts with the nitrating mixture of concentrated acids. So, there’s a greater chance of getting more than one nitro group substituted onto the ring.
- Methylbenzene reacts about 25 times faster than benzene during nitration because of the tendency of the methyl group to push electrons towards the ring.
- Since the reaction is 25 times faster, a lower temperature is used to prevent more than one nitro group from being substituted in methylbenzene. In this case, 30°C is suitable instead of 50°C.
- Since the methyl group is 2,4-directing, nitration of methylbenzene gives 2-nitromethylbenzene and 4-nitromethylbenzene but only about 5% of the product is 3-nitromethylbenzene.

Oxidation of aromatic side chains:
- Benzene and alkanes are not easily oxidised. However, when attached to a benzene ring, the benzylic carbon containing hydrogen undergoes unique oxidation, yielding a benzoic acid.
- Side chains of aromatic compounds are susceptible to oxidation by strong oxidising agents such as aqueous potassium permanganate (KMnO₄) under basic conditions.
- For example, methylbenzene can be readily oxidised to benzoic acid by using aqueous potassium permanganate.
- Benzylic oxidation is also used in the identification of aromatic natural products, especially when spectroscopic techniques are not available.
- An antibiotic formed from penicillium was identified in 1952 with the help of permanganate oxidation.
- This method lends itself well to determining substitution patterns present on aromatic rings. For example, o-, m- and p-xylene will each give a different dicarboxylic acid upon oxidation.

Hydrogenation of the benzene ring (Reduction)
- The aromatic ring undergoes reduction since it’s unsaturated.
- The benzene ring undergoes a reduction in the presence of a catalyst.
- When benzene is heated with hydrogen in the presence of catalysts such as platinum and nickel at high temperature and high pressure, reduction of benzene takes place giving cyclohexane as a product.
- This reduction reaction that takes place in the presence of a catalyst is called catalytic hydrogenation of aromatic rings.

30.01 - Chemistry of Arenes

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