Points to Remember: Aromatic Compounds | Organic Chemistry PDF Download

Benzene: 

A. Preparation:
➤ By Decarboxylation: When sodium benzoate is heated with soda-lime, benzene is formed.

➤ From Phenol: When phenol is heated with Zn dust, benzene is formed.

➤ From Benzene Sulphonic Acid: When benzene sulphonic acid is treated with sodium hydroxide then sodium benzene sulphonate is formed which on fusion with sodium hydroxide produces sodium phenoxide. It on acidification produce phenol which on heating with zinc dust produces benzene.

➤ From Benzene Diazonium Chloride: When benzene diazonium chloride is treated with H₃PO₂ or, Cu/EtOH then benzene is formed.

➤ From Calcium Carbide: When calcium carbide is hydrolysed then acetylene is formed which on passing through heated copper tube then benzene is formed.

B. Reactions:
➤ Nitration: When benzene is treated with cone, sulphuric acid and fuming nitric acid at 55°C, nitrobenzene is formed.

➤ Sulphonation: When benzene is treated with cone, sulphuric acid , benzene sulphonic acid is formed.

➤ Halogenation: When benzene is treated with Br2 in presence of FeBr3, then bromobenzene is formed.

➤ Friedel-Craft Reaction: 
1. Alkylation: When benzene is treated with alkyl halide in presence of anhydrous aluminium chloride, alkyl benzene is formed.

Limitations: 
1. In most cases poly-substituted benzene is formed.
2. In some cases rearranged product is formed. 

2. Acylation: When benzene is treated with acyl halide in presence of anhydrous aluminium chloride then acyl benzene is formed.

➤ Gatterman Reaction: When benzene is treated with CO in presence of HC1 and anhydrous A_ICI₃ then benzaldehyde is formed.

➤ Gatterman-Koch Reaction: When benzene is treated with HCN in presence HCI and anhydrous A_lCI₃ and the intermediate is hydrolysed then benzaldehyde is formed.

➤ Reaction with CI2 in Presence o f UV light: In presence of diffuse light, benzene reacts with Cl2 to produce benzene hexa-chloride (BHC).

Toluene: 

A. Preparation:
➤ From Benzene: When benzene is treated with methyl chloride in presence of anhydrous aluminium chloride, toluene is formed.

➤ From Benzaldehyde: When benzaldehyde is treated with Zn-Hg/HCl or, N₂H₄/K.OH in presence of EtOH then toluene is fonned.

B. Reaction:
➤ Nitration: When toluene is treated with cone, sulphuric acid and fuming nitric acid at 55°C, o- nitro toluene and p-nitro toluene are formed.

➤ Sulphonation: When toluene is treated with cone, sulphuric acid, o-sulfo toluene and p-sulfo toluene are formed.

➤ Halogenation: When toluene is treated with Br₂ in presence of FeBr₃, then o-bromo toluene and p- bromo toluene are formed.

➤ Oxidation Reaction: When toluene is treated with alkaline KMn04 followed by acidification then benzoic acid is formed.

➤ Reaction at Benzylic Position: When toluene is treated with NBS in presence of light and catalytic HBr then benzyl bromide is formed.

➤ Reaction with Br₂ in Presence of UV: When toluene is treated with 1, 2 and 3 mole of bromine in presence of UV light, mono-chloro toluene, di- chloro toluene and tri-chloro toluene are formed. These on treatment with alkali will give benzyl alcohol, benzaldehyde and benzoic acids respectively.

Nitrobenzene: 

A. Preparation:
➤ From Benzene: When benzene is treated with cone, sulphuric acid and fuming nitric acid at 55°C, nitrobenzene is formed.

➤ From Aniline: When aniline is treated with tri- fluoro acetic acid then nitro benzene is formed.

➤ From Benzene diazonium Chloride: When benzene diazonium chloride is treated with NaNO₂ then nitro benzene is formed.

B. Reactions:
➤ Nitration: When nitrobenzene is treated with cone, sulphuric acid and fuming nitric acid at 55°C, m-di-nitrobenzene is formed.

➤ Sulphonation: When nitrobenzene is treated with cone, sulphuric acid ,m-sulfo nitrobenzene is formed.

➤ Halogenation: When nitrobenzene is treated with Br2 in presence of FeBr3, then m-bromo nitrobenzene is formed.

⇒ Reduction of Nitrobenzene: Nitrobenzene in acidic medium give aniline. It under neutral condition reduce to give hydroxylamine. It under alkaline medium reduce to azoxy-benzene then to azo-benzene and finally, to hydrazo benzene.

Aniline: 

A. Preparation :
➤ From Nitrobenzene: When nitrobenzene is reduced with Sn/HCl then aniline is formed.

➤ Cine Substitution: When bromobenzene is treated with sodamide in presence of liquid ammonia, aniline is formed.

B. Reactions:

➤ Nitration: When aniline is treated with cone, sulphuric acid and fuming nitric acid, o-amino nitrobenzene and p-amino nitrobenzene are formed.

➤ Sulphonation: When aniline is treated with cone, sulphuric acid, o-sulfo aniline and p-sulfo aniline are formed.

➤ Halogenation: When aniline is treated with Br₂ in presence of FeBr₃, then m-bromo aniline and p- bromo aniline are formed.

➤ Reaction with Br₂/H₂O: When aniline is treated with bromine water then 2,4,6-tri-bromo aniline is formed.

➤ Reaction with Ac₂O/AcCI: When aniline is treated with acetic anhydride or, acetyl chloride in presence of pyridine then acetanilide is formed.

➤ Reaction with Nitrous Acid: When aniline is treated with nitrous acid then benzene diazonium chloride is formed.

Benzene Diazonium Chloride: 

A. Preparation:
From Aniline: When aniline is treated with nitrous acid then benzene diazonium chloride is formed.

B. Reactions: The reactions of benzene diazonium chloride are schematically given below:

Haioarenes: 

A. Preparation:
➤ Hunsdiecker Reaction: When silver benzoate is treated with bromine in presence of carbon tetrachloride then bromo benzene is formed.

➤ Sandmeyer Reaction: When benzene diazonium chloride is treated with CuBr in presence of HBr, bromo benzene is formed.

B. Reactions:
➤ Nitration: When bromobenzene is treated with cone, sulphuric acid and fuming nitric acid, o- bromo nitrobenzene and p-bromo nitrobenzene are formed.

➤ Suiphonation: When bromobenzene is treated with cone, sulphuric acid, o-bromo benzene sulfonic acid and p-bromo benzene sulfonic acid are formed.

➤ Halogenation: When bromo benzene is treated with Br₂ in presence of FeBr₃, then o-dibromo benzene and p-dibromo benzene are formed.

➤ Cine Substitution: When bromobenzene is treated with sodamide in presence of liquid ammonia, aniline is formed.

Phenol: 

A. Preparation:
➤ From Benzene diazonium chloride: When benzene diazonium chloride is hydrolysed then phenol is formed.

➤ From Benzene Sulphonic Add: When benzene sulphonic acid is treated with sodium hydroxide then sodium benzene sulphonate is formed which on fusion with sodium hydroxide produces sodium phenoxide. It on acidification produce phenol.

⇒ Dow's Method: When chlorobenzene is treated with NaOH at high pressure and very high temperature followed by acidification leads to the formation of phenol.

➤ From Cumene: When benzene is treated with propene in presence of phosphoric acid then isopropyl benzene is formed which is commonly known as cumene. It on treatment with oxygen followed by acidification leads to the formation of phenol.

B.Reactions:
➤ Reimer-Tiemann Reaction: When phenol is treated with CHCl₃ in presence of ale. KOH and followed by acidic work up produce o-hydroxy benzaldehyde (Salicylaldehyde) as the major product.

➤ Kolbe-Schmidt Reaction: When phenol is treated with sodium hydroxide in presence of carbon dioxide followed by acidification then o-hydroxy. benzoic acid (salicylic acid) is formed. But, phenol under similar condition on reaction with KOH instead of NaOH produces p-hydroxy benzoic acid.

➤ Laderer-Manasse Reaction: When phenol is treated with formaldehyde in presence of sodium hydroxide followed by acidification, o-hydroxy benzyl alcohol and p-hydroxy benzyl alcohol are formed.

⇒ Bromination Reaction: The bromination reaction in phenol under different solvent system are given below:

➤ Vilsmeier-Haak Reaction: When phenol is treated with DMF in presence of POCl₃ and the intermediate is hydrolysed then o-hydroxy benzaldehyde and p-hydroxy benzaldehyde are formed.

Benzaldehyde:

A. Preparation:
➤ From Benzyl Alcohol: When benzyl alcohol is oxidised with P.C.C. or, P.D.C. then benzaldehyde is formed.

➤ Etard Reaction: When toluene is treated with chromyl chloride then benzaldehyde is formed.

➤ Stephen’s Reaction: When cyano benzene is treated with stannous chloride in presence of HCI and then the intermediate product is hydrolysed then benzaldehyde is formed.

➤ Swern Oxidation: When benzyl alcohol is treated with DMSO and oxaloyl chloride followed by treatment with triethylamine produce benzaldehyde.

➤ From Dichlorotoluene: When dichlorotoluene is hydrolysed then benzaldehyde formed.

➤ From Styrene: When styrene undergo ozonolysis then benzaldehyde is formed.

➤ Gatterman Reaction: When benzene is treated with CO in presence of HCI and anhydrous AlCl₃ then benzaldehyde is formed.

➤ Gatterman-Koch Reaction: When benzene is treated with HCN in presence HCl and anhydrous AICI₃ and the intermediate is hydrolysed then benzaldehyde is formed.

B. Reactions:
➤ Claisen-Schmidt Reaction: When benzaldehyde is treated with nitrobenzene in presence of NaOH and then the intermediate is treated with acid and heated then a,p-unsaturated aldehyde is formed.

➤ Cannizzaro Reaction: When benzaldehyde is treated with 50% NaOH and then acidification produce benzyl alcohol and benzoic acid.

➤ Wittig Reaction: When benzaldehyde is treated with triphenyl-phosphonium ylide then styrene is formed.

➤ Perkin Reaction: When benzaldehyde is treated with acetic anhydride in presence of sodium acetate then cinnamic acid is formed.

➤ Benzoin Condensation: When benzaldehyde is treated with alcoholic KCN then benzoin is formed which on oxidation with dil.HNO₃ will produce benzil which on reaction with ale. KOH produce benzilic acid.

Benzoic acid:

A. Preparation:
➤ From Toluene: When toluene is treated with alkaline KMn04 and then acidic work up is done then benzoic acid is formed.

⇒ From Tri-chloro toluene: When tri-chloro toluene is hydrolysed then benzoic acid is formed.

➤ From Benzaldehyde: When benzaldehyde is treated with K₂Cr₂O₇ and H₂SO₄ then benzoic acid is formed.

B. Reactions:
➤ Reaction with Diazomethane: When benzoic acid is treated with diazomethane then methyl benzoate is formed.

➤ Reduction: When benzoic acid is treated with lithium aluminium hydride then benzyl alcohol is formed.

➤ Hunsdiecker Reaction: When silver benzoate is treated with bromine in presence o f carbon tetrachloride then bromo benzene is formed.

➤ Reaction with Ammonia: When benzoic acid is treated with ammonia then ammonium benzoate is formed which on heating produces benzamide which on treatment with P2O5 will produce cyano benzene.