of the π electrons in the ring (iii) Presence of ( n + ) π electrons in the ring where n is an integer ( n = , , , . . .). This is often referred to as Hückel Rule .
Some examples of aromatic compounds are given below: . . Preparation of Benzene Benzene is commercially isolated from coal tar. However, it may be prepared in the laboratory by the following methods.
(i) Cyclic polymerisation of ethyne: (Section . . ) (ii) Decarboxylation of aromatic acids: Sodium salt of benzoic acid on heating with sodalime gives benzene. ( .
) (electron cloud) (iii) Reduction of phenol: Phenol is reduced to benzene by passing its vapours over heated zinc dust ( . ) . . Properties Physical properties Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma.
You are also familiar with naphthalene balls which are used in toilets and for preservation of clothes because of unique smell of the compound and the moth repellent property. Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents. They burn with sooty flame. Chemical properties Arenes are characterised by electrophilic substitution reactions.
However, under special conditions they can also undergo addition and oxidation reactions. Electrophilic substitution reactions The common electrophilic substitution reactions of arenes are nitration, halogenation, sulphonation, Friedel Craft’s alkylation and acylation reactions in which attacking reagent is an electrophile (E + ) (i) Nitration: A nitro group is introduced into benzene ring when benzene is heated with a mixture of concentrated nitric acid and concentrated sulphuric acid (nitrating mixture). ( . ) Nitrobenzene (ii) Halogenation: Arenes react with halogens in the presence of a Lewis acid like anhydrous FeCl , FeBr or AlCl to yield haloarenes.
Chlorobenzene ( . ) (iii) Sulphonation: The replacement of a hydrogen atom by a sulphonic acid group in a ring is called sulphonation. It is carried out by heating benzene with fuming sulphuric acid (oleum). ( .
) (iv) Friedel-Crafts alkylation reaction: When benzene is treated with an alkyl halide in the presence of anhydrous aluminium chloride, alkylbenene is formed. ( . ) ( . )