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Wednesday, 15 February 2017

Hydrocarbon X

The compounds that follow the following features are to be considered aromatic:
(i) Planarity
(ii) Complete delocalisation of the π electrons in the ring
(iii) Presence of (4n + 2) π electrons in the ring where n is an integer (n = 0, 1, 2, . . .). This is often referred to as Hückel Rule.

Preparation of Benzene:-

(i) Cyclic polymerisation of ethyne:
(ii) Decarboxylation of aromatic acids:

(iii) Reduction of phenol: Phenol is reduced to benzene by passing its vapours over heated zinc dust

Physical properties:-
1. Aromatic hydrocarbons are non- polar molecules and are usually colourless liquids or solids with a characteristic aroma.
2. Aromatic hydrocarbons are immiscible with water but are readily miscible with organic solvents.
3. They burn with sooty flame.

Chemical properties:-
Arenes are characterised by electrophilic substitution reactions proceed via the following three steps:
(a) Generation of the eletrophile
(b) Formation of carbocation intermediate
(c) Removal of proton from the carbocation intermediate

benzene ontreatment with excess of chlorine in the presence of anhydrous AlCl3 in dark yields hexachlorobenzene (C6Cl6)

Addition reactions of benzene:-
Directive influence of a functional group in monosubstituted benzene:-
1. Ortho and para directing groups and activating- –OH, –NH2, –NHR, – NHCOCH3, –OCH3, –CH3, –C2H5, etc.

2. Meta directing group and deactivating:–NO2, –CN, –CHO, –COR, –COOH, – COOR, –SO3H, etc.

3. Ortho and para directing groups and deactivating- Halogens because of their strong – I effect, overall electron density on benzene ring decreases. However, due to resonance the electron density on o– and p– positions is greater than that at the m-position. Hence, they are also o– and p– directing groups.

CARCINOGENICITY AND TOXICITY:- Benzene and polynuclear hydrocarbons containing more than two benzene rings fused together are toxic and said to possess cancer producing (carcinogenic) property.

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