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Mahatma Phule A.S.C. College, Panvel Prof. Kokil Dhananjay Narayan Dept. of Chemistry Class : S.Y.B.Sc. Paper III {Organic Chemistry}

 Haloarenes  Haloarenes are halogen containing aromatic compounds. There are two types : Nuclear substituted compounds: Halogen is directly attached to the carbon of aromatic ring e.g.

II)Side chain substituted compounds :- Halogen is attached to side chain carbon of alkyl group. These are also called as alkyl aryl halides.

Preparation of Haloarenes :- When benzene is treated with halogens in presence of Lewis acid [AlCl3, FeCl3 AlBr3] to give halobenzene. e.g.

Mechanism :- It is electrophilic substitution reaction. I) Formation of electrophile

II) Attack of electrophile :- III) Loss of proton :-

Halogenation of substituted benzenes (toluene) :- 1) Halogenation of toluene in the dark give ortho and para product.

2) Halogenation of toluene in boiling condition, hydrogen of CH3 gr replaced by halogen successively.

Reactions of Haloarenes :- Nucleophilic substitution reaction in haloarenes [SN¹ and SN²] are not possible because of Electron withdrawing nature of aromatic ring (NO SN²) Back side attack not possible due to aromatice ring (NO SN¹) but under drastic condition, these reactions are possible as follows :-

Electron withdrawing group at o- or p- position favours the reaction.

Mechanism of nucleophilic substitution reactions takes place by two methods

Addition elimination mechanism :- Addition of nucleophile takes place followed by departure of leaving group . [1st addition then elimination takes place]

II) Elimination addition mechanism :- In this mechanism, elimination (departure of leaving group) is followed by addition of nucleophile. In this mechanism benzyne intermediate are formed. [1st elimination then addition of nucleophile takes place ] NH2 is exceptionally strong base in the form of NaNH2 or KNH2 is used.

Mechanism

Cine – Substitution :- In normal aromatic nucleophilic substitution reactions, substitution of group takes place the position of leaving group. But in cine substitution, substitution of group takes place also at adjecent position of leaving group. This is possible only when benzyne intermediate is formed. It can be proved by isotopic labelling of C14.

Following reaction also proves the formation of cine product.

Preparation of grignard reagent from Haloarenes :- Grignard Reagent has Gen. formula R-mg-X, X = Cl , Br, I R=alkyl, aryl group Haloarenes reacts with magnesium in THF [tetrahydrofuran] solvent to form G.R.

G.R. used in preparation of many aromatic comps like carboxylic acid, alcohols. Preparction of carboxylic acid :-

II) Preparation of alcohols :- Aryl magnesium halide [G.R.] when treated than formaldehyde and ketone in presence of ether to give primary, secondary and tertiary alcohol respectively the reactions are as follows

III) Ullmann reaction :- When two moles of aryl halide reacts with copper –bronze alloy, the product is biphenyl derivative. The electron withdrawing gr at aromatic ring favours the reaction and gives good yield. This is Ullmann reaction.

Applications of haloarenes :- Chlorobenzene is used for manufacture of anilines, DDT and phenol. Benzene hexa chloride (BHC) is used to prepare insectiside. Chloronitrobenzene are used to prepare biphenyl derivatives. Aryl G.R. are used to prepare number of synthetically important compounds like carboxylic acids, alcohols etc.

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