CBSE Class 12 Chemistry Haloalkanes And Haloarenes Notes Set E

HALOALKANES , HALOARENES

1- Important terms and concept

R-H +X₂→R-X + H-X

(i) Halogen derivative of alkenes called halo alkenes e.g. R- CH₃

(ii)Halogen derivative of arenes called halo arenes e.g. R- C₆H₅

2. (a) Vicinal di halides – where the two halogens are attached on the adjacent carbon atom.

e.g.CH₂Cl
        |
CH₂Cl

(b) geminal dihalide- where two halogen atoms are attached to the same carbon atom eg CH3CHBr2

2- Important mechanism

(i) Nucleophilic substitution

(ii) Electrophilic substitution

(iii) Elimination reaction

(iv) Carbylamine reaction

(v) Reimer Tiemann reaction

(vi) Wurtz reaction

(vii) Wurtz fittig reaction

Nucleophillic substitution it involves the replacement of an atom or group by another atom or group

A – B + C→A – C + B

It must be remembered that A – B and A – C both are covalent compounds.

(i) In aliphatic system

R – X + OH→ R – OH + XR

– X + O-C₂H₅ →R-O- C₂H₅ + X –

Decreasing order of Basicity CH₃ > NH₂ > O-R > OH- > I- > Br- > Cl -

In general

R – X + :B- R – B + X-

: B- OH-, RO-, CN-, RCOO-, NO₃-, SH-, RSBimolecular

Nuclephilic substitution SN²

(i) It takes place in one step.

(ii) Most of the SN2 reaction are second order but some time when Nucleophillic reagent

is present in excess quantity the reaction is of Its order but still proceeds by SN²

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