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Methods of preparation of Alkyl-halides : : (From Alcohols :champagne:…
Methods of preparation of Alkyl-halides : :
From Alcohols :champagne:
Reaction of alcohols with halogen acids (Order of reaction for a given haloacid is tertiary > secondary > primary)
\(R-OH + HCL -(ZnCl_2)-> R-Cl + H_2O\) \(ZnCl_2\) is needed for primary and secondary alcohols. For tertiary alcohols, the reaction can be done by shaking with conc HCl at room temperature.
Constant boiling with 48% HBr is used for preparing alkyl bromides.
Heating alcohols with KI or NaI in 95% orthophosphoric acid gives good yield of R-I
\(R-OH + NaBr + H_2SO_4 --> R-Br + NaHSO_4 + H_2O\)
Alkyl Chloride can also be prepared by passing dry HCl gas over a solution of alcohol
Reaction of alcohols with phosphorous halides
\(3R-OH + PX_3 --> 3R-X + H_3PO_3\) (X = Cl, Br)
\(R-OH + PCl_5 --> R-Cl + POCl_3 + HCl\)
R-OH -(Red P / \(X_2)\)-> R-X \((X_2 = Br_2, I_2)\) (Phosphorous tribromide or triiodide are usually generated in the reaction mixture)
Reaction of alcohols with thionyl chloride (Darzon's Procedure)
\(R-OH + SOCl_2 --> R-Cl + SO_2(g) + HCl(g) \) (This reaction is preferred because the byproducts are gases )
From Hydrocarbons :fire:
By free radical halogenation of alkanes
\(CH_3CH_2CH_2CH_3 --(X_2 (with) (UV- light) or (heat))--> CH_3CH_2CH_2CH_2X + CH_3CH_2CHXCH_3\)(X - Cl, Br) This reaction gives a complex mixture of isomeric mono and poly halo alkanes, which are difficult to separate, therefore yield is low
From alkenes
\(C_2H_4 + HX --> CH_3CH_2X (X = Cl, I, Br)\)(Addition of hydrogen halides) For unsymmetric alkenes, major product is given by Markovnikov's rule
\(C_2H_4 + Br_2 --(CCl_4)--> BrCH_2-CH_2Br (Vicinal-dibromide, colourless)\) This reaction results in the discharge of reddish- brown colour of bromine, therefore it's used to detect double bond in a molecule
From silver salt of carboxylic acid :red_flag:
\(RCOOAg + Br_2 --> RBr + CO_2 + AgBr\) (Borodine Hunsdicker reaction)
\(2RCOOAg + I_2 --> RCOOR + CO_2 + 2AgI\) (Brinbaun Simolini reaction)
Halogen Exchange :warning:
\(R-X + NaI --(dry-acetone)--> R-I + NaX (X = Cl, Br) NaX \)is precipitated in dry acetone causing forward reaction to take place
\(H_3C-Br + AgF -(heating)-> H_3C-F + AgBr\) This reaction is done by heating R-Br or R-Cl in presence of metallic fluoride (\(AgF, Hg_2F_2, CoF_2, SbF_3\))