Methods of Preparation of Haloalkanes Preparation and Properties
→ From Alkanes : Alkyl halides are almost never prepared by direct halogenation of alkanes because the reaction generally gives a mixture of several compounds.
→ However, certain compounds can form monohalogen product in good yield, e.g., CH4, CH3CH3, (CH3)4C, C6H5CH3, CH3CH = CH2, etc., because in all of them hydrogen atoms to be replaced by halogen are equivalent (remember that allylic hydrogen atoms are much more reactive than the inert vinylic hydrogens).
→ Bromination at allylic and benzylic positions may best be carried out by another brominating reagent, N – bromosuccinimide.
Halogenation of alkanes takes place through free-radical mechanism.
→ By addition of Hydrogen Halide : Alkenes add hydrogen halides to form alkyl halides. In case of unsymmetrical alkenes, hydrogen halides add according to Markownikov’s rule.
CH3 – CH = CH2 + HX → CH3CH-CH3
→ Addition of hydrogen halides to alkenes is an example of electrophilic addition involving carbocations as intermediates.
→ However, addition of H-Br (not HCl, HI and HF) on alkenes in presence of peroxides takes place according to anti- Markownikov’s way, the peroxide effect.
Here the addition takes place via free-radical mechanism.
Addition of halogen to alkene and alkyne
By the reaction of alcohol and Halogen acids:
→ Alcohols are considered the best starting materials for preparing alkyl halides. The most common method is to treat the alcohol with HX. The order of reactivity of the hydrogen halides is: HI > HBr > HCI >> HF.
→ Among the various classes of alcohols, the order of reactivity with hydrogen halides is
Tertiary > Secondary> Primary > CH3OH
→ Tertiary alcohols are so reactive that these are converted to alkyl chlorides in high yield within minutes on reaction with HCl at room temperature, while secondary and primary alcohols give very low yield under these conditions. Therefore, the more reactive HBr is used to get quantitative yield of secondary and primary bromides at elevated temperatures.
→ Tertiary alcohols react with HX by SN1 pathway, while primary alcohols react via SN2. Secondary alcohols may follow either or both of the paths.
In the above reactions, the order of reactivity is as follows :
Halogen acid: HI > HBr > HCl.
→ By the action of thionyl chloride on alcohol : This is the best method for the prepration of alkylhalide from alcohol because with alkyl halides two gaseous byproduct SO2 and HCl are also obtained, SO2 easily escapes and HCl is absorbed by pyridine (C5H5N) and thus leaving behind pure alkyl halide. This reaction is known as Darzen’s reaction.
→ By the action of phosphorus halides : Phosphorus pentachioride (PCl5) and phosphorus trichioride (PCl3 ) form alkyl chloride in following ways:
→ For the preparation of alkyl bromide and alkyl iodide PBr3 and PI3 are used. They are prepared in situ by the action of bromine or iodine on red phosphorus in the reaction flask.
→ From Halogen Exchange Method : This reaction is particularly useful for preparing iodoalkanes. The iodoalkanes are obtained by heating chioro or bromo alkanes with a concentrated solution of sodium iodide in acetone.
→ Sodium chloride and sodium bromide being less soluble in acetone get precipitated from the solution and can be removed by filtratìon. This also prevents the backward reaction to occur according to Le-chatelier’s principle.
→ The reaction gives the best results with primary halides. This reaction is known as Finkeistein reaction. Fluoro alkanes are difficult to prepare directly. These are prepared by treating alkyl chlorides or bromides with salts such as mercurous fluoride (Hg2F2), silver fluoride (AgF), cobalt fluoride (CoF3) or antimony trifluorides (SbF3).
This reaction is known as Swart’s reaction.
→ Prepration from Silver salts of Acids : The silver salts of the carboxylic acids dissolved in CCl4 are decomposed by bromine to form bromoalkanes. This reaction is called Hunsdiecker reaction. The reaction is believed to occur by free radical mechanism.
CH3COOAg + Br2 → CH3 Br + CO2 + AgBr
→ Reaction is used to reduce the length of carbon chain by one carbon atom.
The yield of alkyl bromide follows the order:
1° > 2° > 3°