10. HALOGEN DERIVATIVES - part 02 - Methods of preparation of alkyl halides

10. HALOGEN DERIVATIVES - part 02 - Methods of preparation of alkyl halides

Methods of preparation of alkyl halides :
1. From alcohol : 
  • Most widely used method of preparation of alkyl halide is replacement of hydroxyl group of an alcohol by halogen atom. 
  • Alcohols are available in a wide variety. The hydroxyl group may be replaced by halogen atom using  - 
  1. Halogen acid
  2. Phosphorous halide or 
  3. Thionyl chloride.
a. By using halogen acid or hydrogen halide (HX) : 

  • The conditions for reaction depend on the structure of the alcohol and particular halogen acid used. 
  • The order of reactivity of alcohols with a given haloacid is 30>20>10.
  • Grooves' process : Hydrogen chloride is used with zinc chloride  for primary and secondary alcohols.
  • But tertiary alcohols readily react with concentrated hydrochloric acid in absence of zinc chloride.
Do you know ?
  • Zinc chloride is a Lewis acid and consequently can coordinate with the alcohol, weakening R - O bond.
  • Mixture of concentrated HCl and anhydrousZnCl2 is called Lucas reagent.
alkyl bromides :

  • Constant boiling hydrobromic acid (48%) is used for preparing alkyl bromides.
  • Primary alkyl bromides can also be prepared by reaction with NaBr and H2SO4. 
  • Here HBr is generated in situ.
Alkyl iodides :
  • Obtained by heating alcohols with sodium or potassium iodide in 95 % phosphoric acid.
  • Here HI is generated in situ.
b. By using phosphorous halide : 
  • An alkyl halide may be prepared by action ofphosphorous halide on alcohol. 
  • Phosphorous tribromide and triiodide are usually generated in situ (produced in the reaction mixture) by the action of red phosphorous on bromine and iodine respectively. 
  • Phosphorous pentachloride reacts with alcohol to give alkyl chloride.
Do you know ?
  • Some times during replacement of -OH by -X, alcohols tend to undergo rearrangement. 
  • This tendency can be minimized by use of phosphorous halides. 
  • Straight chain primary alcohols react with phosphorous trihalide to give unrearranged alkyl halides.
c. By using thionyl chloride : 
  • Thionyl chloride reacts with straight chain primary alcohols to give unrearranged alkyl chloride.
  • The byproducts obtained are gases. 
  • There is no need to put extra efforts for its separation. Therefore this method is preferred for preparation of alkyl chloride
2. From hydrocarbon :
  • Alkyl halides are formed from saturated as well as unsaturated hydrocarbons by various reactions. 
  • Halogenation of alkanes is not suitable for preparation of alkyl halides as a mixture of mono and poly halogen compounds is formed.

Addition of hydrogen halide to alkene : 
  • Alkyl halides are formed on addition of hydrogen halide to alkenes. 
Do U Know ?
  • Alkenes form additon product, vicinal dihalide, with chlorine or bromine usually in inert solvent like CCl4 at room temperature.
Do you know ?
  • When alkenes are heated with Br2 or Cl2 at high temperature, hydrogen atom of allylic carbon is substittued with halogen atom giving allyl halide.
3 Halogen exchange : 
  • Alkyl iodides are prepared conveniently by treating alkyl chlorides or bromides with sodium iodide in methanol or acetone solution. 
  • The sodium bromide or sodium chloride precipitates from the solution and can be separated by filtration.
  • The reaction is known as Finkelstein reaction.
  • Alkyl fluorides are prepared by heating alkyl chlorides or bromides with metal fluorides such as AgF, Hg2F2, AsF3, SbF2 etc.
  • The reaction is known as Swartz reaction.
4. Electrophilic substitution :
  • Aryl chlorides and bromides can be prepared by direct halogenation of benzene and its derivatives through electrophilic substitution. 
  • It may be conveniently carried out in dark at ordinary temperature in presence of suitable Lewis acid catalyst like Fe, FeCl3 or anhydrous AlCl3.
  • When toluene is brominated in presence of iron, a mixture of ortho and para bromo toluene is obtained.
  • Aromatic electrophilic substitution with iodine is reversible. 
  • In this case use of HNO3/HIO4 removes HI by oxidation to I2, equilibrium is shifted to right and iodo product is formed. 
  • F2 being highly reactive, fluoro compounds are not prepared by this method.

5 Sandmeyer's reaction : 
  • Aryl halides are most commonly prepared by replacement of nitrogen of diazonium salt.

    Comments

    Post a Comment

    Popular posts from this blog

    6. Biomolecules - part 01 - Carbohydrates

    Image
      6. Biomolecules - part 01 - Carbohydrates Introduction : Our planet is having a wide diversity of living organisms that are classified as - unicellular  (consisting of a single cell; including bacteria and yeast) or  multicellular  having many cells (e.g. plants and animals).  living organisms have cell as the basic structural and functional unit.  The cells have protoplasm containing numerous chemical molecules, the biomolecules. Biochemistry  : Biochemistry is  biological chemistry  that provides us the idea of the chemistry of living organisms and molecular basis for changes taking place in plants, animals and microbial cells.  It develops the foundation or understanding  all biological processes  and communication within and between cells as well as chemical basis of inheritance and diseases in animals and plants. Chemical analysis of all living organisms indicates presence of the most common elements like  carbon, h...
    Read more

    6. Biomolecules - part 02 - Lipids

    Image
      6. Biomolecules - part 02 - Lipids B. Lipids : These are group of substances with greasy consistency with  long hydrocarbon chain containing carbon, hydrogen and oxygen. In lipids, hydrogen to oxygen ratio is  greater than 2:1  (in carbohydrates it is always 2:1). Lipid is a broader term used for fatty acids and their derivatives.  They are soluble in organic solvents (non-polar solvents).  Fatty acids are organic acids which are composed of hydrocarbon chain  ending in carboxyl group (-COOH).  They can be  saturated fatty acids  with no double bonds between the carbon atoms of the hydrocarbon chain.  Palmitic and stearic acids  found in all animal and plant fats are examples of saturated fatty acids. Unsaturated Fatty Acids  are with one or more double bonds between the carbon atoms of the hydrocarbon chain.  Oleic acid  found in nearly all fats and  linoleic acid  found in many seed oils are example...
    Read more

    6. Biomolecules - part 05 - Enzymes

    Image
      6. Biomolecules - part 05 - Enzymes Enzymes : Thousands of different chemical reactions take place automatically at a given time in a tiny living cell. The reactions take place at the body temperature.  If these enzymes were not present in the cell, either the reactions would not occur or if they occur they would occur at a  very very slow rate . Know the scientists : German chemist  Edward Buchner  discovered enzymes by accident.  Buchner discovered that living cells were not necessary but that  yeast  extract could bring about fermentation.  He then coined the term Enzyme (Gk. En = in, zyma = yeast i.e. in yeast). This term is now commonly used for all biocatalysts. Each enzyme catalyzes a small number of reactions, specifically perhaps only one. Substrate :  The substance upon which an enzyme acts is termed as the substrate.  Endo-enzymes :  The enzymes which act within the cell in which they are synthesizedare known as en...
    Read more